W REPORT NO 20S52-fcC02-RO-00
;NAt REF
PORTABLE LASER VELOCIMETER
FOR
STACK VELOCITY MEASUREMENTS
NOVEMBER
L . 0 H E F c J N G E R 5
T T H £ W S a n d H
K r a p c r e ^ f c r
E N V! R O N M 5 N T A L PROTtCTiON A G E N C Y
RESEARCH TRtANGL? PARK NORTH CAROLINA
TRW
--. -
-------
TRW REPORT NO. 20852-6002-RO-OO
FINAL REPORT
PORTABLE LASER VELOCIMETER
FOR
STACK VELOCITY MEASUREMENTS
NOVEMBER 1972
Prepared by
L.O. HEFLINGER, B.J. MATTHEWS, and H. SHELTON
Prepared for
ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NORTH CAROLINA
SYSTfMS GROUP
ONE SPACE PARK REDONDO B E*A CH CALIFORNIA
-------
CONTENTS
Page
1. INTRODUCTION AND SUMMARY 1
2. PORTABLE LASER VELOCIMETER 3
2.1 Background 3
2.2 Velocimeter Operating Principle 5
2.3 Velocimeter Optical System 6
2.4 Velocimeter Electronics 13
2.5 Velocimeter Testing 22
3. LONG RANGE OPTICAL VELOCITY METERS 28
3.1 Introduction 28
3.2 Smoke Optical Properties 31
3.3 Sampl e Vol ume Si ze 32
3.4 Significance of the Smoke Optical
Properties in Conjunction with the
Sample Volume Size 35
3.5 Shot Noise of Detection 36
3.6 The Direct Doppler System 38
3.7 The Fringe Velocimeter: Granularity Noise 42
3.8 Fringe Velocimeter: Required Laser Power 48
3.9 Reticle Velocimeter Noise Estimate 51
3.10 Preliminary Experiments with a
Reticle Velocimeter 53
4. CONCLUSIONS 55
4.1 Portable Laser Velocimeter 55
4.2 Long Range Velocimeters 55
REFERENCES 57
APPENDICES
A SPECIFICATIONS FOR SPECTRA-PHYSICS MODEL
120 GAS LASER WITH MODEL 256 EXCITER 58
B LASER VELOCIMETER DRAWING LIST 59
-------
ILLUSTRATIONS
Page
1. Velocimeter electronics cabinet and sensing head 1
2. Schematic of the interference between two
coherent light beams 4
3. Schematic diagram of laser velocimeter optical
sy s tern 7
4. Laser velocimeter sensing head assembly 10
5. Photograph of velocimeter sensing head showing
component piacement 11
6. Photograph of velocimeter sensing head showing
framework and controls 12
7. Three-quarter view of sensing head 13
8. Oscilloscope traces of PMT signals at the output
of the bandpass amplifier in the breadboard
velocimeter 15
9. Oscilloscope traces of spectrum analyzer signals
obtained with the breadboard velocimeter 17
10. Electronic circuit diagram for the laser velocimeter 19
11. Laser velocimeter sensing head 23
12. Velocimeter electronics package 23
13. Plot of measured versus calculated velocity for
laser velocimeter 24
14. Air and predicted particle (droplet) velocity 27
15. Direct doppler velocity meter 28
16. Schematic of fringe velocimeter 29
17. Reticle velocimeter 30
18. Beam intersection geometry 33
19. Effect of turbulence on direct doppler system 38
20. Granularity noise in the fringe velocimeter 42
-------
FOREWORD
This report summarizes work done under Environmental Protection
Agency Contract No. 68-02-0308, dated 30 June 1971. Activities under
this contract were concerned with: (1) design and construction of a
portable laser velocimeter for stack velocity measurements at close
range; and (2) analysis of optical velocity measurement techniques
over ranges up to 1500 feet.
Technical direction and administration of this project by the
Environmental Protection Agency was provided by Dr. Frederic C. Jaye.
The TRW Project Manager was Mr. Birch J. Matthews. Drs. Haywood She!ton
and Lee 0. Heflinger developed the laser velocimeter instrument and the
analysis for extended range use of optical velocity meters, respectively.
The authors wish to thank Mr. William F. Daley for his valuable
assistance in the design and fabrication of the laser velocimeter.
Mr. Robert F. Kemp also made significant contributions to the initial
design concepts for the optical and electronic systems.
-------
1. INTRODUCTION AND SUMMARY
This report describes the design-development of a short range
laser velocimeter together with an analysis of potential extended range
optical velocity measuring techniques. The work was done under contract
to the Environmental Protection Agency.
The laser velocimeter constructed during this program operates on an
interference-backscatter principle. It is portable and designed to measure
velocities of flowing particle laden gases in power plant ducts or stacks
at relatively close ranges. The instrument is composed of a sensing head
cabinet and an electronics cabinet. These are pictured in Figure 1. The
Figure 1. Velocimeter electronics cabinet (1) and
sensing head (r).
sensing head incorporates a laser illuminator, beam forming optics and photo-
detector. The electronics cabinet contains a signal processing circuit,
spectrum analyzer, as well as power supplies for the laser and spectrum
analyzer. The spectrum analyzer is used to analyze the frequency signals
generated by the sensing head. Following are design and operating charac-
teristics for this laser velocimeter:
-------
Velocity sensing range
t Focal length range
Measuring medium
§ Sensing head weight
Sensing head envelope
Electronics weight
Electronics envelope
Readout (Meter)
Readout (meter)
Power requirements
10 to 125 ft/sec
2 to 15 ft
Fossil fuel stack gas
25 Ib
27-3/4 x 11-3/4 x 6-7/8 inches
25 Ib
12-1/2 x 16 x 7-1/4 inches
Mean velocity, ft/sec
20 sec. time constant
Velocity deviation, ft/sec
5 sec time constant
llOv, 60 Hz, <5 amp
The design, construction and preliminary testing of this
velocimeter are discussed in Section 2.
The second major program task was an analysis of optical velocity
measuring devices for extended range operation. Of interest here was
remote stack plume velocity measurements over distances of 300 to
1500 feet. Three types of optical velocimeters were considered;
namely, Doppler shift, interference-fringe, and a reticle velocimeter.
This discussion is presented in Section 3.
0-100 mv analog signal available.
-------
2. PORTABLE LASER VELOCIMETER
2.1 BACKGROUND
When two coherent beams of light intersect in space, stationary
three-dimensional interference fringe patterns are established. The
fringe spacing A is a function of the wavelength of light A and the
angle Q at which the two beams intersect. This is shown in Figure 2.
In this illustration, both beams are parallel beams of light, as though
from point sources at infinity. Each is traveling at the velocity of
light C, each is of wavelsngth A, and both are coherent to each
other. The two beams of light pass through each other at an angle Q .
Since light is a transverse electromagnetic phenomenon, the direction
of vibration of the electric component is perpendicular to the plane
of the drawing. For plane waves, the electric intensity varies
sinusoidally in strength as one moves down the wave train.
The wavelength A is the physical separation between adjacent
maxima of the traveling electric field. When the maxima of one wave
are superimposed on the maxima of another, the amplitudes add,
constructively. When the maxima of one wave are superimposed with the
minima of the other, the two destructively interfere. If the wavefronts
are of equal amplitude, the net amplitude is zero at these points. The
loci of constructive interference trace out the straight lines shown
in Figure 2. In the case of beams of finite cross section, planes are
actually traced out. These planes, or alternate lines of darkness and
light, are parallel to the bisector of the angle 8 between the direction
of propagation of the two beams. The regions of destructive interference
form periodic planes of absolute darkness. In Figure 2, the loci of
constructive interference are indicated by the parallel lines (regions
of light). Destructive interferences are the blank regions (no light)
-------
EXP j
EXP j
FIRST TRAVELING
WAVE OF WAVELENGTH \
STATIONARY LOCI OF
CONSTRUCTIVE
INTERFERENCE
SECOND TRAVELING WAVE
OF WAVELENGTH \
Figure 2. Schematic of the interference between two traveling collimated coherent
beams of light of wavelength x passing through each other at an angle of e.
-------
in between. Neighboring dark or light regions are separated by a distance
A =
2 sin e/2
As an example, when the two beams pass through one another at an angle
of 60 degrees, the loci of constructive interference are separated from
each other by the wavelength of light. Deep red light has a wavelength
of 0.7 micron or 27.5 microinches.
2.2 VELOCIMETER OPERATING PRINCIPLE
The laser velocimeter described subsequently utilizes the wave
interference properties of two coherent light beams described in the
previous section. Two beams (<1 mm diameter) of coherent light derived
from a helium-neon gas laser (x=0.6328^) intersect in space establishing
an interference fringe pattern. The beams intersect at an angle of
approximately 1 degree (1/60 radian). Horizontal fringe spacing is
0.0015 inch in an intersection volume about 1/2 mm wide and 1 cm long.
Particulate laden combustion gas passes through the region of inter-
ference. A particle moving through adjacent fringes of constant separa-
tion will scatter light periodically. If this scattered light is collected
and focused onto a photodetector, it will produce a coincident pulsating
electric current whose frequency is proportional to the particle's velocity.
In the present velocimeter system, the frequency of light modulation
ranges from 80 Kc to 1 Me corresponding to a velocity range of 10 to 125
ft/sec.
The output of the photodetector is fed into a spectrum analyzer
which is a sensitive receiver that can be electronically swept from
near zero frequency to above 1 megacycle. The input frequency is
indicated by the value of the sweep voltage at the instant an output
signal is obtained. The value of this voltage is stored and displayed
on a meter and read as the proportional velocity. The velocimeter
-------
electronic circuit will search for a lock on any frequency of
sufficient amplitude which is present.
2.3 VELOCIMETER OPTICAL SYSTEM
The laser velocimeter is a single station instrument requiring
only one port or opening in the power plant stack. The signals received
by the optical system are those generated by low angle particulate
scattering in the backward direction.
The optical system is self-aligning which automatically focuses
the two coherent light beams to a crossover point in space. The same
optics collect the backscatter and focus it onto a photodetector. A
schematic of the velocimeter optical arrangement is shown in Figure 3.
The illumination source is a Spectra-Physics helium-neon gas laser
which emits 5 mw power at x =0.6328/a(red light)*. Specifications for
this laser are provided in Appendix A.
This laser was selected on the basis of reliability, plasma tube
life, ruggedness and maximum power output consistent with size and weight
limitations.
From the schematic of Figure 3, it will be seen that the optical
path is folded to obtain a compact design. Collimated light from the
continuous wave helium-neon laser is incident on an adjustable-beam
*
Spectra-Physics Model 120 Gas Laser with Model 256 Exciter. Product
of Spectra-Physics, Inc., 1250 West Middlefield Road, Mountain View,
California.
-------
SLOTTED FIRST
SURFACE MIRROR
CONVERGING LENS,
f. I. = 2.625 IN.
2 SIN (9/2)
\ SOLID STATE
PHOTODIODE
FIRST SURFACE
MIRROR
ADJUSTABLE 240 MM f.l.
ACHROMAT FOCUSING AND
COLLECTING LENS
HELIUM - NEON
GAS LASER
( X = .6328 )
ADJUSTABLE
BEAM SPLITTER
Figure 3. Schematic diagram of laser velocimeter optical system.
-------
splitter. The beam splitter consists of back-to-back 90-degree prisms.
The two-prism interface transmits 50 percent of the beam onto a small
first surface mirror mounted at 45 degrees to the incident beam. The
coated prism interface reflects the other 50 percent of the primary beam
parallel to the transmitted beam. The prism assembly and small first
surface mirror move in pure translation with respect to each other about
a common centerline thus providing adjustable and parallel beam
separation. Beam separation ranges from a minimum of 0.062 in. to 0.80 in.
This range of beam separation allows a constant separation of the
stationary fringe pattern in space of approximately 40 microns.
After division, the parallel beams are turned 90 degrees by a second
first surface mirror and made parallel to the optical axis of the tele-
scope. The beams are converged to a focal point by a 2.625 in focal
length lens. After crossover, the beams diverge and pass through a
0.062 in. split first surface mirror assembly. The split mirror passes
the diverging beams through to a focusing 240 mm focal length achromat
lens. This 4.332-inch diameter lens causes the two parallel beams to
converge in space. The achromat lens telescopes through a distance of
5.50 inches providing a beam crossover location in space from 2 to 15 feet
along the optical axis. Movement of the achromat lens holder is
accomplished with a rack and pinion gear assembly. Fringe spacing is
held constant by manually adjusting the lens travel and beam splitter
to previously calibrated positions.
Backscattered light by particulate in the flowing gas passing through
the stationary interference pattern is collected by the achromat lens and
reflected by the split first surface mirror assembly. This mirror, mounted
at 45 degrees to the optical axis, allows the scattered light to be focused
at an aperture located before a photodetector. With the exception of the
traveling achromat lens, all components of the optical system are precisely
and rigidly mounted to insure that the scattered light is brought to focus
before the photodetector.
8
-------
Figure 4 is a layout drawing of the laser velocimeter sensing head.
This drawing shows the placement of the gas laser chassis and critical
optical components. A tabulation of the sensing head detail drawings is
presented in Appendix B. The photographs in Figures 5 through 7 illustrate
the basic construction and optical component placement of the sensing head.
With the exception of the traveling objective lens, all of the beam forming
optics are mounted on one rigid platform to preserve alignment. The align-
ment of the objective lens is not as critical because the system is self-
aligning with respect to its movement.
In adapting the laser to the sensing head, the outer case was
removed (to save weight) and mounting holes machined in the laser chassis.*
The front end of the laser is rigidly mounted to the optical platform.
A single point pin mount parallel to the optical axis is used at the other
end to permit thermal expansion of the laser chassis.
The laser and critical beam forming components on the optical
platform are mounted to a 1/2 inch aluminum angle frame at three points.
An aluminum sheet metal cover enclosing the optics is attached to the
frame. The three point tie-down of the optics is designed to minimize
any optical path misalignment due to external forces applied to the
frame or cover.
The basic design concept for the sensing head optical system was
predetermined by the requirement for an interference-backscatter instrument.
This being the case, sensing head design was limited to compact packaging
and maintenance of optical path considerations. More attention, however,
was given to selection of a suitable photodetector and design of signal
processing electronics. These subjects are discussed in the section
following.
* TRW Drawing No. SK 1038-12.
-------
. CABLE STOWAGE HOOKS .
SLOTTED FIRST
SURFACE MIRROR
TRAVELING OBJECTIVE LENS
FIRST SURFACE
MIRROR ,
OBJECTIVE LENS
FOCUSING CONTROL
KNOB v
BEAM SPLITTER
ADJUSTMENT
SCONTROL KNOB
70cm
29 ,
Figure 4. Laser velocimeter sensing head assembly.
-------
Beam Splitter
First Surface Mirror
Gas Laser
Traveling Objective Lens
Slotted First Surface Mirror
2.625 In Focal Length Lens
Figure 5. Photograph of the laser velocimeter sensing head assembly
showing the placement of the various components.
11
-------
Optical Platform
Traveling Lens Adjusting Knob
Traveling Lens Mount
1/2 In Angle Frame
Beam Splitter Adjusting
Knob
Figure 6. Photograph of the other side of the sensing head assembly
showing the location of the two control knobs used to focus
the adjustable beam splitter and traveling objective lens.
12
-------
Figure 7. Three-quarter view of the sensing head. The focusing lens
and cylindrical housing are shown in the foreground of the
photograph.
2.4 VELOCIMETER ELECTRONICS
In describing the optical system, it was noted that an adjustable
beam splitter is used to maintain constant fringe spacing within the
limits of the objective lens travel. A fixed beam splitter arrange-
ment was considered. Such an arrangement (variable fringe spacing),
results in excessively high frequency signals at short range; extended
sampling (beam crossover) volumes at long range; and, more sophisticated
signal processing electronics. For these reasons, the fixed beam
splitter design approach was not adopted in the present design.
Initial work on the velocimeter electronics centered on selection
of a photodetector and signal processing system. A survey of readily
available photomultiplier tubes (PMT) and solid state diodes was made.
A type 4473 PMT and a type MRD 500 silicon photo diode were selected
as candidates. The type 4473 PMT is structurally similar to types
931, 1P28 and 1P21 and is, in effect, a 1P21 selected for maximum
"red" sensitivity. The MRD 500 photo diode has a spectral range
13
-------
throughout the visible and near-infrared.* Peak sensitivity for this
diode is about 0.8 micron. Response time is typically <1 nanosecond.
A laboratory breadboard velocimeter system was set-up for the
purpose of generating typical electrical signals. This set-up consisted
of a 3 mw helium-neon gas laser, a beam splitter, simple positive lens,
a slotted first surface mirror, pinhole and PMT assembly, and a 10-inch
focal length, f/5 achromatic objective lens. The optical arrangement
was, in concept, the same as that shown in Figure 3. A photomultiplier
power supply and bandpass amplifier were used with the PMT.
A moving target (smoke simulator) was devised by cutting a circular
groove in a plexiglas disk. A mixture of flyash and coil dope was poured
into the groove and allowed to dry. When rotated by a small motor, the
disk made a useful target for the velocimeter and was capable of
generating known constant or variable velocities within the design
range of the instrument.
The oscilloscope photographs of Figure 8 show traces of a 1P28 PMT
signal at the output of the bandpass amplifier in the breadboard
velocimeter system. These were obtained from the spinning disk. At a
sweep speed of 5 msec/div, Figure 8 shows a large number of spikes,
resulting from point-to-point differences in scattering properties or
in the number of scattering particles in the laser beam at any one time.
The density of particles on the rotating track was intended to represent
a relatively light loading (approximately 1 grain/ft ) in a gas stream.
In Figure 8a, the spikes are nearly symmetrical about the zero
axis. The bandpass amplifier used transmitted frequencies between
Product of Motorola Semiconductor Products, Inc., Type
MRD 500 P-I-N Silicon Photo Diode.
14
-------
(b)
Figure 8. Photomultiplier tube signals at the output of the bandpass
amplifier in the breadboard laser velocimeter. The upper
figure is at a sweep rate of 5 msec/division and shows
scattering from clumps of particles. The lower figure gives
an indication of the autocorrelation of the signal at the
frequency of the velocity-generated signal (about 140 kHz).
about 30 kHz and 600 kHz. At higher sweep speeds, most of the spikes
are seen to contain strong component signals at the laser doppler
frequency corresponding to the velocity of the track. Figure 8b is a
composite of many such traces taken with the oscilloscope trigger level
set to show synchronization with the velocity signal at about 140 kHz.
After 9 or 10 cycles, the signal appears to merge into the statistical
noise in the phototube signal; however, close inspection of the original
photograph revealed a faint but persistent component of this signal
throughout the trace. The task of detecting this signal, therefore,
becomes one of deriving an intermittent signal out of a comparable
amount of noise.
15
-------
Following these preliminary'breadboard tests, consideration was
next given to the means by which the photodetector signal would be
processed. A wide variety of signal processor systems have been used
with laser doppler velocimeters. Of these, three types were used with
the breadboard optical system. Attempts were made to measure the doppler
frequency directly with frequency counters and also with a pulse-averaging
wide-band discriminator. An attempt was also made to use the frequency
counter in a period-averaging mode. Finally, the signals were examined
with a spectrum analyzer. . It quickly became clear that for the first
of these methods to work, considerable pre-processing of the signals
would be required ahead of the counter or discriminator.
A basic problem with the direct-measuring methods is the matter
of signal continuity. If the velocity signal were completely continuous,
the appropriate readout .device would most nearly resemble a frequency-
modulation radio receiver having an indicating meter instead of the audio
circuits. With discontinuous signals, simple burst counting methods will
work, provided bursts are of sufficient length to be processed after they
are recognized. A fairly elaborate burst pre-processor for use with
\^
sparsely seeded gas streams has been described by Lennert, et. al. The
spectrum analyzer, on the other hand, processes all signals, and leaves
the problem of recognition to be dealt with as a subsequent step.
*
A wide-band spectrum analyzer shows that the signal from the
PMT (Figure 8) contains a nearly constant level of .noise within the
bandpass of the instrument together with a recognizable signal at the
laser doppler frequency. Figure 9 is a photograph of a (selected) single
sweep of the spectrum analyzer through the range of 8 kHz to'about
300 kHz showing a recognizable signaTat about 140 kHz. All sweeps do
*
Tektronix Type 1L5, Plug-In Unit.
16
-------
(a)
(b)
(0
jure 9. Spectrum analyzer signals obtained with the breadboard velocimeter. The
upper trace is a single sweep showing an especially strong pulse. The
other traces are composites of about 250 sweeps each showing peaks at
140 kHz and 210 kHz, respectively.
17
-------
not have the same signal to noise ratio; a strong signal such as this
represents a coincidence between the spectrum analyzer local oscillator
and the passage of a particularly reflective particle through the laser
beam intersection volume. Figures 9b and 9c are composites of about
250 such traces each with the rotational speed of the target wheel set
to generate signals of 140 kHz and 210 kHz, respectively. 'Nearly all
of these traces produce pulses which can be recognized by a level
detector set, for example, at the second major division from the bottom.
These results suggested that a velocimeter instrument'having the
greatest versatility could best be made by using a spectrum analyzer as
the primary signal processor. It would, in effect, continuously scan
the range of the instrument (or a selected portion of the total range)
for recognizable signals.
A silicon photo diode was next acquired and evaluated. Use of a
solid state photodetector offered several potential advantages. The
efficiency of the MRD 500 diode i.n converting incident photons (at the
laser wavelength =0.6328ju) to electrons is near unity. Correspondingly,
the quantum efficiencies for PMT's is typically 2 to 3 percent. Further,
the diode offered the desirable feature of being compact and rugged.
And finally, the signal-to-noise ratio for the photo diode and photo
diode circuit appeared superior in testing (at X=0.6328/x) to
comparable PMT systems.
These arguments led to design of a photo diode circuit and
incorporation of a Tektronix 1L5 spectrum analyzer for signal processing.
The resulting electronics circuit is shown in the Figure lO^schematic.
The Motorola photo diode has a back leakage current proportional
to the light falling on it. Its current flows through a 47 kn register
and a bipassed monitor resistance (so the current can be peaked for
18
-------
TRIANGULAR SWEEP GENERATOR
5W. OUT. AMP.
*A/V
82K
SEARCH '
+I5V IK > RECORDER
50FT/S f 150 FT/5
I I (WHITE)
I ? 560 PHOTO
I ? DIODE I
4417 I
FET
L5 TEKTRONIX SPECTRUM ANALYSER
MODIFICATION: JUMPER Bl TO NO.. 7; PULL APART JUNCTIONS BJ, BD, BK, BG
LOOSE COUPLING
TO Its
'.! 7,3pV
i'ZENERS
TYPICAL MC 14336
COMPENSATION
+ 15V -I5V .
4 - HEP 244 - (ANY DIODES OF SUFFICIENT PIV)
Figure 10. Electronic circuit diagram for the laser velocimeter.
-------
alignment purposes). This resistance and the shunt capacity (~4 pf)
give a 3 db high frequency attenuating at about 1 me. The voltage
across this resistor is led to the gate of a 2N4417 Field-effect
transistor operating as a source-follower to drive the capacity of the
coaxial which leads to a 3:1 step up transformer. This series capacity
and shunt inductance give a low frequency cut-off at about 60 kc. High
frequency response is about 1 me with the input capacity of the spectrum
analyzer and cable.
The 1L5 spectrum analyzer is modified as shown on the Figure 10
schematic so as to require least power, to functionally eliminate many
of the analyzer control knobs and assure stable calibration. It is
powered by a light weight power supply (a kilohertz torroid core)
which also supplies plus and minus 15 volts for the photo diode, FET,
and operational amplifiers in the electronic sweep and sample circuits.
Cord line dropping resistors are used to reduce the heat load in the
box and increase the reliability of the transistors in the supply. A
Motorola HEP bridge and large filter capacities supply a voltage that
is regulated to about 75 volts. This voltage drives a DC-DC converter
using high voltage HEP transistors (in parallel to provide current
rating). The inverter is a high efficiency (special square loop tape-
wound core) saturating square-wave oscillator. Multiple air-cooled
secondary windings supply the voltage to full-wave bridges with a
single output capacity. The 225 volts is derived from the 350 volts
and regulated by Zener diodes.
The read-out circuits all use the identical integrated circuit
chip (MC1433G) for simplicity of stocking spare parts. One chip forms
a triangular (not a sawtooth) sweep. This is accomplished by feeding
the integral of the snapping saturated square wave output into the
inverting input and into the sweep output amplifier which supplies
current, to charge the sampling capacitor (whose voltage is proportional
20
-------
to the velocity and drives the output meter through an amplifier). The
amplitude of the sweep (always the same dv/dt) and its mean position is
controlled by the SPOT search-lock switch which changes the
regenerative feedback by 5 and also feeds the meter output voltage into
the circuit so that on "lock", the sweep circuit sweeps +_ 10% about the
signal. When a signal is received from the spectrum analyzer it is
amplified and if large enough, triggers a threshold detector which
charges up the sampling capacity to the voltage value of the sweep at
that instant. A meter then reads the corresponding velocity. If the
signal is lost, the voltage on the capacity will drift towards zero
by the input current of the meter amplifier. (This capacitor can be
recharged to full scale by pressing a reset button).
The required velocity reading range is 10 to 125 ft/sec. To
accommodate this range and provide reading accuracy at lower velocity
conditions, two scales are provided on the one meter; namely, 0 to 50
ft/sec and 0 to 150 ft/sec. The lower scale is provided by feeding
only a third of the sweep voltage to the spectrum analyzer. Precision
resistors provide the accurate negative offset to calibrate the low
scale, and the constant impedance, 3:1 attenuator to match scales.
A second readout requirement is a measure of the velocity
deviation (AV) about the mean value. A peak-to-peak voltmeter allows
this information to be read on the velocity meter. In reality, two
meters are provided on the velocimeter. One is located on the sensing
head. The other is on the electronics cabinet. Values of AV (as well
as mean velocity) are obtained from this remote meter depending upon
*
a selector switch position. Because of the finite number of fringes,
a width to the frequency spectrum exists and so even a single velocity
will show some AV.
*
This meter reads 10 XAV.
21
-------
2.5 VELOCIMETER TESTING
The portable sensing head and electronics cabinet which comprise
the laser velocimeter are seen in the photographs of Figures 11 and 12,
respectively.* Final testing was conducted with this apparatus at the
conclusion of the program to verify basic operating characteristics.
This was done qualitatively with flyash dust and water sprays, and
quantitatively using water droplets and a spinning disc.
Calibration of the velocimeter was accomplished using a spinning
disc in the following manner. An 18-inch diameter smooth white paper
disc backed by 1/8-inch thick acrylic sheet, was installed on the
shaft of an electric motor. The nominal motor speed was 1800 rpm.
The velocimeter was set-up before the disc with the optical axis
approximately normal to the plane of the paper. The objective lens
focus was adjusted until the two incident laser beams merged into
a single spot. The distance between the target disc and entrance
aperture on the instrument was measured and compared with the
indicated objective lens range scale reading. The beam splitter was
next adjusted to an equivalent reading. The distance (radius) from
the motor shaft center!ine to the center of the laser beam spot was
measured before operating the motor.
Disc rotational velocity (shaft speed) was next measured using a
**
Strobotac. The velocimeter was allowed to sweep through the full
frequency range until a signal "lock" was obtained at the lowest
spectrum analyzer sensitivity setting. Velocity readings were made
on both the high and low range meter scales. The remote (electronics
cabinet) meter reading was compared with the sensing head meter. This
procedure was repeated at different target distances to verify the
accuracy of the range scale on the focusing objective lens.
The operation and maintenance of this equipment is described in
Reference 2.
**Product of General Radio Co., Cambridge, Massachusetts. Strobotac
Type 631-VL, S/N 21705.
22
-------
Figure 11. Laser velocimeter sensing head.
Figure 12. Velocimeter
electronics package.
23
-------
After these preliminary tests, the velocimeter was set up a distance
of 10 feet from the target. A series of tests was made at different radii
to check the velocimeter performance over the full velocity range. These
data are plotted in Figure 13. An analysis of the data indicates a stan-
dard velocimeter measurement error of ±1.8 ft/sec over the range of 10 to
125 ft/sec. It is estimated that most of the velocity deviation can be
attributed to small errors in reading the observed measurements. Some dif-
ficulty with the test setup was encountered in making measurements at the
extreme high and low end of the velocity range. As a result, some apparent
data scatter is seen at both ends of the curve in Figure 13.
i
u
O
METER
SCALE
RANGE
SPECTRUM ANALYZER
SENSITIVITY SETTING
0.005 V/CM
0.002 V/CM
0.001 V CM
0.002 V/CM
0.001 V CM
LOW
HIGH
HIGH
LOW
HIGH
ALL DATA RECORDED AT 10 FT RANGE
40 50 60 70 80 90 100
CALCULATED VELOCITY, VG IN FT SEC
Figure 13. Plot of measured versus calculated velocity
for laser velocimeter. Target was 18-inch-
diameter spinning, white paper disc.
24
-------
Additional laboratory tests were conducted using first flyash dust
and later, atomized water sprays. The flyash tests were unsatisfactory
in that the dust generator did not produce a uniform discharge. Rather,
the particulate tended to clump together resulting in very heavy loadings
resembling mostly dense clouds.
A commercial atomizing nozzle was substituted for the dust generator,
with water droplets providing the scattering medium. The water spray
proved more satisfactory in that the water flow rates could be varied by
changes in feed pressure. Velocity measurements up to about 40 ft/sec
were recorded by the velocimeter at ranges of 2 to 5 feet.
A velocimeter test was next made using a TRW low speed wind tunnel.
The two-dimensional test section has a 5- x 24-inch cross section which then
diverges linearly to an 8- x 24-inch cross section. Flow is directed ver-
tically down through the 5-foot-long test section. Four turbulence damping
screens and an aluminum honeycomb section are positioned just upstream of
the tunnel's stagnation chamber. A tunnel contraction ratio of 8:1 further
reduces turbulence intensities.
Both pitot tube and anemometer velocity measurements are used to
measure test section velocities. The constant temperature anemometer is a
TSI Model 1050 single tungsten wire probe. A TSI linearizer is used to
provide analog output signals proportional to the velocity.
For the velocimeter test, the tunnel was operated at a constant speed
core velocity of 58.7 ft/sec at the test section entrance. Air flow in the
test section has a flat velocity distribution with a boundary layer veloc-
ity equal to 90 percent of the core velocity at a distance of 1/2 inch from
the wall.
A water spray nozzle was installed at the entrance to the tunnel test
section. Injection of a water spray into the tunnel air stream provided
scattering centers for the laser velocimeter. These scatterers were intend-
ed to simulate flyash particles in a stationary power plant stack or duct.*
Introduction of flyash or chemical aerosols into the closed-loop tunnel
was rejected for reasons of tunnel contamination.
25
-------
r)
The nozzle was operated at a pressure drop (AP) of 30 Ib/in (limited
by the available supply pressure). At this AP, the injection velocity V.
was calculated from the relationship V. = CD[2gAP/p] ' , where: CD is the
orifice coefficient (assumed to be ~0.75); g is the gravitational constant
2 3
equal to 32.174 ft/sec ; and p is the density of water at 62.4 Ib/ft . The
injection velocity V. was determined to be 50.06 ft/sec.
J
Water droplet velocities as a function of distance from the point of
injection into the air stream were also estimated. A mean droplet diameter
was calculated from the Ingebo correlation . This empirical relationship
yields a volume-number-mean droplet diameter D expressed by the following
equation:
D_./u^^, = s. nu / ii u -t n n~> n i\i ., \ i-,\
where the water jet (orifice) diameter D. and droplet diameter D30 have
units in inches, and the air velocity V . and water jet velocity V. have
units of ft/sec. In this instance, the injector orifice diameters were
0.040 inch and the wind tunnel air velocity was 58.7 ft/sec in the 5- x 24-
inch cross section. From Equation (1) the mean droplet diameter was cal-
_3
culated to be 9.8x10 inch or approximately 250 microns.
Velocimeter droplet velocity measurements were made at axial distances
of 2.3 and 4.6 feet downstream of the water spray injection point in the
tunnel divergence section at a range of 6 feet. Average droplet velocities
of 35 and 40 ft/sec were measured at the two locations. These measurements
correspond to air velocities of 45.2 and 36.8 ft/sec, respectively. Veloc-
imeter measurements at distances further downstream of the spray injection
and at greater range were prohibited due to physical limitations of the
wind tunnel test setup.
The apparent discrepancy between air velocity measurements and veloci-
meter droplet velocity measurements was subsequently analyzed. An expres-
sion was derived to estimate the average particle (droplet) velocity V"
as a function of distance X from the point of injection into the tunnel air
stream. This derived relationship is
26
-------
pf
where:
30
and subscript f
0.92x10
3
'-V
(2)
volume-number-mean droplet diameter
initial droplet injection velocity
drag coefficient for a sphere based upon the
local droplet and air velocities
final condition
initial condition
The results of this analysis are shown in Figure 14 where both air and es-
timated mean droplet diameter (DOQ) velocities are plotted as a function
of distance in the tunnel divergence section. Velocimeter measurements
were within less than 5 percent of the predicted droplet velocities. In
addition, droplet velocities did not achieve equilibrium with the test
section core velocity.
Figure 14.
1234
DISTANCE FROM PARTICLE INJECTION ~ FT
Air and predicted particle (droplet) velocity as
a function of distance from particle injection
into the divergent wind tunnel test section.
27
-------
3. LONG RANGE OPTICAL VELOCITY METERS
3.1 INTRODUCTION
This discussion is concerned with the operation of optical velocity
meters for use on large smokestacks, at ranges of 300 to 1500 feet (100
to 500 meters). The smoke velocity to be measured is assumed to be in
the range of 10 to 125 feet per second (3 to 40 meters per second). The
dust loading is in the range of 1 to 10 grains per standard cubic foot
(2 x 10 to 2 x 10 grams per cm ), and the mean particle diameters
are assumed to have the values of l^m, 10/am, or 50^m. As representative
3 3
specific gravities of the particles, 0.5 gram per cm or 1 gram per cm
is used.
Three types of optical velocity meters are considered here, all
of which are single station instruments which rely on backscatter.
The first type is called the direct doppler velocity meter. Its con-
figuration is shown in Figure 15 where a single beam from a laser is
Laser
Figure 15. Direct doppler velocity meter,
28
-------
pointed at the smoke, and the backscattered light is detected by a
heterodyne detector, using a fraction of the laser's outnut as the local
oscillator for detection. From the output frequency of the heterodyne
detector, the smoke velocity can be obtained when the elevation angle e
is known.
The second type of velocity meter uses two beams from the same laser,
which intersect each other at the region to be sampled (see Figure 16).
In the intersection volume common to the two beams, a set of stationary
interference fringes is formed. These fringes are effectively a set of
"sheets of light." When an object passes through these fringes, the
scattered light pulsates as the object passes from fringe to fringe. By
measuring the pulsation frequency of the scattered light, one obtains
the desired velocity information. This type of velocity meter is
called the "fringe velocimeter."
Fringes or
"Sheets of Liaht
Figure 16. Schematic of fringe velocimeter.
29
-------
This second instrument can also be explained purely in terms of
doppler shifts from the two beams, and hence is also known as a form of
laser doppler velocimeter. However, the fringe explanation is used
because it makes many aspects of the operation more obvious and intui-
tive. The fringes can be observed directly if desired by placing a
stationary card in the intersection region and examining the light on
the card with sufficient magnification to reveal the fringes. Analyti-
cally, the two alternate explanations are equivalent.
The third type of velocity meter is called a "reticle velocimeter"
and is sketched in Figure 17. This instrument is very simple and
can use sunlight as its illumination source. It consists of a lens,
like a telescope objective lens, which forms an image of the smoke on
a reticle. The reticle pattern consists of black stripes on a transparent
glass. Behind the glass is a photodetector. As a bright "blob" of smoke
moves upward, its image passes over the reticle stripes and produces a
pulsating output from the detector. The frequency of the pulsating
Figure 17. Reticle velocimeter.
30
-------
detector output gives the velocity of the smoke. This instrument thus
has considerable similarity to the fringe velocimeter. The reticle at
the detector clays the same role that the fringes at the smoke play in
the fringe velocimeter.
The remainder of this discussion will present information relevant
to the performance of these three types of velocity meters under the
smokestack conditions outlined at the beginning. Since all three of
these meters present their velocity information as a somewhat periodic
signal on a noise background, the system noise is an item of principal
concern.
3.2 SMOKE OPTICAL PROPERTIES
For the performance evaluation, it is important to know the number
of particles in the sampled volume. From the dust loading and particle
data of the introduction, the number of particles per cubic centimeter
may be computed. The results are shown in the following table.
Table I. Number of Particles per cm .
1 GRAIN/fr
10 GRAINS/ft'
Particle
Diameter
1 ym
10 ym
50 ym
Specific
Gravity
= 1
4.5 x 106
4.5 x 103
3.7 x 101
Specific
Gravity
= 1/2
9xl06
9xl03
7 x 101
Specific
Gravity
= 1
4.5 x 107
4.5 x 104
3.7 x 102
Specific
Gravity
= 1/2
9
9
7
x 107
x 10
2
x 10
A second characterization of the particle density is obtained by
computing the mean free path of a photon in the smoke. For present
purposes, it is sufficient to use the geometric cross section rather
31
-------
than the more refined true optical cross section. Thus, the
mean free path is given by , where n is the particle density and a
n w
is the geometrical cross section area of the particle. The mean free
path results are shown in Table II.
Table II. Mean Free Path of Liaht in Smokes
1 GRAIN/ft3
Particle
Diameter
1 ym
10 pm
50 urn
Specific
Gravity
= 1
28 cm
280 cm
1400 cm
Specific
Gravity
= 1/2
14 cm
140 cm
700 cm
10 GRAINS/ft3
Specific
Gravity
= 1
2.8 cm
28 cm
140 cm
Specific
Gravity
= 1/2
1 .4 cm
14 cm
70 cm
3.3 SAMPLE VOLUME SIZE
Before conclusions can be drawn from the foregoing tabular data, it
is necessary tc know the volume and shape of the region sampled. For
the first two instruments, this volume consists of the intersection of
two beams (or of one beam and one observing path). The configuration is
shown in Figure 18.
For a lens of diameter d, the minimum possible width of the beam at
range r is given approximately by
where A is the wavelength of light. For a 4-inch (10 cm) lens at 100
meters range, this gives w = 1/2 mm, and at 500 meters w = 2.5 mm. These
values are based on diffraction-limited performance, and in view of the
turbulent air encountered in the intended operation, it would appear
unwise to base an instrument on this high a performance standard. This
standard of performance corresponds to a telescope of about 100 power or
greater. The military seldom uses telescopes of powers greater than 20
because too often the turbulence renders higher powers useless. Thus,
32
-------
Close-up of Intersection Volume
Far View of Intersecting Beams
Figure 18. Beam intersection geometry.
33
-------
it seems prudent to expect at most a 2.5 mm beam width at 100 meters and
a 12.5 mm beam width at 500 meters, these values being 1/5 of the
diffraction-limited performance of a 4-inch objective, or equal to the
diffraction-limited performance of a 0.8-inch objective.
The length L of the intersection volume is given (to a close approx-
mation) by
where D is the separation cf the two beams at the observing station.
Since very high precision alignment between the two beams is required for
convenient operation, the beam optics must be mounted on a common rigid unit.
For a 2-meter sepe.raticn, about the maximum that seems practical, this
gives an L of 0.25 meter at 100 meters range, and 1.25 meters at 500 meters
range. An instrument with such a large separation would require a special
truck. Moreover, the large separation allows turbulence to affect the
beams independently, which introduces extra noise into the system.
Referring to Table II, where the mean free path is given, it is
clear that for the small particle or heavy loading conditions, severe
attenuation of the beam both on its inward pass and on its outward pass
will contribute to the difficulty of obtaining range resolution. Moreover,
multiple scattering also imposes additional difficulty in obtaining range
information by degrading the signal-to-noise ratio when probing several
mean free paths deep. Because of these difficulties, it appears appro-
priate to concentrate attention first upon a more convenient instrument
which makes no attempt to obtain range information, but simply probes the
plume with a long thin sample volume. The returning information will
then be primarily from the region of the near side with a depth approxi-
mately one-half of the mean free path. Unless otherwise stated, the
remainder of the discussion is limited to instruments with long thin probe
volumes having no range resolution. At a later time, it may be appropriate
to consider more sophisticated instruments with range resolution.
34
-------
3.4 SIGNIFICANCE OF THE SMOKE OPTICAL PROPERTIES IN CONJUNCTION WITH
THE SAMPLE VOLUME SIZE
Three points of importance need emphasis. The first is that the
number of particles in the sample volume is large. Even with the smallest
beam width of 2.5 mm and the lightest loading of coarse particles, there
would still be about 600 particles in the sample volume across a 3 meter
diameter stack. For all other conditions, the number of particles in
the probe volume is much greater.
The second point to note is that the mean free paths are generally much
shorter than the length of the probe volume and of the stack diameter.
Under these conditions, the amount of power backscattered can most easily
be handled by a simple reflectivity coefficient rather than dealing with
the scattering functions from individual particles. Caution must be
exercised with white smokes where multiple scattering may cause complica-
tions, to be noted later. Under these short mean free path conditions,
a change in smoke density does not change the amount of reflected light.
This absence of density dependence is most disadvantageous for the fringe
velocimeter. In the fringe velocimeter, it is desired that as a density
fluctuation, say a small local increase in density, passes by the fringes,
a pulsating return signal is produced. However, in the case where the
mean free path is short compared to the probe volume, no pulsation of the
return is obtained because the density variations simply allow the light
to penetrate a little more or less so that the same number of particles
are always involved, resulting in a constant return. This phenomenon
is referred to as "contrast washout."
A third point to note is that the mean free paths are longer than
the scale of some of the turbulent eddies. This results in a spread of
velocity components along the viewing direction. As will be explained
later, this spread creates a "granularity" noise which is significant in
the laser-illuminated cases.
35
-------
3.5 SHOT NOISE OF DETECTION
One of the principal sources of noise in the instruments under con-
sideration is the shot noise of the detection process. This noise arises
from random fluctuations in the emission times of the electrons at the
photocathode of the detector. In systems using photomultipliers, this
source of noise is often dominant over other sources, such as thermal
agitation noise.
To develop a formula for the signal-to-noise ratio due to shot noise,
the incoming light flux is represented in the form
S(t) = SQ + S^t) + S2(t) .
Here, S(t) represents the light power (in watts) falling upon the detector
and as indicated in the above formula, this is represented as the sum of
three terms. The first term S is the average dc power of the light. The
second term S-,(t) represents those fluctuation components within the
signal pass band of the instrument; that is, S-,(t) is the desired signal.
The third term, Sp(t) represents those fluctuation components outside the
signal pass band. The average values of S-,(t) and Sg(t) are zero, since
S is the dc average of S(t).
The photocathode current is then given by
Kt) = ne {L_[so + S^t) + S2(t)] ,
where n is the quantum efficiency of the photodetector (electrons per
photon), e is the electronic charge, h is Planck's constant, and v is
the optical frequency. After this current is passed through the band
pass filter, there remains the signal,
When this current is passed through a resistor, R, the time-averaged
signal power is then
36
-------
-*-^2 , (3)
(hv)2 ]
where
T
S,2 - J1m 1 SU) dt
I |-H*> T
The shot noise power in the pass band is given by
2eIRAf ,
where e is again the electronic charge, R is the same load resistor, and
Af is the bandwidth of the pass band.* Here I is the average photocurrent
and hence is given by
Combining these, the shot noise power in the pass band is given by
2e2n J- R S Af . (4)
n\> o
Taking the quotient of (3) and (4) , gives the signal-to-noise ratio,
c" 2
Signal Power _ 1 1 1 /5\
Shot Noise Power 2f hv SAf *
This formula can be written in a form so as to have an easily
visualized content. To do this, note that
(5'6)
is the average number of detected photons (i.e., photoelectrons) in the
time interval F The time interval -TF corresponds to the time of one
full cycle at a frequency corresponding to the bandwidth. Thus, one can
wri te
* American Institute of Physics Handbook, Section 6m, "Radiation De-
tection."
37
-------
Signal Power
Shot Noise Power
/Average number of
T_J detected photons
2 \in time interval T>
x At
(6)
The fraction S-/S can be thought of as the modulation index or modulation
depth of the incoming light. It is the ratio of the RMS signal component
to the average light power.
Note that it is the number of detected photons which enters the
signal-to-noise ratio formula. Thus high quantum efficiencies are
desirable, as well as high powers of the light itself. Also, narrow
bandwidths favor the signal-to-noise ratio, for they provide longer time
intervals F for the detection of photons in (6).
Formula (6) can be used to determine the required laser power output
to achieve a given signal-to-noise ratio. However, the pass band Af and the
modulation depth for each system must first be determined.
3.6 THE DIRECT DOPPLER SYSTEM
In order to maximize the signal-to-noise ratio, it is desirable to
keep the pass band as small as possible without losing signal. The signal
width for the direct doppler instrument is determined by turbulence within
the smoke. Referring to the following sketch (Figure 19), the observing line
Figure 19. Effect of turbulence on direct doppler system,
38
-------
is at an elevation angle e. Within the sample volume, the average vertical
component of velocity is denoted by v. This is the velocity which it is
desired to measure. Because of the turbulence, there is added to v a
turbulent velocity, VT- As a rough guide, the turbulent velocities can
be taken to be about 10 percent of the main flow velocity v. These tur-
bulent velocities can be in any direction, and thus some will be along
the sight line.
The component of the main flow velocity along the sight line is
v sin e . Thus, the average doppler frequency is
_ V
When a turbulent velocity v-,- occurs along the line of sight, the frequency
will be
f = -(v sin e ± VT)
A 1
Thus, the spread in frequency due to turbulence is
Af = 2 ^ . (8)
A
Combining (8) with (7), one obtains
- 1
f v sin e
This equation exposes an important limitation of the direct dopoler
velocimeter as applied to the present turbulent smokestack case; namely, if
the elevation angle 9 is too small, then the turbulence causes a spread in
the signal frequency comparable to the basic doppler frequency itself.
Under these conditions, it is impractical to obtain the desired velocity
information.
At steep elevation angles, some determination is possible. For
example, at 45 degreees with the value of VT/V = 1/10 (turbulent
velocities 10 percent of main flow), then Af/f =0.28. With such an
instrument, a 10% determination of average velocity may be possible.
39
-------
The preceding limitation may possibly be overcome with highly sophis-
ticated systems which "track" the turbulent velocity fluctuations, thus
permitting an effective narrow pass band and accurate determination. The
investigation of such a sophisticated system is beyond the scope of the
present report. In this connection, note that due to the fact that the
depth of penetration is larger than some turbulence cell diameters, the
returning signal is not a single frequency shifting about, but a complex
composite with different frequency components simultaneously present from
the different parts of the sample volume moving at different velocities.
For the direct doppler instrument, the best signal-to-noise ratio is
obtained with a reference that is strong compared to the signal. Under
this condition, formula (6) becomes
/Average number of detected\
fel P^cns In the ) . (10)
ytime interval F /
(This formula assumes that the reference source does not contain any excess
noise, a matter of practical concern.)
This formula can be applied to predict the required laser power
output for a direct doppler instrument. Example conditions are as follows:
Elevation Angle e =45 degrees
Slant Range = 100 meters
Reflectivity of Smoke = .1
p
Receiving Aperture = .01 m (4.5 in. diam. lens)
Quantum Efficiency = .1
Wavelength = .5 ym
Smoke Velocity = 40 meters/second
Turbulent Velocity/Flow Velocity = .1
Signal-to-Shot Noise Ratio = 10
These conditions, together with the foregoing formulas, yield the
following:
Center Frequency 57 MHz
Bandwidth 16 MHz
Required Laser Power .02 watt
40
-------
It thus appears that a direct doppler instrument may be a
possibility inasmuch as argon lasers with considerably greater power
than the above are available.
However, there are many further considerations beyond the
fundamental one of shot noise. There are additional sources of noise, for
example, thermal agitation noise, noise associated with the photomultiplier
multiplication process, and noise from fluctuations of the laser intensity
and intermode beating. Background light which is inadequately filtered
out is an additional source of noise. Moreover, operation of a photo-
multiplier at a frequency of 57 MHz requires proper design. For the
direct doppler system to work as described above, it is required that
the illuminated spot on the smoke supplied by the laser be at least as
small as the theoretical resolution limit of the receiving lens. In addi-
tion, turbulence along the path can spoil the resolution, which in turn
can decrease the signal-to-noise ratio. Multiple scattering in the smoke
can also increase the size of the illuminated spot from which the light
returns, and this too degrades the signal-to-noise ratio.
The coherent detection process used in the direct doppler type instru-
ment only functions as described above with light from particles within the
theoretical limit focal volume. This is a cylindrical volume approximately
r 2
A -r in diameter and A (r/D) in length. For the above example, this is a
cylinder 0.5 mm in diameter by 0.5 meter long. Light returning from
particles outside this volume does not have a uniform phase relationship
with the reference light over the full entrance aperture. The result is
that light from particles outside this cylindrical volume contributes
little or nothing to the signal. Such light can increase the noise level,
however. This requirement of a uniphase wavefront is a fundamental limit
imposed by the coherent detection process. The uniphase condition can be
achieved more easily with smaller receiving apertures, but they collect
less light which strains the system in another direction.
Still another demand of the direct doppler system is that either the
laser must have a coherence length equal to the round trip to the sample
volume and back, or that an optical delay be provided for the reference
light equal to the round trip distance. This is necessary to insure that
the reference be coherent with the returning signal.
41
-------
In summary, the direct doppler type instrument appears to have the
following properties: Nearly horizontal paths cannot be used due to the
turbulence contribution. Elevation angles on the order of 45 degrees
appear necessary, and these eliminate long range applications. A one-watt
or greater argon laser is required to give a comfortable signal-to-noise
ratio. Careful attention must be paid to many noise sources and to the
high frequency detector design. The velocity measurement is not likely to
be better than 10 percent, and will have no depth resolution.
3.7 THE FRINGE VELOCIMETER: GRANULARITY NOISE
Before turning to an evaluation of the fringe velocimeter, it is
appropriate to discuss a source of noise relevant to the fringe velocimeter.
This source is called granularity noise because of its relation to
the granularity or speckle pattern produced when a laser illuminates a
diffuse surface. The origin of this noise is shown in the following
sketch (Figure 20):
Diffuse Reflecting
Surface
Detector Aperture
Figure 20. Granularity noise in the fringe velocimeter.
42
-------
In this sketch, a laser beam is shown illuminating a spot of diameter w
on a diffuse reflecting surface. The light reflected from this surface
travels in all directions, and at range r there is placed a large white
viewing screen. It will be found that the light intensity on the viewing
screen is not uniform, but has a speckled or granular appearance. This
stems from the monochromatic nature of the laser.
The intensity at a given point of the viewing screen is found by
adding up the amplitudes from all the different points composing the
illuminated spot of diameter w. This is a phase addition with the ohases
randomized by the diffuse nature of the reflecting surface. At some
points, the phases will largely cancel each other, resulting in a low
intensity or dark spot. At other points, the phases will be largely in
phase, resulting in a bright spot.
The "size", s, of these granules, on the viewing screen, is approxi-
mately given by
c 1 r (1°'5)
S ~ A W '
Actually, the distribution is random, and what is meant by the above is
that two points closer together than s will not have independent inten-
sities, but will be correlated. To the eye, the characteristic apparent
"diameter" of the granules is given by s.
[In the direct doppler instrument, the uniphase wavefront requirement
discussed earlier corresponds to the condition that the receiving aperture
be no larger than the size of one granule, i.e., be of diameter - s.]
In the present smokestack application, the motion of the smoke causes
the granularity pattern to change in time, which creates intensity fluctu-
ations at the detector. These constitute noise, and it is necessary to
evaluate the amount of this noise which is in the pass band of the
instrument.
It can be shown that the RMS fluctuation in light power entering
detector, AP, is given by
43
-------
where PQ is the average light power entering the detector and M is the
number of granules present in the detector aperture. (The foregoing assumes
that the returning light is still oolarized after reflection. If not, M
should be replaced by 2M in Equation 11.) If the detector aperture diameter is
D, then
M = (f) . (12)
Notice that if M = 1 corresponding to D = s, then the RMS fluctuation is
equal to the average light power. By making the receiver aperture large so
that M becomes large, or by making w large so that s becomes small and
hence M large, these granularity fluctuations are reduced, decreasing the
system noise.
To evaluate the significance of this granularity noise, we need to
determine how rapidly the pattern changes, and what fraction of the noise
is in the instrument's pass band.
If the smoke were a rigid mass moving uniformly upward, then when
the smoke has moved upward a distance w, all new particles would be in
the illuminated region, and a new granularity pattern would be present.
Thus, the upper frequency limit of the granularity noise is given by
rigid smoke
where v is the smoke velocity.
However, rigid translation of the smoke upwards is not the only
source of change in the speckle pattern. Recall that in the present
application, the probe volume length is long enough to span more than one
turbulent cell. Thus, the reflector is effectively a set of two or more
partial reflectors which are in motion relative to each other. When the
relative motion is of the order of one wavelength, then the phase addition
is uncorrelated with the previous addition, and a new speckle pattern is
formed. Thus, letting VT denote the turbulent velocity components as
44
-------
before, and recognizing that some of these occur along the line of
sight, the apparent frequency of speckle pattern change due to
turbulence is approximately given by
Inasmuch as VT is approximately 1/10 of v, the frequency given by
(14) is much higher than that given by (13), for realistic values of
w in the present application. Thus, formula (14) is the proper one
to use for estimating the effects of granularity noise in the realistic
case.
By comparison with other aspects of speckle behavior, it is
inferred that the frequency (power) spectrum of this noise is, to
an accuracy sufficient for present purposes, approximately flat from
zero up to f , given by (14). On this basis, the RMS fluctuation
IT13X
of the arriving light due to granularity noise in a pass band of width
Af may be found by combining (11) and (14).* The result is
(RMS Granularity Fluctuation of Arriving Light) = Pa -L f ; (14.5)
a
hence,
T .^^
(RMS Granularity Fluctuation of Arriving Light) = PQ A_ Af . (15)
* To obtain the portion in the pass band, it is necessary to use
the fluctuation power (the square of the RMS). The fraction
Af/f of this total fluctuation power is within the passband.
The square root of this fraction gives the RMS of the fluctuations
in the pass band. An alternate argument leading to the same
result is that the basic fluctuations occur every l/fm3V seconds.
iNaX
The number of basic fluctuations in the time interval 1/Af is
therefore fm,w/Af. The RMS value of the sum of f /Af samples
max max
is /Af/f times the RMS of the basic fluctuation RMS
given by (11), which yields the formula (14.5).
45
-------
To obtain a signal-to-noise ratio, it is necessary to consider the
signal and the pass band Af over which the signal is spread. For the
fringe velocimeter, let A denote the spacing of the fringes and N denote
the total number of fringes across the probe volume's width W. Then:
NA = w . (16)
The output frequency due to particles crossing the fringes with velocity
v is
fsignal = I ' (17)
Since there are N fringes across the probe volume, it can be shown that
the spectral width Af . , of the signal is given approximately by
Afsignal = fsignal ' 1 ' ^
Combining (15) with (16), (17), and (18) yields the granularity noise
in a pass band which just passes the signal. The result, after appropriate
cancellation, is
(RMS Granularity Fluctuations of Arriving
Light in Signal Pass Band) = P
Attention is next turned to the signal. If one particle at a time
passes through the intersection volume, the returning light is 100
percent modulated by the fringes. However, as more particles are simul-
taneously present in the probe volume, the returning light is less
modulated because some particles are "on" the fringes at the same time
that others are "off" the fringes. It can be shown that if the particles
are randomly distributed, the following relation holds:
(RMS Fluctuation of Arriving i i
Light Power Due to Signal) = -=. Pa . (19.5)
m
46
-------
where P is again the average light power returned to the detector, and
a
m is the total number of particles returninq light from the probe volume.
This signal is spread over the frequency interval Af . , given by (18).
signai
Dividing this by (19) yields the following ratio,
RMS Signal Fluctuation \ Jl w ^T M /20>
RMS Granularity Fluctuation/ i2 A v m
The ratio (20) is the ratio between the RMS fluctuation of the arriving
light flux due to the signal, and the RMS fluctuation of the light, due to
granularity, each within the signal passband. After detection, these light
fluctuations become fluctuations in a current (or voltage). Since the power
in the signal or noise is proportional to the square of the current (or
voltage), the signal power to granularity noise power ratio is the square
of the ratio given in (20), namely,
AC Signal Power 1_ w ^T M_ /2i)
Granularity Noise Power 2 x v m
This may be evaluated with the aid of the particle densities given earlier
in this report.
The special case in which the mean free path is short compared to
the stack diameter yields a particularly informative relation. For this
case, the total number, m, of particles returning light is approximately
since is the mean free path, n being the particle density. Moreover,
an n
to first approximation a = 6 , where 6 is the particle diameter.
Substituting these in (21) yields
= ~ M '
47
-------
Using (10.5) and (12), this relation may be written,
AC Signal Power -/iV ^1/5.Y *L (??}
Granularity Noise Power ~\xy v \r/ x ^ '
The factors 6/x and Vj/v are properties of the smoke, and thus for
operation at a given range, the designer has only the lens diameter D
and the probe volume width w with which to secure a favorable signal to
granularity noise ratio. The formula indicates that large values of both
D and w are desirable. It will be seen later that for random particle
distribution, a small value of w is desired to enhance the signal-to-shot
noise ratio. Thus, an optimum w exists which minimizes the sum of
granularity and shot noise powers.
It should be noted that the uniformly random distribution yielding
formula (19.5), on which the above is based, may not give a true
description of the behavior. It seems reasonable that local clumping
due to turbulent eddies may occur, and if these clumps are comparable in
size to the fringes, the clumps may serve as large "particles", giving
rise to much better performance than given by (21) and (22). On the
other hand, for the short mean free path case, there can be a contrast
washout because a region of low particle density simply lets the beam
penetrate further until the backscatter is the same as for a higher
density region. In this case, the true situation can be worse than
predicted by (21) and (22).
Notice that the ac signal power-to-granularity noise power is
independent of the laser power. Thus, one must arrange the system so
that this ratio has a favorable value, and then use the formula for
signal-to-shot noise ratio given much earlier (formula 6) to determine
the required laser power.
3.8 FRINGE VELOCIMETER: REQUIRED LASER POWER
As an illustration, formula (6) or (5) is next applied to a
representative case to estimate the required laser power. Again
assuming a random particle distribution in the sample volume, it. follows
48
-------
that the modulation depth S,/S of the received light has the value -j=
U > p1
where m is again the number of particles in the sample volume. Thus, the
factor Sj2/S02 in (6) has the value 1/m .
The time interval ?- is determined by the signal bandwidth, and is
W
given by - , where v is the smoke velocity to be measured, and w is the
probe volume's width. This follows from formulas (16), (17), and (18).
Substituting these values in formula (6), and incorporating appro-
priate geometric factors for the fraction of laser light collected, the
following is obtained:
Signal Power _ 1_ / p _A_ 1_ w\ 1 (9*\
Shot Noise Power 2 lnp n ° u- ' - ' * ;
where
n = quantum efficiency of the detector
p * reflectivity of the smoke
P. = laser power output
A = area of the detector collection aperture
r = range
hv = energy of a photon (4 x 10 joules for 0.5/urn light)
The quantity in parentheses is the average number of photons detected in
time interval F
At
Denoting the probe volume's length by L:
m = n w2L , (24)
where n is the particle density.
Incorporating this into formula (23):
Signal Power 1_ p A 1 1 1 (25)
Shot Noise Power 2 np L 2 hv v nwL '
If it is further assumed that the mean free path, , is short
compared to the stack diameter, and that the particle cross section a is
approximately 62, where 6 is the particle diameter, then:
49
-------
(Short mean
free path
case)
Signal Power
Shot Noise Power
np
p n ' i- °
L 2 hv v w
Notice that both formulas (25) and (26) show that small values of w
are desirable in order to improve the signal-to-noise ratio. This stems
basically from the fact that small values of w correspond to fewer
particles in the probe volume, which makes the fluctuations proportion-
ately greater. As noted earlier, this is based upon a completely random
particle distribution, and may not apply if turbulence causes local
clumping.
If the particle distribution is truly random, then the small values
of w required by (25) and (26) prevent using large values of w
to combat the granularity noise. In this event, a large receiving
aperture is the only method for providing a large M to combat granularity
noise.
An example of the power required by formula (26) is given below.
The example conditions are similar to those of the direct doppler example;
however, the receiving aperture is increased to keep the granularity noise
down.
Conditions:
Range r =
Reflectivity of Smoke p
Receiving Aperture A =
Quantum Efficiency n =
Wavelength
Smoke Velocity v =
Signal-to-Shot Noise Ratio =
Particle Diameter 6
Probe Volume Width w
Turbulent Velocity/Flow Velocity =
These values yield the following:
Reauired Laser Power = -05 watt
100 meters
.1
Q
.04 m (9 in. diam. lens)
.1
.5 ym
40 meters/second
10
10 ym
25 mm
.1
50
-------
The foregoing does not depend on the number N of frinpes across the
probe volume because it is assumed that the optimum bandwidth is used.
However, to give a feeling of the frequencies involved, it is further
assumed that N = 10, in which case:
Signal Frequency = 16 kHz
Signal Frequency Width = 1.6 kHz
In conjunction with formula (20), the example conditions yield
Signal Power
Granularity Noise Power
= 8
The example conditions are for the near range conditions of
100 meters. For the far range conditions of 500 meters, the situation
o
is more demanding. In formulas (22), (25), and (26) one observes an r
dependence. In addition, w may depend on r, depending on the optical
configuration.
It is to be emphasized that the example conditions are given as a
guide to scale from, not as a design. In particular, the smoke reflec-
tivity may be quite different, and a signal-to-noise ratio of 10 is not
a comfortable design point in view of other secondary noise sources.
3.9 RETICLE VELOCIMETER NOISE ESTIMATE
A performance estimate for the reticle velocimeter is not easy to
make because of unknown smoke properties.
It appears desirable to sample the flow immediately after it emerges
from the stack, for otherwise large vortices of entrained air will compli-
cate the situation and reduce the accuracy.
The contrast of the issuing smoke is not known. When sunlight is
used as the light source, its direction is automatically different from
the viewing direction, and hence the contrast washout mentioned earlier
does not apply. It seems likely that performance can be optimized by
choosing a viewing direction appropriately oriented relative to the sun.
51
-------
As with the fringe velocimeter, the relative signal bandwidth |^
1 " T
is given to first approximation as JT, where N in this case is the number
of reticle lines across the field of view. This first approximation will
be correct if the smoke "blobs" remain intact long enough to traverse the
field of view. If not, the spectrum will be broadened. Since the tur-
bulent velocities are of the order of 10 percent of the flow velocity,
one might estimate that a system with 10 reticle lines across the field
of view would satisfy the condition that the "blobs" remain intact across
the field of view.
Formula (6) for the signal-to-shot noise power can also be applied
for the reticle velocimeter. The illumination power in this case is
supplied by the sun. The following example is given for comparison, but
it is emphasized that the smoke property assumptions are rather arbitrary
and may not be realistic.
Conditions:
Sunlight at 45°
Area Sampled on Smoke
Number of Reticle Lines =
Range =
Smoke Reflectivity
Quantum Efficiency =
Smoke Velocity =
Receiver Aperture
Power density 300 W/m
.01 m2 (.1 m x .1 m)
10
= 100 meters
= .1
= .1
= 40 m/sec
= .01 m2
Contrast Ratio of Smoke,
I
max
- I
mm
I
ave
Number of "Blobs"
Simultaneously in
Field of View
= .1
= 100
The conditions preceding the last two give the following results:
Signal Frequency = 4 KHz
Signal Bandwidth Af = 400 Hz
(Average Number of Detected
aye MUIIIUCI ui ucocv-ucu -i
Photons in Time Interval p) = 6x
10'
52
-------
The last two conditions give the modulation depth, 5./S , as .01, since
there are approximately /TW "blobs" above or below the average "on" the
fringes. Together with formula (6), these results yield
Signal Power 1 fi v ,n7 / mA2 ,nnn
Shot Noise Power" 2 6 x 10 '('01) = 300° '
This result suggests that the shot noise of detection may not pose much
of a problem. This basically stems from the abundance of power provided
by the sun and the narrow bandwidth Af.
Whether artificial light (e.g., sootlights) could be used to permit
night operation is not clear because of the large uncertainties in the
above conditions.
Another source of noise, and quite possibly the primary noise source,
is noise generated by the turbulence modulating the reflectivity of the
smoke. Experiment appears to be the only approach to determine the
magnitude of this noise source.
3.10 PRELIMINARY EXPERIMENTS WITH A RETICLE VELOCIMETER
A very rough set of experiments was done to explore the reticle
velocimeter concept. A 2-inch diameter telescope objective was equipped
with a photomultiplier and a reticle. The output of the photomultiplier
went to an audio amplifier and a loudspeaker.
In a first experiment, a lambsv/ool polishing wheel mounted in a
hand electric drill served as simulated smoke. The wheel was placed
in the sun, and visually would be described as a low contrast target.
At 50 feet, the maximum space available, the speaker output had a
clearly recognizable tone which moved up and down appropriately as the
instrument was slowly scanned along a diameter of the wheel.
Indoor night experiments at a few feet with a dc tungsten lamp
as illumination also performed similarly.
53
-------
The performance of this type of instrument is independent of range
under the condition that the angular field size does not change. This
condition means that the object area sampled increases as the range is
increased.
The instrument also performed well as a velocimeter for automobiles.
At about 75 feet, the passing of each car yielded a distinct tone pulse,
and differences in velocity were readily apparent to the ear. Car head-
lights at a distance of 2 miles at night gave a just detectable tone.
The reticle spacing was such that the tone produced was actually too low
for the pass band of the audio amplifier at this long range.
No smoke plumes were readily available in the Los Angeles area, but an
attempt was made to use the velocimeter on a refinery burn-off flame.
The attempt, done at night, was unsuccessful. The main reason was
that at the nearest available distance to the flame (about 2000 feet),
the field of view of the instrument was larger than the entire flame. It
should have been small enough to sample only a small area near the mouth
of the pipe. In consequence, the principal output consisted of noise
from intensity fluctuations of the flame itself. Another possible reason
for the failure with the flame may be that the flame propagation velo-
cities are higher than the average translation velocity. Thus, the
luminous structure is changing faster than the "blobs" can be swept
across the reticle lines by the average translation. There is reason
to expect smoke to be more coherent in this respect than a flame, due to
the high flame propagation velocities. At times a crackling or frying
noise was apparent, and visually corresponded to a "sparkle" in the
f1ame.
In spite of the failure of the flame experiment, it is still felt
that a properly designed experiment that examines a small area near the
pipe's rim has some chance of success with flame and a good chance for
success with smoke.
54
-------
4. CONCLUSIONS
4.1 PORTABLE LASER VELOCIMETER
A portable laser velocimeter was designed and built to measure particle
laden stack gas velocities based upon backscatter signals generated by the
particulate as it passed through the region generated by two intersecting
laser beams. Only limited testing of the instrument in a laboratory environ-
ment was accomplished due to cost and schedule constraints of the program.
Sufficient work was done to satisfactorily demonstrate its operating range
characteristics. Of significance in these tests was the sensitivity of the
laser velocimeter to signals generated by backscattering of the incident
laser beam on a smooth white paper disc. Application of the velocimeter to
actual stack measurements, however, remains to be demonstrated.
Work on the present velocimeter has led to the conclusion that future
models could be made even more compact and lighter weight. Utilization of
a velocimeter in a fixed installation as a continuous stack gas velocity
monitor would greatly simplify both the electronics and optical system.
Unit costs in even limited production would be considerably reduced.
4.2 LONG RANGE VELOCIMETERS
This study examined the expected performance of three types of
long-range smokestack velocimeters. The discussion attempted to expose
the problems associated with each type of instrument, and to indicate what
parameters are effective in optimizing the performance.
Definite conclusions are not possible because of unknown smoke
properties. The reticle velocimeter appears attractive because of its
extreme simplicity and resulting low cost. Its performance with smoke
is still untested, but it has performed with a rotating wheel simulating
"rigid" smoke. It should be explored further.
The two laser instruments each appear to require a one-watt argon
laser in order to operate well at long distances.
The fringe velocimeter type appears to be the simpler to construct
and can advantageously use a large collection aperture. Its actual per-
formance will depend on the smoke properties; in particular, non-random
clumping may assist its operation greatly. If the clumping does not
55
-------
exist and the instrument is forced to operate on pure random particle
distributions, the principal problem of the instrument then stems from
the small magnitude of the intensity fluctuations, which in turn arise
from the large number of particles simultaneously present in the probs
volume.
The direct doppler instrument will only work at steep elevation
angles due to turbulence in the smoke. This eliminates the really long
range possibilities. The collection aperture is limited in size by
coherence considerations. Multiple scattering may adversely affect
its operation. Very high frequency electronic design is required for
the direct doppler instrument, adding to the difficulty of constructing
the direct doppler instrument.
More sophisticated instruments which give range resolution, and
which function with very light particulate loading have not been con-
sidered in detail. It is felt that some actual experience should be
acquired with a simple long-range system before embarking on the design
of a more sophisticated system.
In interpreting the significance of the noise discussed in this
report, it should be noted that the signal-to-noise ratios given are
power ratios, as is customary practice. Note, however, that most display
devices, such as meters and the oscilloscope deflection of a spectrum
analyzer, are proportional to voltage. Thus, the ratio of deflections
for signal and noise is given by the square root of the signal-power-to-
noise-power ratio.
56
-------
REFERENCES
1. A. E. Lennert, D. B. Brayton, F. L. Crosswy, W. H. Goethert, and
H. T. Kalb, "Laser Technology in Aerodynamic Measurements,"
von Karman Institute for Fluid Dynamics Lecture Series 39, June 1971
2. B. J, Matthews and H. Shelton, "Operation Manual - Portable Laser
Velocimeter for Stack Velocity Measurements," TRW Report No.
20852-6001-TQ-QO, June 1972.
3. R.D. Ingebo, "Drop-Size Distributions for Impinging-Jet Breakup in
Airstreams Simulating the Velocity Conditions in Rocket Combustors,
NACA Technical Note 4222, March 1958.
57
-------
APPENDIX A
SPECIFICATIONS FOR SPECTRA-PHYSICS MODEL 120 GAS LASER WITH
MODEL 256 EXCITER
-------
APPENDIX A
SPECIFICATIONS FOR SPECTRA-PHYSICS MODEL 120 GAS LASER WITH
MODEL 256 EXCITER
Output Power: 5.0 mw s 632.8 nm
Transverse Mode: TEMoo
Warm-up Time: > 3 mw 2 minutes after turn-on
> 5 mw 30 minutes after turn-on
Operating Temperature: 10 to 40°C
Long-Term Pov.-er Drift : < 5$
Altitude: Sea level to 10,000 feet
Beam Amplitude Noise (1-100 KHz): < 3^
Beam Amplitude Ripple (l20 Hz): < 0. 5\>
Beam Polarization: Linear to better than 1 part per thousand
Plane of Polarization: Vertical, adjustable ±20°
Beam Diameter: O.G5 mm at 1/e2 points.'
Beam Divergence: 1.7 milliradians at 1/e2 points
Resonator Configuration: Long Radius
Axial Mode Spacing: 385 MHz
Plasma Excitation: Direct current self-starting
Cable Length (Exciter to Laser): 8 feet (Extension sections available)
Dimensions: Model 120 Laser Head: 3.26" w x 3.66" h x 18.4S1' 1
Model 256 Exciter: 7.25" w x 3.72" h x 9.88" 1
Weight: Laser - T/2 Ibs.
Exciter - 1% Ibs.
Input Power: 115/230v, 50/60Hz, 50va
59
-------
APPENDIX B
LASER VELOCIMETER DRAWING LIST
-------
APPENDIX B
Laser Ve1oc1meter D r a w i ng L1 st
Drawing No. Title
SK 1038-1 Velocimeter Probe
SK 1038-2 Support, Rack & Pinion Lense Assembly
SK 1038-3 Collar, Rack Pinion Assembly
SK 1038-5 Tube, Lense Holder Velocimeter Probe
SK 1038-6 Split Mirror, First Surface Support
SK 1038-7 Holder, Lense 30 MM dia.
SK 1038-9 Support, First Surface Mirror
SK 1038-10 Base, Sub Velocimeter Probe
SK 1038-11 Body, Beam Splitter
SK 1038-12 Laser Modification
SK 1038-13 Cover, Velocimeter
SK 1038-14 Panel, Velocimeter
SK 1038-15 Bracket, Grip Holding Tripod Support
SK 1038-17 Dial Face, Beam Splitter
SK 1038-18 Dial Face, Focus
SK 1038-21 Support, Laser
SK 1038-22 Support, Laser
SK 1038-31 Collar, Front Rack & Pinion
SK 1038-32 Collar, Rear Rack & Pinion
SK 1038-33 Support, Collar
SK 1038-34 Gear, Pinion & Shaft, Collar
SK 1038-35 Bearing, Tube, Lense Holder
SK 1038-51 F'lut, Lense Holder Velocimeter Probe
SK 1038-52 Washer, Lense Holder Velocimeter Probe
SK 1038-53 Rack, Tube Lense Holder
SK 1038-61 Strap, Split Mirror
SK 1038-62 Split Mirror First Surface
SK 1038-71 Nut, Lense Holder 30 MM Dia.
SK 1038-72 Washer, Lense Holder 30 MM Dia.
61
-------
Drawing No. Title
SK 1038-91 Strap, Support First Surface Mirror
SK 1038-92 Mirror, First Surface
SK 1038-101 Spacer, Sub Base
SK 1038-102 Rubber Mount Assembly
SK 1038-111 Pawl, Beam Splitter Velocimeter
SK 1038-112 Link, Beam Splitter Velocimeter
SK 1038-113 Shaft, Beam Splitter Velocimeter
SK 1038-114 Slide, Prism Beam Splitter
SK 1038-115 Slide, Mirror First Surface Beam Splitter
SK 1038-116 Holder, First Surface Mirror Beam Splitter
SK 1038-117 Plate, Body, Beam Splitter
SK 1038-118 Gib, Vee Slide Beam Splitter
SK 1038-119 Gib Spring, Vee Slide, Beam Splitter
SK 1038-131 Window, Lense Focus
SK 1038-141 Focus Dial Window
SK 1038-142 Beam Dial Window
SK 1038-151 Support, Tripod
SK 1038-171 Post, Dial Beam Splitter
SK 1038-172 Pointer, Dial Beam Splitter
SK 1038-181 Pointer, Holder
SK 1038-182 Pointer, Focus
SK 1038-1110 Spring, Vee Slide, Beam Splitter
SK 1038-1111 Spring Drag, Beam Splitter Adjustment
62
------- |