SWRHL-IO!
THE DESIGN AND CONSTRUCTION OF A LASER METER
FOR THE MEASUREMENT OF CW IRRADIANCE AT 623.8 NANOMETERS
by
Richard W. Peterson, M.S.
Wi Ibur F. Van Pelt, M.S.
Harold F. Stewart, Ph.D.
Electronic Products Program
Southwestern Radiological Health Laboratory
U.S. Department of Health, Education, and Welfare
Public Health Service
Environmental Health Service
November 1970
-------
SWRHL-IOI
THE DESIGN AND CONSTRUCTION OF A LASER METER
FOR THE MEASUREMENT OF CW IRRADIANCE AT 623.8 NANOMETERS
by
Richard W. Peterson, M.S.
WiIbur F. Van Pelt, M.S.
Harold F. Stewart, Ph.D.
Electronic Products Program
Southwestern Radiological Health Laboratory
U.S. Department of Health, Education, and Welfare
Public Health Service
Environmental Health Service
Environmental Control Administration
Bureau of Radiological Health
November 1970
-------
ABSTRACT
A description of the design and construction of a meter capable
of measuring scattered levels of irradiance (power density) re-
sulting from the operation of low power, continuous wave, helium-
neon gas lasers is presented. Readily available components are
used and the techniques for calibration and use of the instrument
are discussed. The instrument responds to irradiances of 10 nW/c
or greater depending on the aperture size selected by the user.
-------
ACKNOWLEDGEMENTS
The authors would like to express their appreciation for
the assistance of Mr. Mason MeNinch in the testing and
evaluation of photo conductive detectors and for his aid
in constructing the prototype.
The efforts of Miss Anne Roberts in the preparation of
figures and in reviewing the text are gratefully acknowl-
edged.
^^
-------
TABLE OF CONTENTS
Page
ABSTRACT i
ACKNOWLEDGEMENTS i i
LIST OF FIGURES AND TABLES iv
INTRODUCTI ON I
TECHNICAL CONSIDERATIONS I
CONSTRUCT I ON 3
I. Detector Circuit 3
2. Optical System 5
3. Assembly 7
CALIBRATION AND OPERATION 7
DISCUSSION I I
CONCLUSIONS 12
REFERENCES 14
APPENDIX A 15
-------
LIST OF FIGURES
Figure Page
I. Basic Detector Circuit 3
2. Complete Detector Circuit 4
3. Optical System 5
4. Meter Response vs. Angle of Incidence 6
5. External View of the Assembled Meter 8
6. Internal View of the Meter 8
7. Arrangement of Apparatus for Calibration 9
8. Response of the Meter vs. Relative Irradiance 10
9. Spectral Response 13
LIST OF TABLES
Table Page
I. Aperture Diameters and Areas 6
^V
-------
INTRODUCTI ON
The increasing availability and use of low power helium-neon laser
units has created situations which demand the measurement of irradi-
ance (power per unit area or power density) in order to adequately
assess any possible ocular hazards. Commercially available instru-
mentation is typically designed to provide a large dynamic range of
power measurement. Since ocular hazard analysis requires low level
irradiance determination, we have designed, constructed and tested an
instrument which determines irradiance directly at the helium-neon
laser wavelength, 632.8 nm (nanometers), over a narrow dynamic range
of irradiance values centered about that value considered to denote
the maximum allowable irradiance..
TECHNICAL CONSIDERATIONS
Lasers produce a highly collimated, coherent beam of light. In the
visible region of the electromagnetic spectrum the human eye is the
organ of critical concern. Relatively low irradiance levels can be
concentrated by the focusing action of the cornea and the lens of the
eye, which result in irradiances on the retina on the order of 100,000
times greater than the cornea I irradiance. In evaluating ocular hazards
one is interested only in the measurement of potentially hazardous
exposure levels to the exterior of the eye, and detection instrumentation
must be designed to respond reliably to the cornea I (or low level) irradi-
ance; response to those high levels of irradiance which will produce
observable lesions on retinal tissue is then not necessary. Since direct
laser beam levels generally exceed (by several orders of magnitude)
(acceptable) corneal levels, measurements will routinely be made on
scattered levels. An inexpensive photoconductive detector was selected
for use in the instrument because of its high sensitivity, its speed of
I
-------
response as well as the success achieved in its use in commercially
produced photographic tight meters. While the detector could have
been used to provide a measurement of power only, the measurement of
irradiance was achieved through the use of an aperture of known
dimensions. To minimize the effects of any uneven responses across
the surfaces of the detector, a lens and diffusing screen were neces-
sary to provide uniform surface illumination. Finally, the response
of the detector to a large range of wavelengths required a filter
between the aperture and detector which passes only those wavelengths
at or near the wavelength of interest (632.8 nm).
At this time the U.S. Public Health Service does not advocate or endorse
any published set of laser exposure standards, guides or recommendations.
The design of the prototype meter was therefore based on the premise
that an internally adjustable control would be necessary to center the
response of the instrument at any irradiance which might be typical of
the various published recommendations. This control can be eliminated
at such time that a single hazard irradiance level is adopted.
Experience with optical power meters has indicated that alignment of a
detector for maximum readings is very difficult if the only indication
is on a meter, since the operator must devote his attention to two
points simultaneously. Since the meter is intended for survey applica-
tions, its design includes an audible indicator to allow the surveyor
to devote his attention solely to the alignment of the detector with
respect to the radiation to be measured.
To achieve maximum utility, the meter, which is capable of field oper-
ation, has a minimum weight and no external power requirements. The
size and ruggedness is such that the entire instrument can be operated
and held in one hand.
-------
CONSTRUCTION
I. Detector Ci rcu it:
The detector circuit, in its most elementary form, is the series
circuit shown in Figure I. The conductance of the detector is pro-
portional to the total power of the light incident on the photo-
conductive surface, which is deposited on an inner surface of the
detector. Thus, the current in the meter is related to the incident
Iight power.
photoconductive
detector
battery
meter
Figure 1. BASIC DETECTOR CIRCUIT
The complete detector circuit schematic is shown in Figure 2. To
provide a constant voltage source, a limiting resistor, R|, and
Zener diode have been added. The Zener diode regulates the
-------
potential between points "a" and "b" at 10 volts as long as the
battery voltage remains slightly above this level. Failure of
the battery, B,, can be monitored by constructing a voltmeter
circuit which can be introduced across points "a" and "b" by
S-2, a push-button switch. The variable series resistor, R2,
can be set to provide a full-scale indication when the batteries
are new and the potential "a" to "b" is 10 volts. Any subsequent
test of battery indication which results in less than a full-scale
deflection shows battery degeneration and indicates the need for
replacement of the meter supply batteries. The variable parallel
resistor, R3, provides a method of adjusting the meter response to
the appropriate point such that the midpoint of meter deflection
will indicate that level of irradiance considered to be hazardous.
Figure 2. COMPLETE DETECTOR CIRCUIT
R-l
S-l
S-2
CL-505
-------
Selection of a meter with internal relay points (adjustable from 0
to full-scale triggering) was based on the need for an alternate
indicator mode. This mode is an audible signal furnished by a small
oscillator and speaker which produces a signal at about 3,000 Hz. A
second battery, 62, is used to power this device. The operability
of this segment of the circuit is easily verified by adjusting the
relay trigger point to zero deflection on the meter.
2. Optical System:
As noted earlier, the hazard meter must be designed to respond
to irradiance (power density) and not to total optical power at the
point of interest. This is achieved by using the optical system
shown in Figure 3. Although the aperture must be of known size,
the section of the size is dependent only on:
I) the uniformity of irradiance across the aperture and,
2) the sensitivity required of the system.
Figures. OPTICAL SYSTEM
all dimensions in millimeters
(machined parts aluminum)
instrument case
CL-505 photoconductive detector
diffusing glass
filter
-------
Several apertures have been used and their characteristics are
Ii sted in Table I:
TABLE I
APERTURE DIAMETERS AND AREAS
Aperture No.
1
2
3
4
Di
1 .
1 .
0.
0.
ameter
46 cm
1 2 cm
56 cm
21 cm
Area
1 .
0.
0.
0.
2
67 cm
2
99 cm
25 cm2
2
035 cm
All internal surfaces of the collimator have been sprayed with a
velvet black diffuse paint to minimize internal reflections. The
lens is positioned so that the focal point falls at the diffusing
glass. This provides a well defined acceptance angle and eases
the problem of spurious responses from light entering the detector
from directions not along the axis of the light beam to be measured.
The angular acceptance width to the half maximum response points
(FWHM as indicated in Figure 4) is approximately 12 for all
apertures evaluated.
APERTURE NO. i
APERTURE NO. 2
APERTURE NO. 3
APERTURE NO. 4
0 '" 20 « 10 o 10 20
ANGLE ANCLE
jo 20 10
ANGLE
Figure 4. METER RESPONSE vs. ANGLE OF INCIDENCE
-------
3. Assembly:
The assembled configuration of the laser hazard meter is shown
in Figures 5 and 6. The system is contained in a 5-inch long by
4-inch wide by 3-inch deep aluminum box. No attempt was made in
this prototype to utilize miniature components and it is apparent
that even this small size could be further reduced. The internal
controls, R2 and R-j, are accessible through holes on the bottom of
the case. In operation, these holes should be covered to reduce
stray light leakage to the detector.
CALIBRATION AND OPERATION
The assembled laser hazard meter is calibrated by utilizing an ex-
panded beam from a 2-mW helium-neon laser. Since the irradiance of
the cross sectional plane of the beam is approximated by a gaussian
distribution, expansion of the beam is necessary to assure a nearly
constant irradiance across the aperture. A calibrated power meter,
fitted with a variable iris, is used to determine the irradiance
at the aperture plane. The beam is then reduced to the irradiance
2
assumed to be hazardous (for example, 10 yW/cm ) by inserting cali-
brated optical filters in the beam. A reference power meter,
coupled to the system by a beam splitter, monitors the stability of
the laser output during the calibration procedure. The arrangement
of the calibration apparatus is shown in Figure 7.
The resistor, R3, is then adjusted to give half-scale response. The
linearity of response can be checked by varying the filters in the
beam and noting the response of the laser meter (Figure 8), although
this is not of great importance since the meter is a single point,
threshold device.
-------
Figure 5. EXTERNAL VIEW OF
THE ASSEMBLED METER
Figure 6. INTERNAL VIEW OF
THE METER
8
-------
Figure 7. ARRANGEMENT OF
APPARATUS FOR
CALIBRATION
meter to be calibrated
calibration plane
beam steering
mirrors
reference power meter
\
A.
beam splitter
-------
90
80
70
60
50
40
30
20
10
Figure 8. LASER METER READING vs.
RELATIVE IRRADIANCE
P
5
0
.3 .4 .5
LASER METER READING
.6 .7
.8
.9
10
-------
Finally, the battery metering adjustment, R-,, is adjusted for
full-scale deflection. This calibration procedure requires the
use of a reasonably well-equipped laboratory.
In practice, the operator aligns the meter with the beam of light
at the position of interest. By setting the relay contacts to
trigger at half-scale, the sonic alert will sound at any time the
irradiance exceeds the hazardous level.
The operational life of the meter circuit batteries should be a
minimum of 10 hours as based on the battery manufacturers' speci-
fications if the unit is operated at normal room temperature.
Since measurements are generally of short duration, several months
of routine operation will be obtainable before the batteries must
be replaced. Replacement of the cells in the field (facilitated
by the use of readily available 9V transistor radio batteries)
will not change the instrument calibration because of the Zener
diode regulator circuit.
DISCUSSION
Use of the laser meter has proven to be straightforward. The prime
restriction that the entire aperture be uniformly illuminated
remains. This restriction becomes of lesser importance as the
aperture area decreases. Only beams having diameters less than
the aperture cannot be measured with the meter. Since this condi-
tion generally exists only in the direct beam (which is assumed to
be dangerous) measurement can be made of most scattered light (from
either diffuse or specular surfaces). Specular reflections from
flat surfaces retain the divergence characteristics of the incident
beam and must be analyzed by other techniques.
-------
Aperture dimensions used In the prototype were randomly selected.
If one were to utilize the meter for measurement of the ACGIH
2
guide levels, for example, of 50, 20 and 10 yW/cm (based on
ambient lighting conditions) then three apertures of relative
areas 0.2, 0.5 and 1.0 might be used. It is then necessary to
2
calibrate at only one level (say 10 yW/cm ) and by appropriate
choice of aperture any of the other guide levels would also be
caIi brated.
(2)
The detector selected has a spectral response as shown in Figure 9
When the unit is calibrated at 632.8 nm it should not be used for
other wavelengths due to the rapid change in detector response as
a function of wavelength. The detector has held its calibration
with no measurable change over a period of 60 days. Tests in which
the detector was continuously illuminated over an eight-hour period
2
by 20 yW/cm at 632.8 nm failed to produce any calibration shift.
The detector exhibits a change in conductance of 7 percent over a
temperature range of 0°C to 50°C.
Ambient light does contribute to the meter response. Typical room
illumination contributes between 5 to 10 percent of scale response,
which has been found to be an insignificant contribution because of
the nonlinearity at low levels.
CONCLUSIONS
The feasibility of a portable instrument suitable for detecting
hazardous levels of continuous wave laser radiation at 632.8 nm
has been demonstrated. Utilization of the instrument provides a
convenient method of detecting hazardous levels of reflected or
scattered Iight.
The instrument is stable, easy to use and can be constructed from
readily available electronic components.
-------
100
90
80
70
60
50
40
30
20
10
Figure 9.
SPECTRAL RESPONSE
(reference 2)
400
500 600 700
WAVELENGTH (nanometers )
13
800
-------
The instrument exhibits those properties of cost, utility, and
simplicity of operation which indicate its usefulness for
relatively unskilled field survey personnel.
References:
(I) A Guide for Uniform Industrial Hygiene Codes or Regulations
for Laser Installations, Issued by The American Conference
of Governmental Industrial Hygienists, 1968.
(2) Photoconductive Cell Designers Kit, Essential Technical Data,
Clairex Corporation, 8 W. 30th St., New York, New York 10001,
-------
APPENDIX A
COMPONENTS* USED IN THE LASER METER
Detector: Photoconductive cell, CI airex CL-505
AlI ied Radio 60 D 7458 (I.50)
Batteries (3): NEDA I604D (9V). Allied Radio 18 D
5769 (2.05)
Battery Clips (3): Allied Radio 18 D 5184 (4.20)
Resistors: R-I 500 ohm, 1/2 watt, 20 percent fixed (0.36)
R-2 10,000 ohm, 2 watt, variable (1.60)
R-3 1,000 ohm, 2 watt, variable (1.60)
Switches: S-I SPST, Push Button, Black, Allied Radio
56 D 9947 (0.65)
S-2 SPOT, Push Button, Red, Allied Radio
56 D 4946 (0.65)
Diode: D-I, 10 V Zener reference Diode, Type N758 (1.00)
Meter: M-I, 0-1 mA DC, internal magnetic relay, API
Instruments Model 202M. A lied Radio 51 D
5584 (37.00)
Sonic Alert: Mai lory Sonalert Model SC628. Allied Radio
60 D 8983 (5.50)
Case: Aluminum Minibox. Bud No. CU02I05-A. Allied Radio
42 D 7621 (I.50)
Lens: Piano Convex. Diameter 19.5 mm, Focal length 38 mm
Edmund Scientific No. 94,035 (0.60)
Optical Housing: Machine from stock (see Figure 3).
(cost variable)
Filter: I" x I", cut from Rubylith filter gel.
Diffusing glass: I" x I", but from 4" x 6" piece of 1/8"
opal glass. (1.00)
Paint: 3-M NexteI Velvet Black (2.40 per can)
Equivalent components may be substituted for any of the models
specifically designated above.
* Mention of commercial products used in connection with work reported
in this article does not constitute an endorsement by the Public Health
Servi ce.
15
-------
DISTRIBUTION
1 - 20 SWRHL, Las Vegas, Nevada
21 John C. Vi11 forth, Director, BRH, Rockville, Maryland
22 E. C. Anderson, BRH. Rockville, Maryland
23 Director, Criteria & Standards, BRH. Rockville, Maryland
24 Director, Grants Office, BRH, Rockville, Maryland
25 Robert H. Neill. Program Office, BRH, Rockville. Maryland
26 John G. Bailey, Office of Information, BRH, Rockville, Maryland
27 Roy H. Rohn, Jr., Administration Management, BRH, Rockville. Maryland
28 James W. Miller. Regional Operations, BRH, Rockville, Maryland
29 William A. Mills, Div. of Biological Effects, BRH, Rockville, Maryland
30 George E. Anderson. Chief, Radiation Physics Lab, BRH, Rockville. Md.
31 Harry D. Youmans. Div. of Biological Effects. BRH. Rockville, Maryland
32 Arve H. Dahl, Medical Radiation Exposure. BRH. Rockville, Maryland
33 - 34 Charles L. Neaver, Div. of Environmental Rad., BRH, Rockville, Md.
35 Robert Elder, Div. of Electronic Products, BRH, Rockville, Md.
36 Roger Schneider, Rad. Measurement & Cal. Br., BRH, Rockville, Md.
37 Lavert Seaborn, Intelligence Br., BRH, Rockville, Md.
38 Walt Gundaker, Product Testina & Evaluation Br. , BRH, Rockville, Md.
39 Bob Britton, Compliance Br., BRH, Rockville, Md.
40 William Properzio, X-ray Exposure Control Lab., BRH, Rockville, Md.
41 George R. Schultz, Training Inst., ECA, Rockville, Md.
42 Elizabeth Boeker, Assoc. Dir., ECA, Rockville, Md.
43 C. Bruce Lee, Office Research & Develop., ECA, RocKvilie, Ma.
44 FredericK Erickson, Training & Manpower Develop., ECA, Rockville, Md.
45 Mark H. Barnett, Rad. Health Br., ECA, Rockville, Md.
46 Charles Froom, Rockville Training Sec., ECA, Kockville, Md.
47 Richard Marland, Program Development, ECA, Rockville, Md.
48 Robert M. Hallisey, Regional Rep., Region 1, Boston, Mass.
49 Saul J. Harris, Region II Regional Rep., New York, N. Y.
50 Robert Frankel, Region III Regional Rep., Philadelphia, Pa.
51 Joseph Brennan, Region IV Regional Rep., Atlanta, Ga.
52 Jerome A. Halperin, Region V Regional Rep., Chicago, 111.
-------
Distribution - continued
53 James A. McTaggart, Region VI Regional Rep., Dallas, Texas
54 Gerald A. Jacobson, Region VII Regional Rep., Kansas City, Mo.
55 Paul B. Smith, Region VIII Regional Rep., Denver, Colorado
56 Clifford E. Nelson, Region IX Regional Rep., San Francisco, Calif.
57 Edward J. Cowan, Region X Regional Rep., Seattle, Wash.
58 Morgan S. Seal, NERHL, Winchester, Mass.
59 George Coates, Electronic Products, NERHL, Winchester, Mass.
60 Neil Gaeta, Training, NERHL, Winchester, Mass.
61 Library, NERHL, Winchester, Mass.
62 C. R. Porter, SERHL, Montgomery, Alabama
63 Charles Phillips, SERHL, Montgomery, Alabama
64 Bob Callis, Office of Education & Information, SERHL, Montgomery, Ala.
65 Ralph E. Shuping, Physical Science Unit, SERHL, Montgomery, Ala.
66 Library, SERHL, Montgomery, Ala.
67 Richard Tells, N Y University Medical Center, Tuxedo, N Y
68 Wendell McCurry, Public Health Eng.II, Bur. Environ.Health, Reno, Nev.
69 Mark Jakobson, Physics Dept., Univ. of Montana, Missoula, Mont.
70 Albert Shephard, Georgia Inst. of Technology, Atlanta, Ga.
71 Oliver D. T. Lynch, Jr., Rad. Safe. Spec., Rad.Safe.Br., AEC, Mercury, Nev.
72 Paul Bolton, REECo Indust.Safety, Mercury, Nevada
73 Leroy Garcia, REECo. Indust. Safety, Mercury, Nevada
74 D. W. Hendricks, Radiological Operations, AEC/NVOO, Las Vegas, Nevada
75 Henry J. L. Rechen, Div. of Electronic Products, BRH, Rockville, Md.
76 William Link, BRH Library, Rockville, Maryland
77 D. Hamil, AEC/NVOO, Library, Las Vegas, Nevada
------- |