5ER&
UrritM)
EnvirofVTMntBl Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
EPA-600 2-79-005
January 1979
Bueerch and Development
Examination of
Automatic Data
Reduction Methods for
Particle Field
Holograms
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution-sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-79-005
January 1979
EXAMINATION OF AUTOMATIC DATA REDUCTION
METHODS FOR PARTICLE FIELD HOLOGRAMS
by
J.D. Trolinger
Spectron Development Laboratories, Inc.
3303 Harbor Boulevard, Suite G-3
Costa Mesa, California 92626
Contract No. 68-02-2491
Project Officer
Charles W. Lewis
Atmospheric Chemistry and Physics Division
Environmental Sciences Research Laboratory
Research Triangle Park, N.C. 27711
ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
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DISCLAIMER
This report has been reviewed by the Environmental Sciences Research
Laboratory, U.S. Environmental Protection Agency, and approved for publication.
Approval does not signify that the contents necessarily reflect the views and
policies of the U.S. Environmental Protection Agency, nor does mention of
trade names or commercial products constitute endorsement or recommendation
for use.
ii
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ABSTRACT
Holographic recording techniques provide one of the most powerful par-
ticle field diagnostic tools in existence. A hologram can provide a frozen
three-dimensional image of a particle field through which detailed micro-
scopic examination of individual particles is possible. Frequently, a par-
ticle field may contain many thousands of particles, and it becomes impracti-
cal for the human operator to glean all the data of interest from such a
hologram. For holography to reach its full potential in particle diagnostics,
a three-dimensional image analyzer is required.
The purpose of this study was to examine the feasibility of using exist-
ing electro-optic image analyzers to automatically analyze three-dimensional
image fields and to determine what modifications of existing equipment would
be required to construct such a system.
Sample holograms as well as holograms produced in an actual field holo-
camera were used to make the evaluations experimentally, and well-refined
analytical descriptions of holographic images were used to add to the under-
standing of system requirements. The study established that existing image
analyzers are capable within useful practical limits of locating particle
images in three-dimensions and measuring size and shape factors of the
particles. A plan for integrating such equipment to produce a fully auto-
mated data reduction system is presented.
This report was submitted in fulfillment of Contract No. 68-02-2491 by
Spectron Development Laboratories, Inc. under the sponsorship of the U.S.
Environmental Protection Agency. This report covers a period from 9 March
1977 to 9 September 1977 and work was completed as of 1 January 1978.
iii
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CONTENTS
Abstract ill
Figures vi
Tables vil
Acknowledgement viii
1. Introduction 1
Background 1
Purpose of the Report 2
2. Conclusions and Recommendations 3
3. Review of Particle Field Holography 4
How Holography Offers Advantages 4
Choosing a Holography System 6
4. Automatic Processing of Particle Field Holograms 11
Introduction 11
Existing Data Reduction Technique 12
The Task of Automating Data Reduction 13
Evaluation of the Quantimet System for Holographic
Image Analysis 14
Sample Data from Holograms 35
5. The SDL Field-Portable Particle Holocamera 41
6. A Proposed Automatic Holography Data Reduction System 45
References . 50
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FIGURES
Number Page
i-'-.v.- ifO—
1 Holography of a point source 5
2 In-line hologram of a particle field 8
3 Off-axis hologram of a particle field 8
4 Geometry for Equation (1) 9
5 Intensity distribution in the intensity of an image 15
6(a) Hologram reconstruction setup 17
6(b) Experimental apparatus for machine data reduction of
particle field holograms 18
6(c) Adjusting the Quantimet system for a data run 19
6(d) Recording hard copies from the monitor 20
7 Two different planes of focus from a three-dimensional
reconstructed image as displayed on the "Quantimet" .... 21
8 Definition of image parameters 23
9 Same two planes shown in Figure 7 with all information
below a preset threshold rejected 24
10 One plane of the reconstructed three-dimensional image
of a dust particle field 26
11 Two centimeter change in focus position of the image
field shown in Figure 10 27
12 Two field different focus planes in the droplet 29
13 Fog droplet field reconstruction 30
14 Scanning in focus while processing with two level detection . 31
15 Two focus positions of a pair of 30 micrometer particles
in a dust cloud . 33
16 Same particles as displayed in Figure 15 but shown here
with two level detection 34
17(a) Holographic size analysis - glass beads 38
17 (b) Holographic size analysis - aerosol spray 39
17 (c) Holographic size analysis - dust storm 40
18(a) Field-portable particle holocamera 42
18(b) Side view of the holocamera installed behind a shield
as applied in explosion diagnostics 43
18(c) Front view of the shielded optics package 43
19 Holocamera reconstruction system 46
20 Flow diagram of automated data reduction 47
VI
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TABLES
Number Page
1 Printout of Information Taken from the Image Analysis
of Figure 9(a) 22
2 Printout of Information from Figure 9(b) 25
3 Data for Figure 14(a) 28
4 Data for Figure 14(b) 32
5 Data Associated with Figure 16(a) 32
6 Data Associated with Figure 16(b) 35
ViX
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ACKNOWLEDGEMENT
The author wishes to acknowledge the assistance of Mr. Brian Partridge
of the Cambridge Instruments Company in applying the Quantimet system to the
data reduction of the holographic data. Without his suggestions and assist-
ance, this study could not have been done.
vi-ii
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SECTION 1
INTRODUCTION
BACKGROUND
Holography has provided a new and powerful technique in the study of
particle fields. Holography can be used to freeze the three-dimensional
image of a particle field which covers a relatively large volume. The holo-
gram is then used to reconstruct this three-dimensional image at which time
the image can be probed in great detail microscopically to study individual
particles and their properties, including size, shape, and velocity. The
method can be used to extend the depth of field of the study of a dynamic
particle volume by three or four orders of magnitude over that available to
photography or microscopy.
Although particle field holography has now been used for a number of
years in applications where no other method will work and has become an accept-
ed diagnostic tool, the method still suffers a number of serious limitations
which limit its use or make its use impractical for some studies. Holograms
contain vast amounts of information. Extracting that information can be an
extremely tedious and time-consuming task. If that task can be performed
automatically by machines, then holography could be attractive in many more
types of studies than it is now.
The field of electro-optics has produced a number of two-dimensional
image analyzing systems. For example, an image can be inserted into such a
system by way of a photograph of a negative and the system almost instanta-
neously can convert this image information into a computer-compatible format
such that data processing is very quick. Holographic images are three-
dimensional and, therefore, an image analyzing system must have the capability
of determining another important piece of information. More specifically, the
system must be able to define which plane in the three-dimensional image is in
focus at any given time and is currently undergoing analysis. All the infor-
mation taken from that given plane must be so designated to have arisen from
that particular plane.
During this study we examined the capabilities of existing image analyz-
ing equipment to determine if such equipment was easily adaptable to provide
this requirement which was necessary for automated holographic data analysis.
We paid particular attention to a system produced by the Cambridge Instrument
Company Incorporated (IMANCO System) since the EPA currently has one of these
systems. We found that this system already has the necessary programming
capability to determine when an image is in focus if the image is of suffi-
ciently high quality. Therefore, our next step was to determine a quality
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factor which could be used to describe a hologram capable of producing such
an image. The question, therefore, became not can three-dimensional images
be automatically analyzed, but rather how good must the three-dimensional
image be in order for it to be analyzed by currently existing image analyzers.
More specifically, how good must the hologram be which produced this image.
We then produced a number of holograms of both calibrated particle fields and
typical particle fields, both in the laboratory and in an SDL field-portable
holocamera, to determine how practical such a complete system would be. We
found that when the proper conditions were met by correct field holocamera
design, it is entirely feasible to produce useful holograms in the field which
can be analyzed automatically by computer. It is our belief that such a sys-
tem could be extremely useful in a vast number of requirements of the
Environmental Protection Agency.
A specific limitation of the system which may prevent its widespread use
by a large number of groups at this time is the expense of the overall system.
However, since the EPA already has the image analyzing computer, this should
not be a factor for the use by the EPA.
SDL has constructed and used a wide variety of field-portable holocamera
systems under almost every conceivable naturally occurring environmental
condition. We have found that almost without fail, such a holocamera can be
tailored to meet the problems of any naturally occurring environment. Such
systems have been used in aircraft, in field trailer systems, and in weather
observatories, in conditions ranging from 40°F below zero to over 120°F,
under extreme weather conditions. These systems are capable of producing
holograms of quality sufficient to be reduced automatically. Such systems
will be described in this report.
PURPOSE OF THE REPORT
This is a final report covering a relatively small effort to determine
the feasibility of practical use of automated image analysis systems in re-
ducing holographic particle field data. In Section 3 we provide a brief
explanation of particle field holography and its variations after which the
discussion is extended to the properties of reconstructed images. This pro-
vides the necessary background for the search for an automated holographic
data reduction technique. In Section 4 we describe an image analyzing com-
puter system which was chosen for further experimental work and describe
experiments which were carried out on actual reconstructed images from a
variety of holograms of particle fields. In Section 5 we describe a typical
field-portable holocamera which can produce holograms of the quality neces-
sary for use in an automatic data reduction system. In Section 6 a more
detailed layout of the holocamera recording and data reduction system is
presented.
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SECTION 2
CONCLUSIONS AND RECOMMENDATIONS
During the course of this study, we have produced a wide variety of
holograms. These holograms range in quality from the ultimate or ideal qual-
ity that can be produced in a laboratory over a very limited known particle
field to the quality that arises from an operational portable field holocam-
era which is used in routine data collection in studies ranging from weather
to explosion diagnostics.
Taking holograms produced for this study, we concluded that existing
optical image analysis equipment was sufficient to reduce images reconstruct-
ed from holograms produced in a good field holocamera. Therefore, we have
concluded that the equipment necessary to automatically reduce holographic
data exists in various components and needs only to be assembled and inte-
grated for application to holography. We have shown that the TV resolution
is sufficient for a wide range of interests of holographic images, the system
is capable of rejecting common types of noise which normally would make data
reduction difficult, the system is capable of eliminating common speckle
type of noise, the system is capable of sizing particles of interest, and the
system is capable of determining when a particle is in focus.
The remaining task is to design such an integrated system in detail so
that the remaining components can be procured and assembled into a working
unit. At this stage the computer software for that specific system must be
developed. The principal item to be procured in this development is the
computer-driven x,y,z traverse onto which the hologram is mounted. This
traverse could be driven by a number of different computers. In fact, the
Hewlett-Packard desk top calculator which we used during this study could
perform that task, although it would be limited in speed. A more preferable
system would be a minicomputer which would allow the data handling as well
as the traversing capability to be greatly enhanced. We would recommend the
EPA make plans to assemble such a system for use with their existing
Quantimet 720 image analyzer such that an automated holographic data analyz-
ing system would be available for use in particle field studies.
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SECTION 3
REVIEW OF PARTICLE FIELD HOLOGRAPHY
HOW HOLOGRAPHY OFFERS ADVANTAGES
The purpose of this section is to review the current state-of-the-art
and to present examples of applications of holography in the study of parti-
cle fields.
An optical hologram is a special type of recording of a light wave which
stores all the information necessary to reconstruct an essentially identical
light wave at a later time. The three-dimensional image of a field is viewed
through a hologram as if the hologram were a window behind which the actual
field was frozen in time. It is a simple matter to convert this virtual
image (which lies behind the hologram) to a real spatial image (which lies on
the viewer's side of the hologram). A hologram is formed by mixing coherent
light scattered from the field of interest (object beam) with a reproducible
mutually coherent (reference) beam, Figure l(a), and recording the intensity
of the sum on a photographic emulsion or other recording medium (there are
many types) . When the hologram is once again illuminated by the reference
beam, Figure l(b), the hologram, acting as a special type of diffraction
grating, changes the reference beam into several components, one of which is
nearly identical in information content to the original object beam. The re-
constructed beam is never exactly identical to the original beam simply be-
cause the recording and reconstruction processes are not ideal. Optical
noise and aberrations cause resolution limits on the process which are thor-
oughly discussed, for example, in Reference 1.
When particles can be trapped on slides, replicated in plastic, or
frozen in space, conventional methods of viewing and photographing are usu-
ally satisfactory, if not superior. When they cannot be trapped, more uncon-
ventional methods are necessary. The need for in situ characterization of
particles in all fields of science has spurred the development of many tech-
niques based on light scattering and extinction laws, nucleation phenomena,
shadow detection, and measurement of other properties which can be related
back to particle size, number, and type. In many cases large numbers of
particles are involved and statistical properties of the entire sample are of
more interest than individual particles. In such cases the detailed examina-
tion of individual particles may not be required. Holography is particularly
powerful when it is desired to examine individual particles in a distribution.
All optical imaging measurements, holography included, are resolution
limited by the laws of diffraction. The precise limit of resolution for any
optical sy^rem depends upon a somewhat arbitrary definition of resolution;
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Liser Light
(a) In-line and off-axis hologram
of a point object.
Off-Axil Hologram ~B"
Rnllitilgi
(b) Forming the image with the
hologram.
Figure 1. Holography of a point source.
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however, the same
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An in-line hologram is made by passing coherent light through a volume
of particles and recording the resulting wavefront (Figure 2). Part of the
light is diffracted by the particles and the remainder passes through the
field unscattered. The unscattered (reference) light interferes with the
scattered (object) light and is recorded on a photographic medium. An off-
axis hologram is formed by introducing a reference wave which is separate
from the light passing through the object field onto the recording (Figure 3).
The laser beam is split initially in two. One portion illuminates the object
field, while the other serves as the reference beam. Many geometries are
suitable for off-axis holography. The object field can be illuminated dir-
ectly with the laser beam or with laser light that has been diffused by a
frosted glass or similar diffusing medium.
The object wave in both in-line and off-axis cases is reconstructed by
illuminating the developed recording with the reference wave. Equation (1)
predicts the position of the reconstructed image as a function of the wave-
length, radius of curvature, and position of the reference wave. (Figure 4
illustrates the geometry for Equation (1).)
-1
I1 + "2 ¥
\z Xn z
\ p 1 r
M'1
A-z I
1 o/
A z +X2z z
x. = +- x -T x+ — x (1)
a. Xn z o X.. z r z p
1 o 1 r p v
X? z. X_ z. z.
yi = + \:~ry0 ~ x7^yr ^v
1 o 1 r P
These equations result from a first order analysis of thin holograms and are
not accurate when large changes in wavelength or reference wave curvature
are encountered. A more general analysis is treated in Reference 1.
There are no hard, fast rules for choosing one specific type of system
for a given application. In-line holography is the simplest type of holog-
raphy and perhaps should always be considered first in particle field studies.
However, there are many cases in which off-axis holography is applicable and
in-line holography is not. Such cases exist when the particle number density
or their total cross section is very large or when the object field signifi-
cantly modulates the phase of the electromagnetic radiation. A sufficient
amount of light (typically about 80 percent) must pass through the field with-
out modulation to serve as a reference wave to produce a good in-line
hologram.
When a field is illuminated by diffuse illumination, the resulting re-
constructed image is easier to view with the naked eye. Associated with
diffuse illumination are speckle patterns which cause problems in high reso-
lution viewing. In any viewing plane the background of illumination is not
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Direct _
Illumimtion
Scittend Light
- Hologram
- Interfmnx
bemem
ScittnedMd
Unscattereif
Light
Figure 2. In-line hologram of a particle field.
Holognm
Figure 3. Off-axis hologram of a particle field.
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+y
x y z
ooo
Hologram
Producing the Hologram of a Point
sub o ~ object wave
sub r ~ reference wave
sub p — reconstructing wave
sub 1 ~ reconstructed wave
x y z
P P P
Reconstructing the Point Image
Figure 4. Geometry for Equation (1).
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uniform; it is formed by speckle points of varying size and separation. The
mean speckle separation is about twice the resolution of the original record-
ing system. Therefore, the resolution limit of the holography system must be
five to ten times smaller than the smallest particle to be observed so that
particles can be distinguished from speckle.
To see the smallest particles in the reconstructed image requires a
total system of low noise. Even those particles which are much smaller than
the system resolution limits can be seen as poii'its of light whose brightness
can be associated with size. The task is to separate these points from the
optical noise. One of the most powerful methods for seeing particles in the
submicron and micron size range is the holographic recording of scattered
light only, against a dark background. The particle field is illuminated by
a pencil' or sheet of light and the hologram of scattered light is made at
some angle away from the axis of the illuminating beam.
The hologram itself is often the poorest quality optical element in the
system because the emulsion adds phase and amplitude noise to the recording
and reconstruction process. This element can be improved by such techniques
as liquid gating and special photographic procedures^. Such methods are
tedious and are avoided in most applications.
Holograms possess most of the properties of lenses. In practice,
however, lenses are better imaging elements than holograms and are combined
with them to attain higher resolution and larger working distance. When the
field of interest must be located at a relatively large distance from the
nearest optical element, lenses or mirrors are normally used to image the
field closer to the hologram to maintain high resolution. A lens hologram
combination can be used in such a way to completely correct aberrations in
the lens during reconstruction. Although high magnification is possible in
principle in holography without lenses, the most successful applications have
resulted from combinations of conventional with holographic magnification.
A well-designed holography system will normally produce ultimate image qual-
ity and resolution which is slightly poorer than a conventional optical sys-
tem which is focused on an object plane, but it will do this over a volume
in a single recording. A detailed review of particle field holography is
given in References 2 and 3.
10
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SECTION 4
AUTOMATIC PROCESSING OF PARTICLE FIELD HOLOGRAMS
INTRODUCTION
There are two basic limitations of holography systems for particle field
studies which are suffered in almost all current applications. First, data
recording rates are too slow. One sample volume, typically on the order of
a liter, can be recorded every few seconds. This rate is too slow to provide
ample statistics in many cases. Second, data reductions from holograms re-
quire tedious, time consuming procedures which make it difficult to reduce
even the quantities of data generated by existing systems.
The technology for increasing data recording rate is here; however, it
would make little sense to increase the data recording rate without first in-
suring that methods to rapidly reduce the data are forthcoming. Therefore,
it is reasonable to first remove the holographic data reduction bottleneck
and define the quality of data required for rapid data reduction. This will
tell us how good the recordings must be and how many can practically be
reduced.
Two different kinds of holography data must be considered, depending upon
the type of light which is used to illuminate the particle field. The field
may be illuminated with diffuse light or with direct laser light.
To illuminate a particle field diffusely, the laser beam is expanded onto
a frosted glass or other diffuser which is placed behind the sample volume.
A reference beam is carried around the sample volume where it is mixed with
the light from the sample volume onto the hologram. The hologram looks like
a uniformly exposed piece of film, appearing to contain no information at all.
The reconstructed image appears to contain dark, shadow images against a
diffuse light background. As previously explained, the problem with this
method occurs in high resolution studies.
Diffuse illumination has some advantages. The reconstructed image can
easily be viewed with the naked eye, with the particle field appearing against
a bright background. A particle is illuminated from a relatively wide angle
of directions, and accordingly can be viewed over that wide angle. This
provides two benefits. First, since forward scattered light is greatest, it
provides for a wider angle over which forward scattered light can be recorded.
This makes a more uniform recording which would exhibit higher resolution,
except for the effects of speckle. Second, the particle image, being backed
by a diffuse field, has essentially no out-of-focus shadow. The out-of-focus
image blends into the background in such a way that it cannot be distinguished
11
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from the background when defocused by typically 100 particle diameters
(depending upon the system). This feature provides a simply defined focusing
property of the image.
Direct light holograms are produced when the particle field is illumi-
nated by light directly from the laser with lenses being used to expand the
beam large enough to fill the sample volume. The reference beam is either
(1) the unscattered beam passing through the sample volume or (2) a separate
beam introduced onto the hologram by another path around the sample volume.
In either case, the hologram appears as a more or less uniformly exposed
piece of film with diffraction patterns and fringes superimposed. For each
particle in the volume from the laser to the film there is a diffraction
pattern (a coherent shadow) which is a set of more or less circular concentric
rings. When reilluminated by the laser, each of these diffraction patterns
diffracts light into a particle image in its original position in 3-D space.
When passing through the focused plane of an image, the diffraction
pattern collapses into a sharp high contrast image which is bright against
dark for in-line holography and dark against bright for off-axis holography.
After passing through the focused plane, the defocused image (shadow or dif-
fraction pattern) expands in size and loses its characteristic shape ulti-
mately appearing as a somewhat circular set of diffraction rings bearing no
resemblance to the original particle.
EXISTING DATA REDUCTION TECHNIQUE
The reconstructed 3-D image of the sample volume is scanned either by
moving the hologram and passing the image over a viewing device or by moving
the viewing device through the sample volume image. The simplest type of
viewing device is a microscope which the viewer scans by hand. This technique
is the one we use in the field for hologram quality evaluation. However,
viewing large numbers of reconstructed holographic images through a microscope
by eye is extremely tiring. In most laboratories, extensive data reduction
is accomplished by converting the coherent reconstructed image (normally a
red image from a HeNe laser) to an incoherent black and white image by pro-
jecting it into a closed circuit TV system. Final viewing of the image is
done on the TV monitor. When focused on the TV monitor, the image may be
studied, sized, photographed, or sketched.
Consider a typical particle field hologram from our field portable holog-
raphy system. The sample volume has a cross sectional area of about 25 cm2
and a length of 15 cm. A 100 micron particle should appear about 1 cm across
on the TV monitor for comfortable viewing; that is, a magnification of about
100. The TV magnifies electronically by 25, the remainder is achieved opti-
cally upon reconstruction either with lenses or holographically (see Equation
(1)). The actual cross sectional area viewable on the TV screen represents
an actual area of about .05 cm2 under these conditions. Therefore, 500 in-
depth scans are required to analyze the entire hologram. This can take an
operator an entire day, depending upon what data he is collecting.
Considering the fact that in a single day's operation of the system hun-
dreds of holograms can be recorded, the data handling problem becomes
12
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cumbersome. Normally the data output comprises photographs, number counts,
size distributions and, in some cases, spatial and velocity distributions.
THE TASK OF AUTOMATING DATA REDUCTION
Consider the task performed by the human operator. In general he must
scan through the 3-D field while watching image characteristics. In the case
of direct light holograms, he can tell directly from the hologram what the xy
positions of particles will be by locating diffraction patterns. Some of
these particles are contamination on windows and in the optical train. If
two holograms made in the same system at different times are placed side by
side, he can mentally subtract one from the other leaving only diffraction
patterns which are not common to the two. These represent the particles in
the sample volume. He could immediately count the number of particles (or
determine velocities) directly from the hologram without reconstruction. To
see the particle he must locate the focused position, z, during reconstruction.
The human operator uses the following characteristics to decide upon the fo-
cused position:
(1) The diffraction pattern converges to form an image;
(2) The light distribution becomes more intense in the image;
(3) Image edges become defined;
(4) He sees an expected shape or combination of shapes emerge;
(5) Optical noise is mentally rejected because it has different
characteristics than the image.
Every criteria used by a human operator can be stated mathematically and
can be sensed optically and/or electronically. Therefore, in principle, the
reduction of holographic data can be done by machine. In fact, for almost
every criteria there exist several machine processes which can, in principle,
perform the human task. Determining factors in the rapidity with which a
human operator can function in holographic data reduction are memory and
response to learned characteristics. Both factors are poor during the first
hour or so of a data reduction session. They peak for a period of only an
hour or so and then decline as the operator becomes tired or succumbs to
monotony, resulting in variable judgment and poor efficiency. A well-pro-
grammed machine would excel in this respect because it could exercise the best
knowledge of a skilled operator with more speed, consistency and endurance.
A higher quality hologram requires less skill and judgment in data re-
duction and, therefore, can be automatically reduced by a less complicated
machine. As the quality of a hologram get poorer, so does the practicality
of devising an automatic data reduction machine. (In fact, a certain point
is reached whereupon the required level of expertise of a human operator
increases.) One of the tasks in automatic data reduction is to determine
practical machine limitations for performing the above tasks so that a mini-
mum quality hologram can be specified.
During this program, we examined a number of possible methods to accel-
erate data reduction. The most promising immediate approach was the exten-
sion of existing two-dimensional image analysis techniques. We performed
experiments with the IMANCO "Quantimet" image analyzer located at the
13
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Cambridge IMANCO Instrument Company, Mountain View, California, office. The
results are described in the following subsection.
EVALUATION OF THE QUANTIMET SYSTEM FOR HOLOGRAPHIC IMAGE ANALYSIS
Existing Work
Shortly after we had begun this investigation for the Environmental
Protection Agency, a group of scientists with whom we had been in touch work-
ing for the Meteorological Office in the United Kingdom had independently
chosen the IMANCO system to apply to the data reduction of holograms produced
in weather and in environmental studies. In Reference 4, they describe a
series of experiments in which they analyzed holograms-with this*system. The
information gained from those studies actually gave us a head start in the
conduct of our own studies.
As previously described, the new problem involved in adapting the IMANCO
system to holographic data reduction is to force this system to differentiate
between particles which are in focus and out of focus, therefore, allowing a
scan automatically of the third dimension'. They showed, for example, that the
existing IMANCO system was capable of performing the first three tasks which
are outlined in the previous subsection. Consider, for example, the intensity
distribution illustrated in Figure 5, which represents the cross-sectional
intensity in the image of a particle as the particle passes through focus.
Three levels were set into the IMANCO system to be used in image detection.
Level 1 was set to be greater than the normal background noise. The IMANCO
system, therefore, rejects any optical information below this level and does
not display it on the TV monitor. Level 2 is set at some value higher than
Level 1 but lower than the maximum intensity of an in-focus image of the
particle. Level 3 was set to be higher than the intensity in the in-focus
image of the particle. The determination of focus is made in the following
way. When the image intensity exceeds Level 1 but not Level 2, it can be
assumed that the particle is very badly out of focus and the system can con-
tinue scanning toward focus. When the intensity in the image increases to
the point where it exceeds Level 2, it can be assumed that the image is ap-
proaching its in-focus condition and the scanning system can be slowed down.
When the image nears its in-focus condition, the diameter of a particle as
determined by the extent of the image going out to Level 2 should be approx-
imately the same as that determined from Level 1. When the difference be-
tween the Level 1 diameter and the Level 2 diameter is at a minimum, the
particle can be considered in focus, and the size determined by an average of
the Level 1 and Level 2 diameter should be a good representation of the par-
ticle size. That information is recorded for that particular particle image.
For some cases, an out-of-focus condition produces a higher intensity at the
center of the particle image than away from the center, and these cases are
rejected when the intensity in the image exceeds Level 3. The IMANCO system
has the correct programming to allow the determination of minimum values such
as that described here, and an output is available to drive interface equip-
ment which would, in turn, control traverse systems. During the reported
studies, a Hewlett-Packard 9830A was interfaced with the Quantimet to perform
the task of driving the traverse which held the hologram under study.
14
-------
Intensity
Level. 3
Portion of Inag*
Betw»«n Lavels 162
Radial Distance
(a) In-Fooxm Im»«« intensity Distribution.
Intensity
Level 3
Level 2
Level 1
Radial Distance
(b) Out of Focus.
Intensity
Level 2
(c) Badly Out of Focus.
Figure 5. Intensity distribution in the intensity of an image.
-------
Experimental Investigation
To further evaluate the ability of the "Quantimet" system in hologram
image analysis, we produced a wide variety of holograms. Three basic types
of holograms were produced: (1) ideal laboratory quality under clean condi-
tions (these were intended to represent the best case recordings), (2) holo-
grams produced in the SDL field holocamera of controlled particle fields
(controlled size and number density), and (3) holograms produced in the SDL
field holocamera in a variety of natural environments.
Our intent was to determine what limitations would be exhibited by the
"Quantimet" system. At first, we had planned to begin with the ideal best
case holograms to provide a baseline. We decided very quickly that this
would not be necessary. We observed that the Quantimet could, in fact, ana-
lyze almost all of the holograms we had produced. Some limitations will be
illustrated. Therefore, we advanced to the holograms produced in the field
tests, spending most of our time using the Quantimet to analyze actual field
recorded holograms of particles.
The system used was the Cambridge IMANCO Quantimet 720 image analyzing
computer interfaced with a Hewlett-Packard 9830A desk top calculating computer.
All experiments with the Quantimet were conducted at the Cambridge Instrument
Company's Mountain View office with the assistance of Mr. Brian Partridge, a
Cambridge employee. The required holographic reconstruction equipment was
transported to Mountain View for the experiments.
Figure 6(a)-(d) shows the experimental apparatus. The reconstructing
laser was a Spectra-Physics Model 124 HeNe laser producing 15 mw of laser
power at 633 nanometers wavelength. This beam was expanded and collimated
after which it illuminated the hologram. All holograms were produced on 4x5
inch Agfa-Gavaert, Scientia emulsions. Both Agfa 10E75 and 8E75 plates were
examined. Both in-line and off-axis recordings were produced. Only the in-
line recordings were examined since a larger laser is required for off-axis
reconstruction. An adjustable neutral density filter allowed us to adjust
laser power at the hologram to optimize the interface between the reconstruc-
tion system and the TV camera.
The reconstructed image produced by these holograms is a one to one re-
construction of a 50 mm by 150 mm cylindrical volume which starts 10 mm from
the hologram and extends to a distance 160 mm from the hologram. This image
was magnified and relayed into the TV camera by a high quality copy lens,
specially selected for this problem. As pointed out in Reference 4, cleanli-
ness is an important factor with all optical elements in this system.
Figure 7 shows the face of the Quantimet monitor when a reconstructed
three-dimensional image is presented to the associated TV camera. This is
the "raw" image with none of the image processing capability engaged. Two
different focus planes are illustrated. These figures can be used to explain
the meaning of the various features in the figure. The inner frame is the
active region for the system. The system can be described as a matrix of
picture points 800 points wide by 625 points high. Therefore, it is conven-
ient to use dimensions of picture points. In the top figure, a dimensioning
16
-------
TV Camera
Adjustable
. mt*r
Hewlett-Packard
Desk Top
Calculator
IMANCO
Analyzer
Figure 6(a). Hologram reconstruction setup.
-------
-
Figure 6(b). Experimental apparatus for machine data reduction
of particle field holograms.
-------
-
Figure 6(c). Adjusting the Quantimet system for a data run.
-------
I
<
Recording hard copies from the monitor.
-------
x - From 0 to 800 Picture Points
y - From
0 to 625
Picture
Points
(a) z = 5 cm
NOTE: The hologram was made in an SDL field-portable holocamera in heavy fog.
Figure 7. Two different planes of focus from a three-dimensional
reconstructed image as displayed on the "Quantimet."
-------
raster has been displayed on the monitor. The small elements are spaced by
10 picture points (p.p.). The number in the top left corner displays how many
features (particles) were counted (six for this case). The top righthand num-
ber is the last dimension measured by the system.
We have identified certain regions of the image to illustrate its appear-
ance as particles pass through focus. Region A contains a diffraction pat-
3-
tern which is the out-of-focus coherent image of the particle shown in region
A, . Region B contains an in-focus image which is out of focus in region B, .
These images are 20 micron diameter water droplets.
The program which we used determined the x and y positions of each fea-
ture, its area (A), perimeter (PERI), horizontal feret (FH), vertical feret
(FV), 45° feret (FE), 135° feret (FW), horizontal projection (PH), vertical
projection (PV), ratio of area to perimeter, and ratio of area to perimeter
squared. Figure 8 illustrates the definition of each of these dimensions.
When the image is first presented to the TV camera, it appears as in
Figure 7. The noisy background characteristic of holographic images is
clearly evident. The first processing step is to set a threshold value of
intensity below which all information is rejected. Figure 9 shows the images
resulting from Figure 7 when this is done. This rejects entirely the small
out-of-focus images whose intensity is spread over the Fraunhofer diffraction
pattern. See, for example, regions B and A^. Larger particles are still
3. D
detected over a rather large depth of field as can be expected. However, so
long as size and not position is needed, the size of the particle image does
not change drastically for a quite large depth of field, so it could be meas-
ured with reasonable accuracy in either figure (although it is truly in focus
in only one thin planar region).
In Figure 9(a) six features were counted, while in Figure 9(b) only five
were counted. A marker appears on the monitor beside each feature which is
being analyzed. This will be useful to an operator allowing him to quickly
evaluate the validity of what is being analyzed. A typical printout of data
is shown in Table 1. (Dimensions are given in picture points, p.p.)
X
TABLE 1. PRINTOUT OF INFORMATION TAKEN FROM
THE IMAGE ANALYSIS OF FIGURE 9(A)
AREA PERI FH FV FE FW PH PV
A/P
A/P'
1.
2.
3.
4.
5.
6.
235
241
473
678
482
321
163
273
355
526
643
670
101
154
194
79
34
485
39
47
56
34
23
82
11
15
18
10
8
26
12
14
17
10
8
24
16
18
18
14
8
30
14
18
20
14
12
30
12
14
17
10
8
24
12
15
18
10
8
26
2.590
3.277
3.464
2.324
1.478
5.915
0.066
0.070
0.062
0.068
0.064
0.072
The particles actually ranged in size from about 10 to 50 microns.
22
-------
Vertical
Feret
Horizontal Feret
Vertical Projection = a + b
Horizontal
Projection
Figure 8. Definition of image parameters.
23
-------
No. of Features
Processed
Last Programmed Process (1)
Performed and the Measured Value (93)
z = 5 cm
z = 7 cm
Figure 9. Same two planes shown in Figure 7 with all information
below a preset threshold rejected.
-------
When we changed the focus position to that leading to Figure 9(b), the
following printout resulted.
TABLE 2. PRINTOUT OF INFORMATION FROM FIGURE 9(B)
X Y AREA PERI FH FV FE FW PH PV A/P A/P2
1.
2.
3.
4.
5.
283
286
521
456
366
162
275
353
516
674
315
310
227
11
596
53
64
55
13
92
13
20
18
4
28
14
20
16
3
26
16
24
22
8
34
16
24
22
4
30
16
20
16
3
26
16
20
18
4
30
5.943
4.844
4.127
0.846
6.478
0.112
0.076
0.075
0.065
0.070
Comparing the data in Tables 1 and 2, the first two particles are the
same. The particle diameters measured vary by 15 percent and 25 percent in
horizontal feret respectively. The nearness in equality of vertical and
horizontal feret are characteristic of spheres. The third particle is the
same in both figures and so are the measured parameters to within a few
percent. The fourth particle is different in the two figures and size is the
reason. The fourth particle image in Figure 9(a) is small and did not carry
enough intensity into the camera to be above the threshold in the setting for
Figure 9(b) where it was considerably out of focus. The same is true for
particle No. 4 in Figure 9(b) and No. 5 in Figure 9(a). The sixth particle
in Figure 9(a) is the same as No. 5 in Figure 9(b). This is the largest
particle in the set. Its size is determined within 7 percent the same for
the two cases. In tb.e actual setup the true focus position of these images
could be easily seen to within better than one millimeter, though it is not
completely evident in the photographs.
The key to successful implementation of such a system in holographic data
reduction is its capability to handle noise. Figures 10 and 11 illustrate
some of the studies in noise characteristics and system response. Several
kinds of problems are illustrated here; and one by one we discuss their re-
moval by the system processing functions. In Figure 10 the intensity across
the image varies. When this variation is severe, picture "shading" correc-
tion capability can compensate for this effect. We did not find this neces-
sary on data we were analyzing.
Speckle noise is normally a serious factor in image processing. We
found in every case examined that speckle noise could be almost entirely re-
jected by the proper setting of a so-called resolution limit. Speckle noise
is made up of intensity distributions of extremely small "points" of light.
When the speckle size is much smaller than particles of interest, it can be
rejected by existing instrument functions (compare Figure 10(a) and 10(b».
The two images in the bottom right were circled by diffraction rings,
even after processing. Again this effect is caused by the brighter image on
the righthand side of the screen and can be removed by shading correction.
We studied the focusing problem and the possible system functions which
can be used to drive an automatic scanning system to focus. We found several
25
-------
(a) Entire unprocessed image as presented to
the image analyzer.
(b) Image after rejecting all information below
a preset threshold.
Figure 10. One plane of the reconstructed three-dimensional
image of a dust particle field.
26
-------
(a) Unprocessed.
(b) Information above a preset threshold.
Figure 11. Two centimeter change in focus position of the
iraage field shown in Figure 10.
-------
ways to achieve this, but one in particular was rather simple and existed in
the normal system programs.
Figure 12 shows recordings at two different reconstructed image planes.
The system was set to examine that part of the image which lies between two
intensity levels. The result is shown in Figure 13. It can be seen that
each image is now represented by an annulus of light. This annulus of light
associated with the original images varies much more rapidly with z position
and the properties varying are easier to quantize than the entire image.
Figure 14 includes some interesting comparisons at two z positions for
several particles. The image marked A is present as an annulus in Figure
cl
14(a). In Figure 14(b) this image (marked A.) is out of focus 2 cm. The
intensity in the image reaches Level 1 but not Level 2. Therefore, it is
shown as a solid image. The image marked B^
14(a) than it is in Figure 14(b).
increases in Figure 14 (b).
is closer to focus in Figure
The thickness of the annulus, therefore,
Tables 3 and 4 are the printout data for these two cases.
TABLE 3. DATA FOR FIGURE 14(A)
X Y AREA PERI FH FV FE FW PH PV A/P
1. 379
2. 253
3. 254
4. 492
5. 489
6. 118
7. 675
8. 691
9. 338
324
343
10.
11.
12. 351
13. 356
14.
15.
305
316
16.
17.
18.
19.
356
308
354
331
80
160
274
347
353
375
460
518
637
639
640
645
656
662
663
663
671
671
674
17
26
70
17
11
23
12
7
18
30
8
1
22
8
28
17
20
26
120
22
52
86
31
23
29
26
4
21
24
12
4
23
12
47
16
22
21
172
5
10
15
9
10
6
6
1
7
9
5
1
5
4
12
4
5
6
27
4
9
16
10
16
6
5
2
3
6
2
1
8
2
17
15
7
7
29
8
14
18
6
12
8
8
2
8
12
4
2
10
4
20
10
6
10
32
8
14
18
12
16
8
6
2
6
8
4
2
10
4
8
16
10
8
26
6
17
27
10
5
8
7
2
4
5
2
1
8
2
17
6
7
7
36
7
17
28
10
9
9
10
1
8
9
5
1
6
4
13
4
7
6
42
0.773
0.500
0.814
0.548
0.478
0.793
0.462
1.750
0.857
1.250
0.667
0.250
0.957
0.667
0.596
1.063
0.909
1.238
0.698
0.035
0.010
0.009
0.018
0.021
0.027
0.018
0.438
0.041
0.052
0.056
0.063
0.042
0.056
0.013
0.066
0.041
0.059
0.004
The third particle in Table 3 is the first in Table 4. We will describe
each measured parameter in more detail in a more appropriate figure later.
However, these figures show other important points. In Table 3, 19 features
(particles) are described. Starting with particle No. 9 through 19, all
particles are very close to the same x-y position (i.e., around x = 320,
y = 650). These are all, in fact, describing
region marked C . The
28
-------
Figure 12. Two field different focus planes in ths droplet.
-------
(a) Unprocessed image presented to the image
analyzer.
(b) Only intensity lying between two gray levels
is displayed and processed.
Figure 13. Fog droplet field reconstruction.
30
-------
(a) z = 0
(b) z = 2 cm
Figure 14. Scanning in fomc while processing with twu level aetection.
-------
large diffraction ring had broken into what was interpreted as,particles.
This "erroneous" result was deliberately left in this picture for purposes of
illustration.
TABLE 4. DATA FOR FIGURE 14(B)
2
X Y AREA PERI FH FV FE FW PH PV A/P A/P
1. 356
2. 594
3. 839
4. 722
5. 440
274
356
401
618
646
137
122
18
22
4
73
113
28
35
10
17
19
6
6
4
18
21
5
8
1
22
24
10
10
4
22
24
8
10
4
22
37
7
13
1
25
37
10
11
4
1.877
1.080
0.643
0.629
0.400
0.026
0.010
0.023
0.018
0.040
The diffraction ring can be rejected by the several methods mentioned,
or it can be removed by the operator using a light pen. Any area on the
screen which appears objectionable by an operator can be circled and removed
entirely from the processed data. In Table 4 this particular region was
moved outside of the detected area and rejected.
Figures 15 and 16 illustrate in more detail the properties of an image
when passing through focus. We picked two reconstructed images of dust
particles and magnified them to show more detail. They lie in close vicinity
in x and y, but are considerably separated in z (about 1 cm). Figure 15
shows the unprocessed images. The particle size was about 30 microns. The
top particle is in focus in Figure 15(a) while the bottom is in focus in
Figure 15(b).
Figure 16 illustrates the system output when the two intensity levels
are set and detection limited to between the levels. Comparing Figures 15
and 16 clearly shows how much better the focus position can be found when
the annulus is displayed instead of the entire image. This is related to the
fact that the gradient of the intensity at the image edge is what is used by
an observer in finding focus. The thickness of the annulus is a direct meas-
ure of the intensity gradient at the image edge and, therefore, provides the
operator more direct information. In the usual case, an operator must look
at the intensity at the image edge and mentally determine changes in gradient.
This can be done only if the image is being scanned, a reason why it is hard
to illustrate in a report.
Tables 5 and 6 describe the processed images in Figure 16.
TABLE 5. DATA ASSOCIATED WITH FIGURE 16(A)
X Y AREA PERI FH FV FE FW PH PV A/P A/P2
1. 386 330 487 455 57 70 72 66 150 152 1.070 0.002
2. 549 621 1142 397 56 60 60 64 109 133 2.877 0.007
32
-------
(a) Top particle in focus.
(b) Bottom particle in focus.
Figure 15.
Two focus positions of a pair of 30 micrometer
particlco in a ciuat cloud.
-------
(a) Top particle in focus.
(b) Bottom particle in focus.
Figure 16. Same particles as displayed in Figure 15 but shown
here with two level detection.
-
-------
TABLE 6. DATA ASSOCIATED WITH FIGURE 16(B)
X Y AREA PERI FH FV FE_ FW PH_ PV_ A/P A/P2
1. 494 349 1476 644 72 101 78 82 121 220 2.292 0.003
2. 623 629 421 420 58 56 70 58 119 144 1.002 0.002
Here we describe in detail the signficance of the measured parameters.
x,y Positions—
The intensity distribution (particle image) numbered as 1 and 2 corre-
sponds to the same particle in both figures. The x and y positions would be
identical if the z-scan was precisely normal to the TV vidicon. Since it
was not, the positions on the screen vary somewhat (more in x than in y).
Area—
The area of the annulus was thought to be a direct measure of in-focus
position. We found this partially so, but we did observe cases where area
actually gets smaller outside of focus. This is a property of the diffracted
light field of the particle.
Perimeter—
Perimeter, like area, can get smaller outside of focus, especially for
a non-spherical particle with complex edge detail. Particle No. 2 is an
example.
Horizontal, Vertical, 45°, 135° Feret—
These are the same for a spherical particle. As shown here, they de-
scribe quite well the particle shape.
Ratio of Area to Perimeter—
We found this ratio to be an excellent guide to determine focus position.
Recalling that the dimensions are in picture points, this ratio would equal
unity when the annulus is two picture points wide because the area is equal
to the number of picture points in the annulus times one, while the perimeter
is equal to the number of picture points. Notice that A/P = 1.070 in
Figure 16(a) and 2.292 in Figure 16(b) for the top particle. In Figure 16(a)
the ratio A/P = 2.877 for the bottom particle while A/P = 1.002 in Figure
16(b). Through a number of studies we concluded that this ratio is an excel-
lent parameter for finding focus.
The most logical approach appears to be to program the calculator to
search for minimum A/P and to record the data at that value of x position of
a traverse.
SAMPLE DATA FROM HOLOGRAMS
The primary objective of this study was to determine feasibility for
automating data reduction. Most of the effort was directed to the under-
standing of the properties of an individual image. In the ultimate applica-
tion, a primary interest is in the overall particle field; namely, statis-
tical samples, distributions and descriptions of the overall field.
35
-------
While this study was in progress , a large number of holograms of par-
ticle fields were produced, not only in support of this project but also in
support of three other projects, the task of which was to provide diagnostics
of particle fields. One of the projects involved the use of holography to
analyze fuel dispersions in explosive clouds. The third involved a study of
combustion of fuel particles.
In this section we present sample data from those projects to illustrate
the ultimate capabilities of holography in this type of study as well as to
illustrate current limitations. In one of these studies six holograms of a
fuel droplet cloud were analyzed in great detail. Many hours were spent in
manual analysis, counting and sizing thousands of particles and studying
their distribution in space. In the other holograms the study was limited to
a statistical count.
Commonly a hologram contains the images of thousands of particles, more
than enough for a statistical sample. The number of counts required to ob-
tain a representative sample depends upon (1) the type of distribution,
(2) the accuracy of each measurement, (3) the required accuracy of the dis-
tribution and the mean, and (4) a priori knowledge about the particle field.
Although a detailed discussion of the statistics of particle measurement is
beyond the scope of this report, a few approximate relations assist with the
discussion here.
Consider a Gaussian distribution in diameter (a log normal distribution
would perhaps be more true to nature). If, for example, we desire to deter-_
mine the mean diameter we must make n measurements. Then our mean diameter D
has a 68 percent chance of lying within + S of the true value, 95 percent
chance within + 2S where S , the "standard error of the mean," is
— mm '
approximately
(2)
and S is the standard deviation of the n measurements about the mean.
If ,_for example, we examine a particle field with a Gaussian distribu-
tion of D = 25 microns, and a standard deviation of 10 microns, the probabil
ity is 68 percent that error of the mean will be less than + 10 percent if
17 measurements are made to compute the mean. To improve the probability to
the 95 percent level requires 65 measurements. In actuality, the required
number of measurements to be made can only be determined after enough meas-
urements have been made to allow reasonable estimates of S.
In a given recorded volume, it is normally not necessary to analyze the
entire volume. Only enough counts to provide an accurate representative
sample are required. And this can be determined through statistics similar
to that described above. In each z-scan, a known volume is swept through
providing a total number and size distribution for that volume. This pro-
duces a number density. A second z-scan produces another set of data like
36
-------
the first. The number of scans required can again be determined after a few
scans are made by applying the above equation.
The xy positions are normally picked so that a significant space sepa-
rates the volumes and no chance for overlap exists. However, when number
densities are very low, it is advantageous to count every single particle in
the hologram. In fact, more than one hologram could be required in the
extreme.
Figure 17(a)-(c) illustrates representative particle size distributions
taken from holograms produced in this study. Figure 17(a) was taken from a
sample of nominal 30 micron diameter glass beads. A comparative count made
with a microscope is shown with this case. Figure 17(b) is the analysis of
an aerosol generator, while Figure 17(c) is the combined analysis of three
holograms made on a windy day in the Mojave Desert.
37
-------
Nominal 30 Micron Diameter
Glass Bead Sample
Microscope Sizing
Holographic Recording
of a Dispersed Sample
26 27 28 29 30 31 32 33 34 35 36 37 38
Diameter — Microns
Figure 17(a). Holographic size analysis - glass beads.
38
-------
Average Number Density 530/orf
10 20 30 40 50 60 70 80 90 100 110 120 130
Diameter ~ Microns
Figure 17(b). Holographic size analysis - aerosol spray.
39
-------
Average Number Density 150/cm~
75-
70-
65-
Cf\ _
oU ••
55-
50-
lu 45-
8 40-
M
| 35-
on _
30 *
O c _
iO
20-
i r*
15"
10
5-
r
^
••
••
W
^^^^^ — BB1111111
•
w
•
•r
•t
•
••
^
1 1 1
1 1 1
10 20 30 40 50 60 70 80 90 100 110 120 130
Diameter ~ Microns
Figure 17(c). Holographic size analysis - dust storm.
40
-------
SECTION 5
THE SDL FIELD-PORTABLE PARTICLE HOLOCAMERA
SDL has developed and has refined during the past three years a field-
portable particle holocamera for in situ diagnostics of particle fields in
the size range from a few microns up to hundreds of microns in diameter.
This system has undergone a wide variety of field tests during which time its
refinement and modification to simplify its operation and to improve its re-
liability have continued. Figure 18(a)-(c) shows the system and a field
installation. The heart of the system is a pulsed ruby laser which delivers
a single or double pulse of light having a time duration of approximately 15
nanoseconds and a total energy output to about 100 millijoules. In the
double-pulse modej two such pulses can be produced with time separations
ranging from a few microseconds up to approximately one millisecond. A
helium-neon laser is built into this unit and passes precisely along the line
of the ruby laser beam for the purposes of alignment and alignment check.
The ruby laser beam is approximately two millimeters in diameter when passing
from the laser. The first element it hits is the beam splitter which splits
off a portion of the light to serve as a reference wave. The remaining por-
tion of light passes through the beam splitter to a second beam splitter
where a small portion is taken from the laser beam for the purposes of power
output measurement. After this, the remaining light beam passes into a beam
expander whereupon it is expanded to a two-inch diameter collimating lens
which collimates the light and passes it out through a probe to a 90° turning
mirror whereupon the laser beam passes through a shuttering system across the
sample volume.
The purpose of the shutter system is primarily to keep the optics clean.
We have tried many methods to keep the optics free of dust and contamination
and we can find no method better than keeping the optics covered except for
a fraction of a second during which the laser pulse exists. The laser is
controlled to fire when the two protective shutters, labeled 9 in Figure 18(a),
are in their full open position. The sample volume is approximately two
inches by six inches deep. After the beam passes through the sample volume,
the scattered and transmitted light are turned 90° by a turning mirror back
into the collection system.
The collection system consists of an image transferring lens system
which transfers the image of the sample volume back to a position near where
the hologram is made.
The reference beam passes from the beam splitter just after the ruby
laser to a system of mirrors after which it is beam expanded and collimated
and then turned to mix with the object wave at the hologram.
41
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N>
(£) Mirror (§)
(2) Beamsplitter (?)
(5) Detector ©
(4) Beam Expander/Spatial Filter©
(D Alignment Laser (HeNe) ©
Ruby Laser
Film Plane Shutter
Film/Plate Holder
Shutter
Port for Auto Collimator
External Laser Alignment
Viewing Port
Sample Volume
Purged/Protective Housing
Figure 18(a). Field-portable particle holocamera.
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Figure 18(b)
wwK^W.-iwwwffiss^Hjgojjmjj
5ide view of the holocamera installed behind a
shield as applied in explosion diagnostics.
Figure 18(c). Front view of the shielded
optics package.
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With this system, either in-line holography or off-axis holography is
possible. Simply removing the reference wave converts this to an in-line
holocamera. We have found in a number of cases that holograms made in this
system with the reference waves can be used in either mode, in-line or off-
axis, and sometimes there are advantages to being able to do both.
The basic configuration of this holocamera is not especially novel.
Nevertheless, a number of design features built into this system were found
important in its utility in a field-type environment. The entire system is
housed in an aluminum sealed box. The box is purged with dry nitrogen to
keep contamination outside. A cover lifts off for access to the inside.
A small hatch allows access to the film plane for exchanging photographic
plates. The entire optical system inside the rectangular portion of the box
is mounted on a secondary reinforced surface which is vibration isolated by
a rubber mounting from the outside parts of the box. This allows a certain
amount of torquing of the outside of the box without misaligning the internal
optics.
The alignment of the ruby laser can be done without removing the cover
of the system. This is done by inserting the optical alignment tool through
the sides of the box and alignment adjustments to the ruby laser are extended
by flexible cables to the external part of the box. The actual condition of
alignment can be observed by peering through peep holes at the rear of the
box to the frosted glass on the back of the hologram holder observing the
reference wave and the object wave as produced with a helium-neon laser. As
a final check, the ruby laser itself can be fired and the actual beams which
will be making the hologram can be observed on the frosted glass at the back
of the plate holder through this porthole labeled 12.
The packaged optical system weighs approximately 100 pounds. The power
supply which controls and powers the ruby laser is in a separate container.
It weighs approximately 200 pounds. This entire system fits in a small
portable trailer which is used to transport the system to various places for
our application.
We found that the holograms produced in this system in most naturally
occurring environments were of sufficient quality to be reducible by the
Quantimet image analyzer. This is not to say, of course, that the problem
of automatic data reduction is completely solved. A considerable amount of
work is left in automating the features which we have described which are
necessary to make the image analyzer perform the data reduction task.
The holograms which are described in the previous section which were
ultimately used to perform the studies with the Quantimet system were made
in this system with the reference beam removed. We were not able within the
scope of this study to do a similar analysis for the off-axis case.
44
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SECTION 6
A PROPOSED AUTOMATIC HOLOGRAPHY DATA REDUCTION SYSTEM
Based upon these results, we are now in a position to design an auto-
matic holography data reduction system. The purpose of this section is to
present such a conceptual design based on the use of the Quantimet 720 image
analyzer currently in operation at the EPA facility. We will assume that the
holograms for which this system is designed are at least of the quality of
those tested during this program.
Figure 19 is a schematic layout of a suggested reconstruction system,
while Figure 20 is a schematic layout of the electronic and control system.
The hologram would be mounted in a computer controlled xyz traverse. It
would be illuminated by a collimated helium-neon laser, and the reconstructed
image would then be reimaged and magnified onto the TV camera. Therefore,
the proper positioning of the hologram in its x, y, and z dimensions would
place the three-dimensional image onto the vidicon surface. At any instant
the image analyzer can be addressed to provide the information describing
the intensity distribution on the surface of the TV screen. At any one
instant this information includes those parameters which were mentioned in
Section 4 for images for both in focus and out of focus and, to a certain
extent, noise on the TV camera. For any one given set of holograms of a
particular quality an operator must intervene at the beginning to adjust the
image analyzer such that its adjustment falls within the optimum operating
range dependent upon the quality of the reconstructed image. Such adjustment
could, in fact, be computer controlled; although we believe at this stage
computer control of everything in this process is not necessary nor desirable.
We feel that a certain amount of operator judgment will always be necessary
in this data reduction just as it is in almost any type of data reduction of
field data.
The operator initially sets the thresholds on the TV screen such that a
given threshold is well above the noise level and, at the same time, consid-
erably below the level of intensity of the smallest particles which are
desired to be analyzed. This defines the particle and the display concerning
the x-y position of the particle. At this point, sizes of those particles
which are in focus or nearly in focus would normally be printed out as data.
A third dimension, however, must be located to insure that only particles in
focus are analyzed and retained as data.
As has been described, this can be accomplished in a number of ways with
existing data outputs from the Quantimet 720. The experiments performed
here suggest that the two-level detection method would be a principal method
in performing this task. The computer which drives the hologram in its xyz
45
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Collimator
HeNe Laser
Plate or
Film Holder
High
Resolution
Camera
To Image
Analyzer
To
Controller
Traverse
Figure 19. Holocamera reconstruction system.
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z-Scan
Medium Speed
Store Data,
Compute A/P
Preliminary Setup
Set xyz limits.
Calibrate for size.
Set Thresholds.
Set z scan speed
controls.
Figure 20. Flow diagram of automated data reduction.
47
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position would be set to position the hologram at a given x-y coordinate and
then start the translation in the z dimension. The annulus defined by the
two levels of detection in this method would then be continuously analyzed in
the computer with reference to its ratio of area to perimeter. When this
ratio becomes minimum, the particle would be designated as an in-focus par-
ticle and the information describing that image at that time would be accepted
as data. This would include its xyz positions, its particle dimensions, and
the parameter area over perimeter.
The z scan of the hologram can be carried out at a fairly high rate
until one reaches the proximity of a particle. When the holographic image is
far removed from focus, no image intensity is detected on the TV screen, and
at any such time the z scan can be run at a fairly high rate. When the first
level of detection is surpassed by an intensity distribution, this is an
indicator that a particle is approaching its focus. The scan speed could at
that point be reduced. When the second level of threshold is surpassed by
the intensity, the focus position is even more closely being reached. At
this point, the computer-driven traverse could be slowed even further. The
ratio of area over perimeter of this annulus would be continuously analyzed
by the computer in search for a minimum. All data in this region of the scan
would be stored in a buffer memory until the focus position has been determined.
When the ratio of area over perimeter is minimized, that data associated with
that position would be accepted as retained data and would be transferred to a
recording disc or tape. The remaining data in the buffer memory could, at
that point, be erased or simply renewed. Since the xyz position of all par-
ticles is recorded, the computer can be programmed not to accept two particles
in the same xyz position such that an overlapping of the volume being scanned
could be rejected.
When the end of the holographic volume was reached, the computer would
drive the traverse in an x dimension until a new region on the hologram was
examined once again in its z position. It appears that a hologram of the size
which we are describing here could be reduced in this fashion in a matter of
time ranging from a few minutes to one hour, depending upon how many particles
were in the recording.
A number of other program operations should be included to correct for
possible discrepancies, some of which have been described in Section 4. For
example, when very large particles exist in a volume, the diffraction rings
around these particles are often counted as particles. We have shown that one
can reject this noise to some extent by setting resolution of the system to a
value at approximately equal to the smallest particle of interest. Another
method which we believe would be required in this study is to have a volume
predetermined which would be designated as the smallest allowed volume to con-
tain more than one particle. If such a volume was designated, then any such
volume containing more than one particle would be rejected by the computer.
We have found in these studies that noise is characterized typically by this
kind of information. Where noise does exist in a reconstructed image, it
appears as a number of particles clumped in a small volume.
The very reason for having performed this study has actually prevented
us from getting certain statistical information which will be necessary to
48
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refine such a system. One needs, after the construction of an automated holo-
graphic data reduction system, to perform analysis on statistical samples of
holograms such that factors can be fine tuned on some statistical basis. Our
conclusions so far have been strictly based on specific small number samples.
The computer must be programmed to set certain limits on holographically re-
constructed images which are based upon statistics derived from performing
studies with the system itself. The initial setup of such an automated sys-
tem would depend upon the quality of the hologram being reduced. However, it
is reasonable to believe that a given set of holograms made under similar
conditions would retain the same setting as that for the initial hologram.
We know, however, this is true only to the extent that the particle field be-
ing diagnosed is similar. The ultimate noise in the reconstructed image is
not only dependent upon the hologram quality, but it is also dependent upon
the numbers and sizes and distributions of particles in the field of interest.
49
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REFERENCES
1. Champagne, E. Non Paraxial Imaging, Magnification, and Aberration-
Properties of Holography. J. Opt. Soc. Amer. 57 (1957).
2. Trolinger, J. D. Laser Instrumentation for Flow Diagnostics.
AGARDograph 186 (1974). 121 pp.
3. Trolinger, J. D. Particle Field Holography. Optical Engineering 5,
p. 383 (1975).
4. Bexon, R., Bishop, G. D., and Gibbs, J. Aerosol Sizing by Holography
Using the Quantimet. A Cambridge Instrument Company Publication.
50
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
REPORT NO.
EPA-600/2-79-005
2.
3. RECIPIENT'S ACCESSION NO.
TITLE AND SUBTITLE
EXAMINATION OF AUTOMATIC DATA REDUCTION METHODS FOR
PARTICLE FIELD HOLOGRAMS -
5. REPORT DATE
January 1979
6. PERFORMING ORGANIZATION CODE
A'JTHCRJS!
J.D. Trolinger
8. PERFORMING ORGANIZATION REPORT NO.
PERFORMING ORGANIZATION NAME AND ADDRESS
Soectron Development Laboratories, Inc.
3303 Harbor Boulevard
Costa Mesa, CA 92626
10. PROGRAM ELEMENT NO.
1AD71? BK-AS (FV 77)
11. CONTRACT/GRANT NO:
68-02-2491
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Sciences Research Laboratory — RTF, NC
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park. N.C. 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final 3/77-1/78
14. SPONSORING AGENCY CODE
EPA/600/09
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Holographic recording techniques provide one of the most powerful particle
field diagnostic tools in existence. A hologram can provide a frozen
three-dimensional image of a particle field through which detailed microscopic
examination of individual particles is possibe. Frequently, a particle field may
contain many thousands of particles, and it becomes impractical for the human
operator to glean all the data of interest from such a hologram. For holography to
reach its full potential in particle diagnostics, a three-dimensional image analyzer
is required.
The purpose of this study was to examine the feasibility of using existing
electro-optic image analyzers to automatically analyze three-dimensional image
fields and to determine what modifications of existing equipment would be required
to construct such a system.
Sample holograms as well as holograms produced in an actual field holo-camera
were used to make the evaluations experimentally, and well-refined analytical
descriptions of holographic images were used to add to the understanding of system
requirements. The study established that existing image analyzers are capable within
useful practical limits of locating particle images in three-dimensions and
measuring size and shape factors of the particle. A plan for integrating such
equipment to produce a fully automated data reduction system is presented.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
COSATI Field/Group
*Air pollution
*Particles
Examination
*Data reduction
*Analyzers
holography
13B
14B
09B
14E
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
21. NO. OF PAGES
59
22. PRICE
EPA Form 2220—1 (Rev. 4-77) PREVIOUS EDITION is OBSOLETE
51
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