EVALUATION OF POLARIZER FOR USE IN OBLIQUE AERIAL PHOTOGRAPHY

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP71B00822R000100240001-1
• Release Decision: RIPPUB
• Original Classification: K
• Document Page Count: 59
• Document Creation Date: December 14, 2016
• Document Release Date: October 1, 2002
• Sequence Number: 1
• Content Type: REPORT

STAT
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STAT
EVALUATION OF POLARIZER
FOR USE IN
OBLIQUE AERIAL PHOTOGRAPHY
by
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STAT
STAT
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The purpose of this project was to test and evaluate the use of a polarizer in obligee
aerial black-and-white photography and to determine whether the results of such testa
indicate that general use of the polarizer would be beneficial. A series of controlled
flight tests were made. On each flight, a specific target was chosen and two s :multi -I--
eous photographs of the target - one polarized and one nonpolarized - were taken.
Selected negatives and enlargements of these photographs are presented herewith fo:
visual comparison. The results confirm that a polarizer can affect the contrast of
aerial photographs and that the contrast is, in most cases, improved. In somz situa -
tions, however, contrast is actually reduced. Examples of both are cited.
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Section
Title
Page
I INTRODUCTION . . . . . . . . . . . . . . . . . . . . . 1
II THEORETICAL DISCUSSION . . . . . . . . . . . . . . . . 3
III FLIGHT TEST PROGRAM . . . . . . . . . . . . . . . . . 9
B. Camera Installation . . . . . . . . . . . . . . . . . . 11
C. Flight Tests . . . . . . . . . . . . . . . . . . . . . . 15
1. Flight 1 . . . . .. . . . . . . . . . . . . . . . . . 15
2. Flight 2 . . . . . . . . . . . . . . . . . . . . . . 19
3. Flight 3 . . . . .. . . . . . . . . . . . . . . . . . 19
4. Flight 4 . . . . . . . . . . . . . . . . . . . . . . 19
IV COMMENTARY ON SELECTED PHOTOGRAPHS . . . . . . . 25
A. Explanation . . . . . . . . . . . . . . . . . . . . . . 25
B. Selected Enlargements and Commentary . . . . . . . . . 28
C. Commentary on Negatives . . . . . . . . . . . . . . . . 43
V CONCLUSIONS AND RECOMMENDATIONS . . . . . . . . . . 53
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Fig. No.
Title
Flight Specification
Flight Specification
Flight Specification
Polarization vs Scatter Angle for Rayleigh Scatter . . . . . _ . . ;
Definition of Sun Bearing . . . . . . . . . . . . . . . .
Engineering Flight Pattern . . . . . . . . . . . . . . . . . i :.
Cultural Flight Pattern . . . . . . . . . . . . . . . . . . .
Equipment Installed In Test Aircraft . . . . . . . . . . . . . .
Mount With Equipment Installed, Set at 30? Oblique Angle . . . . t 7
Mount With Equipment Installed, Set at 70? Oblique Angle . . . . t 8
Enlargement No.
(Flight 1) . . . . . . . . . . . . . . . . .
(Flight 2) . . . . . . . . . . . . . . . . . .sl
(Flight 3) . . . . . . . . . . . . . . . . .
Flight Specification (Flight 4) . . . . . . . . . . . . . . . . . ;}
Diagram of Photographic Geometry . . . . . .
Enlargement No.
Enlargement No.
Enlargement No.
Enlargement No.
Enlargement No.
Enlargement No.
Enlargement No.
1 . .
2. .
3 . . . . . . . . . . . . . . . . . . . . . .
4. . .
5 . . . . . . . . . . . . . . . . . . . . . .
6. . .
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This is the final report of a flight test program to evaluate a manually oriented
polarizer for use in oblique* aerial photography. The program considered only black-
and-white photography using Kodak type 3401 film. Two sun elevations and two atrr es-
pheric haze conditions were included, and the camera orientation was varied. the
flight test program included a variety of situations which can occur in an aerial photo-
graphic mission.
The results of the program are presented as original negatives with enlarged prints
and annotation of selected negatives. The results include examples of each of three ways
in which a polarizer can affect the contrast of an aerial photograph. A discussion of
these three ways is included in Section II of this report.
The results of the program confirm that a polarizer can affect the contrast of aerial
photographs. In most instances there is an improvement in contrast. However. in some
situations the contrast is actually reduced. Examples of both cases are cited in the
prints.
It is absolutely necessary to refer to the set of original negatives supplied iv--J',
this report in order to obtain maximum information from the data presented
herein.
* Oblique angles are measured from the vertical throughout this report.
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THEORETICAL DISCUSSION
The purpose of a polarizer in aerial photography is to improve the contrast of tie
optical image at the film plane of the camera. It is the optical contrast rather than the
photographic contrast (or gamma) which is affected by a polarizer. The photograpr; c
contrast is completely independent of polarization effects, Of course, the -:ontrau,t of
the resulting photograph is improved when the optical contrast is increased.
There are three fundamental ways in which a polarizer can affect the e nn.tras
aerial photographic imagery. First, a polarizer can be used to reduce the contras
degrading effect of atmospheric scatter or haze. Second, a polarizer can he used to
reduce the flare caused by specular reflections from water. Third, a polarizer ca ii
be used to alter the optical contrast between objects in the scene whose imsaL',es ar-
unequally polarized. We shall consider each of the three ways in more depth.
A polarizer is useful in penetrating haze when the haze radiation is pol:,rized
differently (either in magnitude or direction) than the image-forming radiation fron
the scene. Then, the polarizer can be oriented perpendicularly to the direction of
haze polarization, improving the ratio of image-forming radiation to haze r:,diatio a
The result is that the optical contrast, especially in the shadows, is increased.
Two types of scatter combine to form the haze encountered in aerial photography.
The first type is the result of scatter by molecules and other particles whose dimensions
are small compared to the wavelength of the scattered photons. It is this ty?;)e of s^alter
which causes the blue sky and the blue haze which is observed on a clear da\. Mi- ie alar
scatter is highly polarized in certain directions and can be greatly reduced tv a limit rly
oriented polarizer.
The theory of molecular scatter was developed by Lord Rayleigh` and is often rknerred
to as Rayleigh scatter. The most useful result from this theory is that the polari2ati4un P
of light after single scattering events is related to the scatter angle H by the equatio~r
1. Max Born and Emil Wolf, Principles of Optics, Third (Revised) Edition,
Pergamon Press, 1965, pp. 652-656.
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t; sin` 0
1 +- sin' 0
(1)
The scatter angle is defined as the angle between the direction of travel of a photon,
before and after the scattering event. The direction of polarization is with the electric
vector perpendicular to the plane of observation, i. e. , the plane in which the incoming
and scattered photon travel. A plot of Equation (1), along with an illustration of the
scatter angle, is presented in Figure 1. Notice that the polarization is unity at a scatter
angle of 90 degrees. A polarizer is very effective against Rayleigh scatter when the
scattered radiation is this highly polarized.
The second type of particle which is important in atmospheric scatter is the aerosol
particle with dimensions in the neighborhood of, and larger than, the wavelength of the
scattered light. The most common particles of this type are condensed water vapor;
but other such particles include smoke, dust, volcanic ash, and smog.
The theory of aerosol scatter was developed by Mie; therefore scatter by this type
of particle is called Mie scatter. The polarization of M,ie scatter is a complex function
of the optical constants of the material, the particle size, and the scatter angle. Because
these particles are statistically distributed in size and space, as well as time, a quanti-
tative prediction of the gross scattering properties is not worthwhile, at least not for
our purposes. There are, however, two useful qualitative results from the Mie theory
which are relevant to this program. First, the scattered radiation is partially polarized,
but never as highly as molecular scatter. Second, the scatter is polarized in the same
direction as Rayleigh scatter. Thus, a polarizer is oriented in the same direction to
minimize the haze resulting from either Rayleigh or Mire scatter.
In the atmosphere, the situation is more complicated than simple individual scatter-
ing events by two kinds of particles. The concentration of aerosol particles can vary
widely. Also, the polarization, as predicted by Rayleigh and Mie, is lowered by the
occurrence of multiple scattering. As a result, even on the clearest day, the sky polar-
ization perpendicular to the sun is not unity. The probability of multiple scatter is a
function of the aerosol concentration of the atmosphere.
'Thus, the polarization of atmospheric haze is a function of the scatter angle (for
first scatter events) and the concentration and distribution of aerosol particles. These
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 t t L t i l 1. 1 I l r t
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80 9Q 10O 110 120 136 140 150 160 170 180
Scatter Angie t B?
x Ul k ih ~jUL1 Deacter -ngie r:or k ayieigh Scatter
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two variables are included in the experimental evaluation of the haze penetrating poten-
tial of a polarizer.
From the above discussion, it can be inferred that the polarizer will be most effec-
tive when looking perpendicularly to the sun on a clear day. Under these conditions, a
polarizer should increase both the contrast and the slant range of photographic visibility.
On the other hand, on a hazy day, with its associated increase in multiple and aerosol
scatter, the polarization of atmospheric scatter is going to be low; and it is doubtful
that a significant improvement can be achieved using a polarizer.
.In addition to the polarization of haze, some of the: scene light is also partially
polarized. The principal source of polarized light, is specular reflection by dielectric
surfaces.
The theory of dielectric surface reflections predicts the polarization as a function
of angle of reflection when the dielectric constants of the two materials (e. g. air and
water) are known. Again, the polarization is with the E vector perpendicular to the
plane of observation. The polarization of reflected light is unity when the angle of
incidence is Brewster's angle. The polarization decreases monotonically to zero as
the angle of reflectance goes to either zero or 90?. The Brewster's angle eR for an
air interface is given by
eg = arc cot ( 1/n)
where n is the index of refraction for the reflecting medium. The theoretical prediction
of polarization as a function of the angle of reflectance is cumbersome and will not be
presented here. It may be found in Born and Wolf ;# but not in an easily used form.
Polarization from dielectric reflections occurs in aerial photography in two impor-
tant ways. The most important is in the surface reflections from water. Specular reflec-
tions of the sun from water will cause serious blooming in the neighborhood of the image
of the specular reflection. It also contributes to general flare in the lens since it is a
strong source of nonimage-forming light. This specular reflection can be reduced and
the contrast of the neighboring imagery greatly enhanced by a polarizer.
2. Ibid. , pp. 43-45
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The second way in which polarized specular reflections can affect photographic
contrast is when a texture difference exists between an object and its background. For
example, the painted lines on a parking lot could have a similar diffuse photos,,raphic
reflectance to the pavement. However, in the direction in which specular reflections
from the sun occur, the lines may be much brighter because of the specular reflections.
In this situation, a polarizer oriented to penetrate the atmosphere would actually reduce
the contrast of the lines.
All three sources of polarized light (i. e. , atmospheric scatter, specular reflections
from water, and specular reflections from cultural targets) have been observed in *hu
flight test results.
The typical aerial photographic situation includes both atmospheric scattc r and
specular reflections. Except when looking into (or away from) the sun, the pclariza>i.bn
of light from specular reflections from a horizontal surface will not be in the same
direction as that from atmospheric scatter. Consider the case of the sun near the
horizon and the target also near the horizon, 90? from the sun. In this case, the ph me
of observation for atmospheric scatter is horizontal while the plane of observ