INVITED PAPER THE ULTRA HIGH PRECISION STEREO COMPARATOR

• Document Type: CREST
• Collection: NGA Records (Formerly NIMA)
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78B05171A000100010033-2
• Release Decision: RIPPUB
• Original Classification: K
• Document Page Count: 25
• Document Creation Date: December 28, 2016
• Document Release Date: September 8, 2003
• Sequence Number: 33
• Content Type: PAPER

A
Approver Release 2003/12/04: CIA-RDP78B 71A000100010033-2
THE ULTRA HIGH PRECISION STEREOCOMPARATOR
THEORY, METHODS AND INSTRUMENTS OF RESTITUTION COMMISSION II
Xlth INTERNATIONAL CONGRESS OF PHOTOGRAMMETRY
Lausanne, Switzerland
8-20 July 1968
0h as
-~ L)
`crc\ e % S a- 's
svuw~i
?`tom C i1"~
c k- Vt~ ~ S 5 l b i^~-- ip.) c. S
Declass Review by
Approved For Release 2003/12/04: CIA-RDFfffab1d1000100010033-2
 Approved For ReVhe 2003/12/04: CIA-RDP78B05171A09-00010033-2
ABSTRACT
This paper describes a new photogrammetric instrument used
for the precise measurement and viewing of stereo imagery.
The state of the art has been materially advanced by the pro-
vision of automatic correlation of the imagery into a stereo pair of
views.
The instrument substantially assists the operator in produc-
ing an accurate and :high volume of measurements particularly under
conditions where the operator is scanning or changing the field of view.
The automatic operation is performed with a computer program-
med for the geometry of the photographic system in conjunction with an
electro-optical scanning correlation device.
* Invited Paper for Theory, Methods and Instruments of Restitution Commission
II, at the XI International Congress of Photogranimetry, Lausanne, Switzerland,
8-20 July 1968.
STAT
Approved For Release 2003/12/04: CIA-RDP78BO5171A000100010033-2
 STAT Approved For Release 2003/12/04: CIA-RDP78BO5171A000100010033-2
Next 1 Page(s) In Document Exempt
Approved For Release 2003/12/04: CIA-RDP78BO5171A000100010033-2
 Approved For Release 2003/12/04: CIA-RDP78B0517 A000100010033-2
low 1W
PREFACE
This paper describes an instrument developed by the
The instrument is known as the Ultra High Precision Stereo-
comparator, and is a microscope type measuring device controlled by a
computer and photoelectric scanning.
The measuring is performed by determining the x-y positions
of each of two stages through the use of laser interferometers.
The measurements relate to a stereo pair of photographs, as
an example, where a pair of illuminated reticle spots create a floating
reference spot for x-y-z mensuration.
The instrument further advances the science of photogrammetry
and it is interesting to review the historical background of the Stereo-
comparator.
HISTORICAL DEVELOPMENT
The Ultra High Precision Stereocomparator was made possible
only through the long term developments of analytical photogrammetry
and by the evolution of the high-speed electronic computer.
In. trying to trace backwards through history the developments
that contributed significantly to our Stereocomparator, one is impressed
by the fact that there is really no starting place from which it can be
said that an idea germinated and grew into the Stereocomparator. We
are thus faced with an arbitrary decision as we go backwards in time to
stop at some point from which we can, by stretching our imaginations,
say that without this particular concept, the Stereocomparator as we
know it, would never have been produced.
One convenient stopping point in our historical research is
when, in our early literature, we find inquiring minds probing the whys
and wherefores of perspective in the arts of drawing and painting. We
are going to ascribe to Leonardo da Vinci the first concepts which led
to our Stereocomparator.
Prior to the year 1500, Leonardo da Vinci and other artists
were much concerned with questions and problems of perspective as
Approved For Release 2003/12/04: CIA-RDP78B05171A000100010033-2
 Approved For Rase 2003/12/04: CIA-RDP78B051710100010033-2
applied to painting and drawing on a flat surface. This resulted in the
publishing and dissemination of various conclusions and thoughts con-
cerning graphical perspective.
These ideas of the artists were taken up by professional math-
ematicians of the time, and the concepts of projective geometry resulted.
Projective geometry is, of course, the basis for photogrammetry. For
the next 400 years, much time and effort was expended on the analytical
solution of the geometrical problems that had been recognized.
Additionally, the development of the Stereocomparator is an
offshoot of a parlor pastime of some 136 years ago. In 1832, Sir Charles
Wheatstone invented a. device that he called a stereoscope. This instru-
ment showed a pair of views, such as portraits, seen from two different
aspects which simulated the left and right eye view. The device had a
definite scientific interest, and Sir David Brewster presented papers
about it in 1843 and 1844 to the Royal Society of Edinburgh. The optical
concepts of this simple device established the principles of stereo view-
ing, essential to the Stereocomparator.
During 1900 and for a several year period, Sebastian Finster-
waller began to apply analytical mathematical principles to mapping
from aerial photographs.
Unfortunately for the workers in this field, the electronic com-
puter had not been developed., and the mathematical concepts that were
employed were based on the performance abilities of the laborious hand-
computing techniques then available.
Paralleling the development of the analytical techniques, the
practical course of photogrammetry continued through the use of specially
designed instruments which performed many of the laborious hand calcu-
lations by opto-mechanical means.
The origin of the modern mathematical solution of the problems
of analytical photogrammetry occur in a 15-year period, beginning in
about 1934, with the publications of Earl Church. Professor Church
was still limited by desk-calculator techniques, but nevertheless he
was able to put the complex mathematical concepts into manageable
units for solution.
By 1960, Hellmut Schmid had developed the analytical proce-
dures from the vector notation to the matrix notation with direct appli-
cation to the high speed electronic computers. Since this date, many
workers have contributed significantly to the classification, expansion
of detail, and the improvement of computer programs which will provide
analytical computations of the highest accuracy.
The concept of our Stereocomparator utilizes the direct appli-
cation of these analytical methods to the computer control of the optical
viewing system without the necessity for recourse to the older instru-
mented opto-mechanical method of solution for the photogrammetric
equations.
We have established from the mathematical concepts of the
Stereocomparator that we can employ an automatically controlled variable
Approved For Release 2003/12/04: CIA-RDP78BO5171A000100010033-2
 Approved For Release 2003/12/04: CIA-RDP78BQ71A000100010033-2
i-OW
optical system to maintain stereo viewing of non-ideal stereo pair
imagery.
The Stereocomparator has thus uniquely solved the critical
problem of providing mensuration accuracy of the highest order to
accommodate the developing needs of photogrammetric science.
THE STEREOCOMPARATOR SYSTEM
The Stereocomparator device is actually a system made up of
a series of complex subsystems. The Stereocomparator instrument
itself, with its moveable stages, adjustable optics and binocular view-
ing eyepieces, is the heart of the installation. The necessary servos
and operator controls and the measuring systems are contained in seven
electronic racks, an operator control console, and pneumatic and air
control system rack.
The viewing optical system consists of the following major
subassemblies:
The Illumination System.
The Objective System.
The Reticle Spot Projection System.
The Zoom Magnification System.
The Ana morph System.
The Image Rotating System.
The Eyepiece Switching System.
In addition, there is a phototube scanning or image analysis
system and a photoelectric light intensity control system. The optical
systems, related to the image dissector tube for the image analysis
system and the photo-multiplier tube for the light intensity control
system, are relatively minor.
The Illumination System
The illumination source is a 450 watt xenon compact arc lamp.
The condenser and projection system associated with the light source
must maintain the proper optical relationship for the appropriate settings
of the variable objective and zoom system.
In order to accommodate the two switching positions of the
objective lenses, there are two sets of switching lenses and one set of
switching faxed diaphragms. These devices switch automatically by
means of electronic servos, when the objective lenses are switched.
Mechanical stops are provided so that the terminal locations for the
switchable elements are accurately positioned. This is because of the
necessity of obtaining close optical alignment through the entire optical
system.
In order to accommodate to different positions of the zoom lens
system, a variable diaphragm is provided in the illumination system and
the final condensing lens assembly is made adjustable. These systems
are controlled by electronic servos from the zoom lens system in the
Approved For Release 2003/12/04: CIA-RDP78B05171A000100010033-2
 Approved For Rele 2003/12/04: CIA-RDP78B05171A0.1,00010033-2
main optical path. By this means, the light source arc is brought to a
focus as required beneath the film plane.
Objective Lens System
Two objective lenses are provided: one with a focal length of
40mm and the other with a focal length of 80mm. These lenses are re-
quired to accommodate the continuously variable 10:1 magnification
range system to provide an overall magnification range of 20:1. Thus
the two objective lenses, as they are switched in or out of the system,
represent a change in the magnification of the system of 2:1.
The eyepieces are of a fixed 10:1 magnification, and taking
into account the zoom magnification range, the overall magnification
for the instrument is lOX to 200X.
The objective lenses are controlled by an electronic system
which drives a motor, which in turn drives a Geneva mechanism. The
purpose of the Geneva mechanism is to position the objective lenses
as accurately as possible on the optical axis. To maintain this position
as closely as possible during focussing of the objective lenses, a unique
system is provided which preloads the lens elements in the lens barrel,
sideways to the barrel, in such a manner that there is no sideplay as the
lens elements are aligned vertically during the focussing operation.
In addition, the Geneva mechanism is latched into its two
terminal positions where the 40mm and the 80mm lenses are in their use
positions. This latch arrangement consists of a lever arm with a tapered
wedge-shaped head which falls into a tapered socket, locking the entire
Geneva mechanism against rotation. The action of the Geneva drive
includes unlatching the locking mechanism prior to rotation of the Geneva
cams, and then relocking by inserting the tapered latch device after the
Geneva drive has reached its new position.
The Reticle Spot Projection System
Immediately adjacent to the objective lenses is a beamsplitter
which provides for the injection of the reticle spot into the main light
path. The beamsplitter is located as close as possible to the objective
lenses, since position measurements of the stages are made with respect
to the reticle spot. Thus the further the reticle spot injection beam-
splitter is from the film plane, the greater the potential deviation of
measurement error will be. Further, the reticle spot must be injected
prior to the zoom lens, anamorph lens, image rotator, etc. , in the main
optical path, since any eccentricity of the optical axis, as these sub-
sidiary elements are adjusted, would result in a movement of the reticle
spot with respect to the film plane and thus create a measurement error.
The reticle optical system essentially duplicates the entire
adjustable optical features of the main optical path. In addition, there
are added optical features required by the necessity for adjustment of
the reticle size with respect to the field of view.
The reticle spot itself is a bright illuminated disc in the center
of the field of view of the eyepiece. The illumination system consists
Approved For Release 2003/12/04: CIA-RDP78B05171A000100010033-2
 Approved For~Release 2003/12/04: CIA-RDP78BO51 000100010033-2
of a high pressure xenon enclosed arc lamp supplying light to an aper-
ture in a diaphragm, by means of an array of optical condensers.
The size of the aperture is so arranged that at the end of the
reticle projection system the size of the reticle spot is approximately
diffraction limited. The reticle spot is thus of minimum optically prac-
tical size to provide a clearly defined disc.
A light intensity control is provided for the reticle spot which
permits the operator to follow the illumination intensity level of the
main optical path. The light intensity is controlled by means of a pair
of rotating light filter discs. The filter discs consist of a pair of
neutral density optical filters capable of continuously changing the
filter density in the light path, over the range of 2:1.
In addition, the light path must pass through a colored filter
holding arrangement which switches a filter in or out of the system by
means of an electric motor. This provides either a "white" reticle
spot or a "red" reticle spot, at the operator's selection. It is thus
possible to use a highly contrasting spot color to assist in identifying
the reticle spot in the field of view.
Next in the reticle system is a zoom lens system, This remote
operated zoom element adjusts, by operator selection, the diameter of
the reticle spot over a range of 4:1, from the size limited by diffraction
to four times larger.
The reticle spot image next passes through a second zoom lens
arrangement capable of adjusting the diameter of the reticle spot over
a 10:1 range. This zoom lens is operated by a servo drive which is
slaved to the servo system of the movable zoom subassembly in the
main optical path. Thus, as the main zoom magnification is increased,
the apparent reticle size remains constant through the compensating
action of the reticle zoom system subassembly.
Next in the reticle projection system is an anam orph lens assem-
bly arranged for changing the diameter of the reticle spot on a given axis
over a range of 2:1. This system is servo operated and slaved to the
anamorph system in the main optical path. Thus as the anamorph system
in the main optical path changes by a specific degree of expansion, the
reticle system anamorph compensates by reducing the diameter of the
reticle spot along the same axis of stretch.
These several systems are linked together by the various servo
arrangements to better than a 1% error.
The next component in the reticle projection system is an image
rotating Pechan prism. The axis of this image rotator is servo controlled
and slaved from the axis of the anamorph system in the main optical path.
This arrangement compensates for main optical path anamorph distortions
of the reticle spot and maintains a truly round reticle spot in the field of
view as seen at the eyepiece.
It will be apparent from the foregoing description of the reticle
spot optical system that the possibility of off-axis deviation of the
Approved For Release 2003/12/04: CIA-RDP78BO5171A000100010033-2
 Approved For lease 2003/12/04: CIA-RDP78BO517 00100010033-2
reticle spot is extremely high, considering the necessity for shape and
size compensation through various optical parameter variations. In
other words, unless further corrective measures were included, it could
be expected that the reticle spot would move substantially off axis as
the various reticle optical system elements were adjusted. This, of
course, would result in measurement error since the reticle spot position
would be changed with respect to the field of view.
The problem has been solved by placing a reducing afocal sys-
tem in the reticle spot exit path. This system reduces the reticle spot
to 1/50 of its original size before injecting it into the main optical path.
This reduction of 1/50 has the effect of reducing all off-axis deviations
to 1/50 of their original amount. The resulting final deviation of the
reticle spot with respect to the field of view is therefore less than 1/4
of a micrometer as the reticle spot optical parameters are varied.
The Zoom Magnification System
The lOX zoom system for the main optical path is located in a
vertical axis immediately above the reticle spot injection beamsplitter.
This zoom subassembly consists of five clusters of lens
elements, three of them moving. Two of the moving lens elements are
attached to one carriage and the other movable lens elements are attached
to a second carriage. The two carriages are driven from the same cam
shaft, and their position. is carefully computed to place the zoom elements
within the necessary accuracy for proper zoom operation. The movable
lens elements slide vertically and therefore the system is mechanically
loaded in one direction on the cam, and thus backlash is not a problem.
The system is guided vertically with bias loaded rollers on an accurately
straight shaft, and thus there is essentially no radial backlash.
The positioning cam shaft contains two variable spiral grooves
machined into a long shaft. Cam followers ride in the grooves and cause
the carriage elements to travel along the cam shaft spirals. The rotat-
ing cam shaft is driven with a servo system and has potentiometer read-
outs of position. These potentiometers provide feedback position signals
to the computer which controls the entire optical system, and to the
slave linkage servo arrangement which drives the reticle spot 10:1 zoom
assembly and the main illumination zoom assembly.
Microswitches are arranged on the potentiometer driving gears
so that the system cannot be driven beyond its maximum travel position.
A brake is provided to hold the cam in its fixed position after servo
movement. The servo driving motor contains an integral tachometer and
an integral speed reducer.
The upper end of the zoom system carries the corner prism
which turns the optical path through 900 from the vertical axis to a
horizontal axis
The Anamorph System
The anamorph system in the main optical path is for the pur-
pose of correcting obliquity in the viewing aspect of the object. This
Approved For Release 2003/12/04: CIA-RDP78BO5171A000100010033-2
 Approved For elease 2003/12/04: CIA-RDP78BO5171A000100010033-2
'Ve
optical system consists of four sets of prism assemblies which are
placed transverse to the optical axis. The prisms are mounted on four
separate rotating shafts driven from a common servo drive. The shafts
are driven by precision gears which provide accurate placement of the
prism assemblies with respect to each other.
A potentiometer is provided to supply a signal to the computer
to show the position of the anamorph prisms. The servo motor is
equipped with an integral gear reducing head and a tachometer. A brake
is provided to hold the prism system stationary between operations of
the servo system.
In addition to the motion of the anamorph prisms with respect
to each other, the entire assembly of anamorph elements with their drives
and the position readout potentiometer are rotated about their optical axis.
This is done through a second set of precision gears and bearings and a
servo motor system with integral gear reducer and tachometer. Readout
of this position angle of the anamorph system is provided by means of
precision potentiometers which supply signals to the computer and to the
slave servo in the reticle projector system. A brake is arranged to hold
the anamorph position fixed between servo commands with respect to the
optical axis.
Since the angular position of the anamorph about the optical
axis is infinite in either direction, the anamorph system is provided with
electrical slip rings to supply electric power for the servo drives and the
signal feedback system on the anamorph prism positioning system. These
slip rings are of the low-noise, low-speed variety and are not especially
complicated
The Image Rotating System
After passing through the anamorph system, the main optical
path passes through a Pechan prism image rotating system. The prism
is carried on precision bearings and is driven through precision gears
by a servo motor with integral gear reducer and tachometer. A brake
holds the system in position when the servo system is not in operation.
Precision potentiometers are provided to supply signals to the computer
which show the position of the image rotator.
The purpose of the image rotator is to orient the left eye and
right eye fields of view with respect to each other so that proper stereo
fusion is possible. Stereo fusion is attained through the elimination of
Y-direction parallax in the two fields of view, and this situation is
realized through the proper orientation and position of the pair of images
in the fields of view.
The Eyepiece Switching System
The eyepiece assembly is the point at which the right and left
eye channels come together for presentation to the operator for stereo
viewing. The previous descriptions of the optical system have described
specifically only the one eye channel; however, there are two complete
systems, one for each eye, identical to the one system that has been
described.
Approved For Release 2003/12/04: CIA-RDP78BO5171A000100010033-2
 Approved For Rase 2003/12/04: CIA-RDP78B05171100010033-2
The light level at the eyepieces is under operator control with-
out affecting the main optical system light level or the level of illumina-
tion at the scanning phototubes. For this purpose, two light level con-
trol filter discs with a transmission range of 5:1 are provided.
One of the requirements of the Stereocomparator instrument is
that it be capable of viewing through a film of density 3.0. This means
that the light reaching the operator's eyes has to be controlled to limit
accidental excessive brightness., Thus, if a hole or zero density patch
in a film of 3.0 density were to be suddenly moved into the field of view,
the light level would be increased by a factor of 1000. While there would
be no possibility of permanent damage to the operator's eyes, neverthe-
less it was considered that this increase in the illumination level would
be a source of discomfort to the operator and would be highly objectionable
The light level control phototube provides a means of measuring
the average light level and includes the capability of sensing sudden
high light level fluctuations. However, the light level control filter disc
device contained in the main illumination system would not respond rap-
idly enough to prevent discomfort for the operator. The problem was
solved by providing a pair of opaque shutters in the main optical path at
the eyepieces, controlled by the photomultiplier light level control photo-
tube .
These shutters have a 1 millisecond response time in shutting
off excessive light from the operator's eyes. In addition, the opaque
shutter system is so arranged that when the illumination level returns to
a normal value the shutters will automatically move out of the optical
path and the operator may again see the fields of view.
The eyepiece assembly is provided with a means of switching
the eyepieces relative to the field of view; that is, the two eyepieces
may be switched so that both look at either the right eye channel or the
left eye channel. Additionally the eyepieces may be switched so that
each eye sees its respective channel or so that the right eye sees the
left eye channel and the left eye sees the right eye channel, This system
flexibility is provided by a series of prisms mounted on a rotating disc
assembly which cause the various optical switching arrangements de-
scribed above to be performed.
Provision is included in the eyepiece assembly for adjustment
of interpupillary distance, for vertical angular adjustment and for hori-
zontal angular adjustment between the two eyepieces. Thus, the so-
called "squint angle" can be accommodated by the vertical adjustment,
while the horizontal adjustment is provided primarily to eliminate opera-
tor eye fatigue in conjunction with the adjustment of interpupillary
distance.
Scanning and Li hg t Level Control
The Stereocomparator optical system contains two automatic
image plane scanning features. These consist of a photomultiplier tube
which maintains the illumination level at a constant average value by
adjustment of the main illumination system, and an image dissector
phototube which scans the field of view and produces a specially shaped
Approved For Release 2003/12/04: CIA-RDP78B05171A000100010033-2
 Approved For Release 2003/12/04: CIA-RDP78BQ571A000100010033-2
variable raster whose output controls the computer and automatically
maintains stereo fusion for the operator through servo adjustment of the
optical system.
A beamsplitting prism assembly is provided immediately after
the image rotator in the main optical path to provide images for these two
phototube systems. A second beamsplitting prism splits the secondary
ray into two separate rays. The beam which supplies illumination to the
light level control photomultiplier tube is diffused to produce an average
level of illumination. The phototube measures the light level and uses
the information to operate a servo link which controls the adjustable
filter discs in the main illumination system.
The second phototube beam is focussed on the photocathode of
a high sensitivity image dissector phototube which then scans the field
of view in a precisely controlled pattern. The output from the left eye
field of view image dissector tube is compared with the output from the
right eye field of view image dissector tube. These comparative outputs
are analyzed in a special electronic computation system called an image
analyzer which then produces a series of control signals to the computer.
The computer thereupon provides necessary and appropriate commands to
the various elements of the main optical system which in turn make the
various servo operated optical adjustments that create stereo fusion for
the operator.
Thus within the corrective capability of the optical system,
the fields ov view of the right and left eye channels are automatically
corrected for scale, skew and orientation differences as a means of
maintaining proper stereo fusion for the operator.
The Film Drive and Platen
The Stereocomparator is designed to use roll film up to ten
inches in width carried on 500 foot film reels. The glass film platen is
10" wide by 20" long and is mounted between the film reels. Immediately
below the platen and adjacent to the operator, "cold" light tables are
mounted for overall direct scanning. This allows the operator to select
and position the desired film frame area in a convenient manner prior to
detailed viewing or measuring. The film drives are servo controlled and
provide a uniform film tension with a speed of travel of up to 350 feet
per minute in either direction.
A vacuum film clamping arrangement is provided which self-seals
both ends of the platen and along both edges of the film. The edge seals
also serve as film guides during motion of the film. They are mounted on
a positioning device to adapt the seals to varying widths of film. The
vacuum system seals the film tightly against the platen in about 5 seconds.
The Stages and Guides
The film platens are installed on a granite stage which is guided
for x and y coordinate motions by a granite "Tee" assembly. The guiding
surfaces are granite against anodized aluminum air bearing pads.
A granite base block approximately 5 feet wide by 7 feet long
Approved For Release 2003/12/04: CIA-RDP78BO5171A000100010033-2
 Approved For Reldhe 2003/12/04: CIA-RDP78B05171A0 00010033-2
by 1-1/2 feet thick and flat to within 50 millionths of an inch supports
the movable stage assembly on air bearings.
The air bearings are of the load compensated variety. This is
required to minimize the effects of film transfer between reels at the
platen. If the platen were to tilt under the change of weight distribution
then frequent refocussing of the objectives could become an operating
problem.
Granite is used as a stage and base material for several reasons.
The primary reason is based on the dimensional and shape stability of
granite as a structural material. The granite is completely stabilized and
does not warp or otherwise deform with time. In addition, it is stable to
changes in temperature as the large mass used for the Stereocomparator
is not affected by short time temperature oscillations.
The fact that the granite arrangement results in a heavy assembly
is advantageous for the vibration absorption equipment. That is, external
vibrations are more easily damped out.
The Laser Interferometers
The high precision measuring accuracy for the stage position is
realized through the use of laser light and a Twyman Green interferometer.
There are two laser units used, one for each of the measuring stages.
The output of each of the Helium-Neon lasers is split into two
beams, one provided for each of the x and y measuring axes. The two
beams are directed to a pair of interferometers - one for the x and one for
the y axis. The interferometers include long mirror reflectors , whereby
the interferometer readout photocells are installed directly opposite the
objective lenses and perpendicular to the x and y axes respectively.
By locating the interferometers centrally with respect to the objective lenses,
the effects of misalignment of the stages during motion is minimized.
The output of the interferometer light is in the form of light and
dark bands. The direction of motion of the light bands across the face of
the photoelectric field effect transistor pickup cells which comprise the
receptor system of the interferometer, determines the direction of stage
motion. Thus by proper arrangement of phase analysis equipment, the
counting system can be directed to count up or count down correctly.
It is apparent that counting interference fringes from He-Ne laser
light results in a very unhandy unit of measurement. Therefore a special
purpose computation counter was provided to transform the interferometer
counts into 0.1 parts of a micrometer. These counting devices read out
digitally directly in micrometers and tenths of a micrometer for optimum
operator and computer convenience. The least count of the system is thus
0.1 micrometers and the counting registers record counts over the full 10
by 20 inches of travel of the stage, as the stages are moved about by the
operator or through the computer control.
The interferometers thus record exactly the position of the stages
with respect to their starting position and with respect to each other.
This numerical information is transmitted to the computer or to the card
Approved For Release 2003/12/04: CIA-RDP78BO5171A000100010033-2
 Approved F rpelease 2003/12/04: CIA-RDP78B054000100010033-2
punch or to other equipment, as desired, by means of appropriate logic.
The Structural Supports
There are two complete three-piece granite stage assemblies in
the Stereocomparator system, one for each of the two eye channels.
The two granite base slabs are supported on a strong and rigid
U-shaped structural steel frame. Each granite base slab is supported by
a 3-point suspension from the steel frame. In order to maintain one of the
granite bases level with respect to the other granite base, the structural
steel frame is supported by four pneumatic combination level supports
and vibration absorbers. These units absorb vibration down to almost
one Hertz and will maintain the overall system level to within a few mil-
lionths of an inch. The two granite base blocks not only support the
stages but also provide a rigid mounting for the optical system.
The Optical Bridge
The optical bridge which carries the entire optical assembly is
in three sections. The two outboard sections are supported by the base
granite while the center section carrying the eyepieces is supported from
the structural steel frame which supports the granite slabs.
This three-piece optical bridge was designed so that the optical
system is not dependent on the absolute position of the granite base
blocks with respect to each other. Some movement of these granite blocks
is allowable and has no effect on the measuring accuracy of the system
since the granite blocks also carry the Twyman Green interferometer and
the laser system.
The center portion of the optical bridge supports the eyepieces
and the various subassemblies of the optical system which occur in the
collimated light region of the main optical path. Thus relative movement
between each of the eye channels in this portion of the optical system
does not result in changes to the measuring accuracy since the relation
between the reticle spot and the field of view is unchanged.
The Ancillary Equipment
The electronic equipment for driving the servo motors, for read-
ing out stage positions for the computer, and for performing the numerous
subsidiary tasks of the Stereocomparator are housed in three double elec-
tronic racks approximately seven feet high. An additional double rack of
similar size is provided to house the controls and the special compressed
air distribution equipment for supplying air to the air bearings on the
granite stages. Additionally this cabinet contains the controls for the
platen vacuum film holddown system, electric power control and the elec-
trical distribution systems.
A teletypewriter is included in the Stereocomparator system for
the purpose of transmitting computational data into the computer and for
communicating special instructions to the computer for requirements such
as stage x-y prepositioning coordinates .
Approved For Release 2003/12/04: CIA-RDP78B05171A000100010033-2
 Approved For Rase 2003/12/04: CIA-RDP78B05171A%0100010033-2
A card punch and reader is also provided as part of the system
in order to provide a card record of data at times when other on-line
data acquisition equipment is not available.
METHOD OF OPERATION OF THE STEREOCOMPARATOR
In general, photograrnmetry has developed over the years with
cartography as its prime objective. The Stereocomparator is firmly not
a map-making tool. It is a mensuration device for determining, through
a computer, dimensional parameters and other special information for
selected viewing points in transparent imagery. The use of the instru-
ment is not limited to simple stereo photography of terrain since the
inherent flexibility of the equipment and its variety of operating modes
result in many different applications .
In the case of stereo type terrain photographs, the Ultra High
Precision Stereocomparator automatically provides a stereo view from a
pair of geometrically similar, or even partially dissimilar, stereo photo-
graphs. These photographs must be in the form of film transparencies,
either negative or positive. The optical system for the instrument con-
tains two identical optical paths for viewing the two transparencies.
Each eye is thus provided with a means of optically transforming the two
different photographic views to make them compatible for stereo viewing.
The variable optical elements of the two optical systems are
operated through servo motors controlled through a computer and an
electronic scanning system. The views are thus transformed by the
optical system and presented to the left and right eyes of the operator
in the form of a stereo pair. The instrument is limited by the optical
parameters, to the degree of optical correction it can perform. That is,
it can only operate successfully with imagery that reasonably approaches
the criteria for a stereo pair of photographs.
The Stereocomparator can be considered fully flexible for all
different types of photography, provided an appropriate mathematical
program can be written for the computer. At present, programs for pan,
strip and frame photography are available.
In addition to limitations from the parameters of the optical com-
ponents, there is a further limitation in that not all of the true optical
transformations have been provided in the instrument. For example, the
instrument does not include a means of varying the scale as a function
of position in the field of view. That is, the scale of the field of view
may be changed as a whole, but not selectively with position in the field.
Because of this limitation, stereo fusion may not be attainable over the
entire field of view.
We are told that a 11 degree included angle at the eye contains
all the information necessary for the eyes to maintain a state of stereo
fusion. The actual field of view of the instrument is 40 degrees at the
eye. This merely means that for photographs substantially deviating from
a stereo pair then true stereo viewing may only be achieved over the central
1/3 of the field of view.
Approved For Release 2003/12/04: CIA-RDP78B05171A000100010033-2
 Approved For, Release 2003/12/04: CIA-RDP78B05171A000100010033-2 NOW
This difficulty is not particularly significant since the
Stereocomparator is primarily a mensuration instrument equipped with
a reticle spot at the center of its field of view . Thus the requirements
for stereo fusion and mensuration are compatible at the center of the
field of view and the theoretical failure of the instrument to provide
wide angle optical transformations to provide true stereo fusion do not
significantly affect the use, accuracy or operation of the instrument.
The general historical background of the Stereocomparator has
been pointed out and the design of the other various pieces of equipment
comprising the system has been described. In addition, the arrangement
of the optical system was considered in some detail, and thus having
treated the subsystems, the Stereocomparator system as a whole will be
examined.
The Stereocomparator has three primary modes of operation:
(a) manual, (b) computer operated, and (c) computer operated with image
analysis (i.e . , with correlation). These modes differ in many respects:
(a) The manual mode provides the operator with controls by which
he can adjust the optics and move the stages to suit his needs. He can
thus traverse the field of view and produce stereo fusion in a stereo pair
of photographs and make measurements as required of the x, y and z axis.
There is a sub-mode associated with the manual mode which
utilizes the computer program to assist the operator in the orientation of
the various controls of the machine. This in no way obviates the oper-
ator in the adjustments of the controls, but it is of substantial assistance
when making the adjustments to have the computer direct the adjustments
to the proper control element with the proper motion. For example, the
photography in the field of view may present the subject in an upside down
position which the operator can correct by rotating his image rotation
elements. Thus the image would appear in the proper position for viewing.
However, in order to make the field of view appear to travel upwards in
the eyepieces, the operator would have to traverse the stages in a direc-
tion which would normally cause the images to go downwards in the eye-
pieces. By the time the anamorph and the zoom functions for both eyes
in a stereo operation are manually compensated, the advantage of having
the computer perform the proper orientation for the operator becomes im-
mediately obvious.
(b) The computer controlled mode can be described as semi-automatic
in that the operator performs certain key functions with respect to updating
the computer parameters. In this mode the computer program with the
proper numerical parameters and computations is inserted into the com-
puter. The program considers the geometrical parameters of the stereo
pair of photographs being viewed, including the special characteristics
of the camera equipment and distortions and deviations present in the
finished film. If all the parameters were exactly and absolutely known,
it would theoretically be possible to scan large photographs and auto-
matically maintain stereo fusion for the operator.
In practice, this is not possible. There are enough uncertain-
ties so that the computer cannot be exactly programmed. Further, the
amount of computation necessary to maintain an exact program is prob-
ably beyond the capability of any but the most elaborate computing
Approved For Release 2003/12/04: CIA-RDP78BO5171A000100010033-2
 Approved For Re bse 2003/12/04: CIA-RDP78B05171ANN. 100010033-2
equipment. Therefore, while this could presumably be considered a
desirable goal, it cannot presently be attained practically.
In order to take care of this lack of information condition, the
Stereocomparator is so arranged that the parameters for the program are
continuously being upgraded. The operator performs this function by
adjusting the various optical and stage position controls for optimum
stereo-viewing conditions. The computer is arranged to sense this
change in parameters, or coefficients, and to perform additional compu-
tations which utilize the operator's adjdstment of the controls in order
to establish the most accurate basis for interpolation and extrapolation.
This means that the operator can not only measure accurately over a
small area under full computer control, but can then scan along greater
distances by performing manual adjustments as required. Thus, when
the operator sees the stereo image begin to fail, the stereo fusion can be
recovered by the operator controls and the machine either used for mea-
surement again in the computer corrected small area or else the operator
may continue the scanning operation under updated computer control.
One point that becomes apparent is that unless the majority of
the mathematical parameters are known concerning a particular pair of
photographs, the computer mode of operation alone does not maintain
stereo fusion and the extent of operator participation necessary for up-
grading the computer becomes too great a burden. Further, the initial
adjustments for stereo viewing must be performed by the operator since
the Stereocomparator has no provision for identifying the absolute posi-
tion parameters in a given pair of photographs.
Prepositioning is provided in the system as a further aid in the
computer control mode. For prepositioning it is necessary for the oper-
ator to place a grid over each of the photographs, and establish by grid
coordinates a series of up to six points of interest. These grid coor-
dinates may then be typewritten into the computer memory by the oper-
ator's teletype. Then, when the photographs are placed on the platen
of the Stereocomparator and the prepositioning controls are operated,
the computer will drive the stages to these various preselected positions.
(c) An additional mode consisting of computer control plus auto-
matic correlation is provided for maximum operating convenience. In
this mode, a pair of image dissector phototubes are used to scan the
same fields of view that are presented to the operator's eyes. The out-
put from these tubes supplies electronic information to the servo drives
utilizing a special purpose computer called an image analyzer or corre-
lator. This special computer performs various mathematical calculations
whose output determines the geometry of the raster for the scanning of
the image dissector tubes and additionally supplies optical and stage
position data to the computer. The computer then readjusts the Stereo-
comparator optical-mechanical servo systems which cause the scanning
geometry of the raster to return to an arbitrary condition. This normal
raster condition is defined as that which produces stereo fusion.
An image dissector scanning system of this type has a relatively
limited pull-in range, and additionally the computer must perform many
computations for each stage position or optical system change. The
film images must be within about five percent deviation from the stereo
fusion condition before the image analysis equipment can be effective.
Approved For Release 2003/12/04: CIA-RDP78B05171A000100010033-2
 Approved,For Release 2003/12/04: CIA-RDP78BO 1A000100010033-2
It is clear that the addition of the image analyzer to the com-
puter control mode is of tremendous advantage to the operator from the
point of view of convenience, time saving and eye fatigue. It is thus
not necessary that all the parameters be known for the photographic
system and the operator is required only to perform initial adjustment
for stereo fusion to about 95 percent accuracy.
SUMMARY
From the foregoing discussion, it is apparent that the primary
purpose of the Stereocomparator is to enable an operator to rapidly and
accurately scan and measure stereo photographs, over a wide range of
film and acquisition parameters.
The time previously necessary for an operator to adjust and
optically correct for stereo fusion has been reduced from many minutes
to fractions of a second, and thus the productive output of measurements
using the Ultra High Precision Stereocomparator is vastly greater than
lesser instruments not equipped with its automatic features .
All controls are placed conveniently in front of the operator in
a control console. Above the control console are the eyepieces, and on
each side are the two film stages. The operator is thus in the most con-
venient position for viewing and control of the Store ocomparator .
The instrument is equipped for precise mensuration with a least
count of 0. 1 micrometer and with an overall accuracy of one part in
100 , 000 plus one quarter of a micrometer.
The science of photogrammetry has been advanced substantially
by the availability of the Ultra High Precision Stereocomparator.
Approved For Release 2003/12/04: CIA-RDP78BO5171A000100010033-2
 Approved For Release 2003/12/04: CIA-RDP78BO5171A000100010033-2
?25X1
Approved For Release 2003/12/04: CIA-RDP78BO5171A000100010033-2
 Approved For Release 2003/12/04: CIA-RDP78BO5171A000100010033-2
MONOSCOPIG
MECH.MEASURtNO
I64 TD 2?
MONOSCOPIC
OPTO-ELECTRICAL
MEASURING
5 To 10 MICRONS
COMPUTER OPERATED
STEREO
I To 5 MICRONS
.ACCURACY
CORRELATION OPERATED
STEREO
Ito 5 MICRONS
CLtGTRONIC CORRELATION OPERATED
14 To I MICRON
9 7 N
Approved For Release 2003/12/04: CIA-RDP78BO5171A000100010033-2
 Approved For Release 2003/12/04: CIA-RDP78BO5171A000100010033-2
OPTICALLY REMOVES TILT DISTORTION &'Y PARALLAX`
HENCE PERMITS STEREO VIEWING SO THAT M1=ASUREMENTS
MAY bE MADE DIRECTLY ON TILE PHOTOGRAPHS
STEREO PAIR WITH X.Y,&Z MEASUREMENTS
MADE DIRECTLY ON THE PHOTOGRAPHS
Approved For Release 2003/12/04: CIA-RDP78BO5171A000100010033-2
 Approved For Release 2003/12/04: CIA-RDP78BO5171A000100010033-2
ROTATOR
ANAMORPH
.:.........................
-RETICLE
?ZOOM
?ANAMORPH
*ROTATOR
.........
`OPTICAL SWITCHING
?STEREO