740 PHOTOGRAMMETRIC ENGINEERING

+ .035(25) + .12(35) + .015(45)+ .015(45)

= 19.26 minutes

Thus a quantitative comparison of theprevious cost per photograph processed of$4.425 and through-put time of 21.025 min­utes can be made by decision making per­sonnel in order to determine the desirabilityof such an addition to Rectifier magnifica­tion. In this manner it is possible to quantita­tively evaluate equipment design in terms of sys­tem performance.

CONCLUSION

This paper has presented a specific exampleof the utilization of operations research in thedesign of a specific photogrammetric supportsystem. By the use of similar techniques, the

A Slit-Scan ElectroOptical Rectifier*

expected performance of any system can bedetermined provided the distributions of inputvariables, possible system states and thespecified output quality requirements areknown or can be reasonably estimated. Inthis manner, not only may the initially ex­pected system performance be determined,but also the effect of changes in equipmentdesign on the system performance may bequantitatively evaluated. Thus, more precisemeasures of worth can be introduced intoboth system and equipment design decisions.

ACKNOWLEDGMENTS

The most helpful comments and sugges­tions of Mr. Arthur Faulds and Mr. RobertBrand t concerning the photogrammetric as­pects of this paper are gratefully acknowl­edged.

SAMUEL W. LEVINE,

Fairchild Camera and Instrument Corp.,Defense Products Div.,

Syosset, N. Y.

ABSTRACT: A unique machine is described which has the capability of rectifyingpractically all types of oblique aerial photography. The rectifier takes advan­tage of optical projection for transforming the photographic information, andelectronic computer controlled distortion for achieving dimensional restitution.By combining these two techniques a machine results which produces rectifiedphotographs having very high resolution, excellent dimensional characteristics,high speed of operation, and flexibility in regard to the types of oblique photog­raphy which can be processed.

Oblique frame, vertical panoramic, and oblique slit scan photography can berectified. Oblique panoramic can be rectified by special adaptation. A largerange of focal-lengths and oblique angles can be accommodated by the rectifier.A resolution figure of 80 lines-per-millimeter should be achieved in the rectifiedphotograph with speed of operation being less than 15 minutes for 100 squareinches of copy.

INTRODUCTION resolution being obtained. In addition the

T HE requirement for improvements in requirement for reconnaissance and mappingequipment for rectifying oblique aerial has increased substantially.

photography has become of increasing im- Historically, rectification equipment hasportance in the past few years. This has come been limited to the optical projection type.about because of the increased use of a variety \Vith a few exceptions only oblique-frameof types of aerial photography and of the high type aerial photography can be processed by

* Presented at the Society's 27th Annual Meeting, The Shoreham Hotel, Washington, D. c., March19-22, 1961.

A SLIT-SCAN ELECTRO OPTICAL }mCTI FIER 741

the optical rectifier. Even with this restrictionthe optical rectifier is limited in resolutionand in range of focal-length and degree of tiltthat can be accommodated by a single instru­ment.

Several panoramic optical rectifiers havebeen built or are now being designed, but ineach case they are limited in resolution, inmaximum scan angle, to a single focal-length,and to vertical photography.

In order to produce a rectifier that is moreversatile and does have the required highresolution, advanced electronic techniqueshave been applied to the problem. Thisincludes digital computer techniques as wellas video electronics for photographic imagetransformation.

Hycon Manufacturing Corp. has demon­strated a universal type electronic rectifierwhich can rectify all types of photography.Electronic scanning as well as recording isused. Dimensional rectification is accom­plished by servo-drives operated from pre­punched tapes. A digital computer preparesthe punched tape by solving the transforma­tion equation.

Fairchild Camera and Instrument Corpora­tion has delivered to Rome Air DevelopmentCenter (RADC) a spot scanning electro­optical rectifier which accepts oblique framephotography and produces positive prints onpaper up to 36 inches by 48 inches in size.This machine can rectify frame photographyfrom one and one-half inches up to one hun­dred inches focal-length and tilt angles up toeighty-five degrees from the vertical. Scan­ning is accomplished by an oscillating mirrorand the video processed photographic in­formation is recorded with an ultrasonic light

modulator. A buil t-in electronic analog com­puter plus a mechanical computer transformsthe dimensions required for rectification.

Both the Fairchild and Hycon electronicrectifiers have limitations in resolution andspeed of operation. In each case the copy isscanned by a spot of finite size. The photo­graphic information within the spot is inte­grated by a photomultiplier tube into avideo electronic signal having an averagevalue representative of the information in thespot. The limiting resolution is, then, the sizeof the scanning SI)Ot. If attempts are made todecrease the spo'£'size for increased resolution,there arise prool~ms of signal to noise andspeed of recorUing.

These problems are overcome in the designof an electro-optical rectifier now being de­signed for RADC. In this design photographicinformation is transposed from oblique copyto rectified print by optical projection. Trans­formation of both dimensions of the copy isaccomplished using servo drives and punchedpaper tape derived from a digital computer.

RECTIFIER-SYSTEM OPERATION

A schematic of the slit-scan electro-opticalrectifier is indicated in Figure 1. Oblique copyis placed on a flat, glass platen that may berotated to the correct swing angle. The copyis scanned by a very thin line of light pro­jected from an illuminated slit. A high-pres­sure mercury-vapor tube is used as the lightsource and an elliptical mirror images theslit in the plane of the copy. Exposure con­trol is obtained by moving a variable densityfilter between the light source and slit.

The copy is moved past the projected slitand as this is done the scanned information is

PROGRAMMINGTAPE

-RECORDINGCYLINDER(IMAGE)

.FIG•.J. Schematic of slit scan electro optical rectifier.

742 PHOTOGRAMMETRIC ENGINEERING

SAMUEL V·i. LEVI~E

projected to a recording cylinder by a high­resolution imaging lens. Rotation of the re­cording cylinder is preprogrammed to cor­relate with the rate of scan of the copy bythe illuminated slit. In this manner thephotographic content of the oblique copy istransferred to the rectified recording.

Rectification in each dimension of thephotograph is accomplished by utilizing twodifferent optical techniques. In the directionalong the slit the required change of dimen­sion is obtained by correctly positioning thelens and copy. In the direction perpendicularto the slit, the required change of dimensionis obtained by relative motion between copyand recording.

By these means the photographic informa­tion is transformed in an optical projectionsystem wher~ the limiting factors on resolu-

tion are the resolving power of the projectionlens and recording film. Dimensional rectifica­tion is obtained by programmed motion ofthe lens and copy position, and of the relativemotion between copy and recording.

An artist's rendition of the rectifier is givenin Figure 2. As may be noted, the scanning-slitis set in a vertical direction. This is done toreduce the load on the servo system drivingthe copy holder. Copy in the form of rollfilm may be loaded on the copy holder so thatit will not be necessary to cut out separateframes for rectifying. The recording cylinderloading is accomplished in a darkened room,but the lights may be turned on when loadinghas been completed. The electronics arelocated in the lower part of the machineand in an auxiliary rack located nearby.

GEOMETRICAL BASIS FOR RECTIFIER DESIGN

Application of this technique to rectifica­tion may best be understood by consideringthe analytical geometric aspects of rectifica­tion. In all oblique aerial photography, witha few exceptions, there is a so-called principal­line or its equivalent. Lines perpendicular toit are linear in dimension within themselves.

Single-frame oblique photographs have aprincipal-line along which the scale from copyto rectified print constantly changes. Per­pendicular to this principal-line are lineswhich are linear. That is, equal unit lengthalong these lines repesent equal increments oflength on the ground. The scale from line toline does change.

In vertical panoramic photography the linealong the direction of panoramic scan is com­parable to the principal-line in the single­frame photograph. All lines perpendicular tothis line are linear within themselves. Tipped

FIG. 2. Electro optical rectifier.

A SLIT-SCAN ELECTRO OPTICAL RECTIFIER 743

FIG. 3. Schematic of slit scan.

n-i--~~-·---D

DIMENSIONAL CHANGE PERPENDICULAR

TO SLIT

In order to transform the dimension fromcopy to recording along the principal-line,relative motion between copy platen and re­cording cylinder is utilized. In Figure 3 is in­cated the slit scanning the copy, the projec­tion lens, and recording surface. The object tolens distance and lens to image distance isshown such that the magnification factor isunity. It can be seen that if the rate of copyscan is equal to the rate of movement of therecol-ding surface, then no change in size fromcopy to recording is realized.

panoramic photography has no such compari­son and rectification here requires a separatetreatment.

Oblique slit photography has the principal­line analogy although it is not singularly de­fined. Lines perpendicular to the analogousprincipal-line direction are linear withinthemselves as is determined from the factthat the speed of the vehicle is constant.

The common factor in all three types ofphotography is that perpendicular to a prin­cipal-line are lines which are linear. This factis used in the design of the rectifier in thatdistance along the scanning-slit is linear andnon-variable. In order to rectify, the copy ismounted on the platen so that the principal­line is perpendicular to the slit and thereforelines perpendicular to the principal-line arealong the slit length direction and will belinear.

If the recording surface rate of movement istwo times the rate of the copy scan, then anenlargement of two to one is realized in thedirection of scan-that is, perpendicular tothe length of the slit. It is to be noted that inthe direction along the slit-perpendicularto the scan direction-there is not change ofsIze.

When the object to lens and lens to imageposition are arranged so that a two to onemagnification is obtained, and the rate ofcopy scan is equal to the rate of movement ofthe recording surface, then in the direction ofscan only a one-to-one magnification is ob­tained as previously. At a two-to-one ratio ofspeed of the recording surface to copy scan­ning a two-to-one enlargement is obtained.

It can thus be seen that change of dimen­sion in the scan direction is independent of theoptical magnification characteristic of theprojection lens.

When the ratio of recording to copy speed isdifferent than the optical magnification ratio,there is a small image smear effect in the re­cording that has a degrading effect on resolu­tion in this dimension. By using a very nar­row scanning slit this is minimized and in themachine being designed the slit width is 10microns. Reference to Figure 4 demonstratesgraphically the effect on image smear of usinga very small slit. On the left is represented aresolution pattern having bars and spacesequivalent to resolution lines, and having thesame width as the scanning aperture. Theletters above each pattern represent the suc­cessive positions of the resolution pattern intime. The projection lens is set for a magni­fication ratio of one-to-one and the resultingrecording is shown at the right. For this ex­ample a recording speed of 1.5 is used com­pared to the scan-speed of unity. As eachsuccessive position of scan and record is

RECORDINGLEN'COPY

COPY SPEED=1

RECORDING SPEED=1.5

.'" IM.'fDCI .. "Ie D It: f'G HI'" I(

FIG. 4. Image smear effect.

744 PHOTOGRAMMETRIC ENGINEERING

WIDTH OF THE SLJT. 0.01 MM

100%

80%

100 LlNES!MM

25 UNES/MM

10 LlNES/MM

COPYRESOLUTION

80'

5.76070'

2.922

60'

2.00

400 500

1.304 1. 557

30'1.153

20'

1.063

10'

1.015

0% L....,;;;~~;;;;;;;;;;;;;;:::::==;:::=...L-_-'-_----L __l..-_...1.-__

o·1.00

80%

FIG. 5. Resolution of rectifier in scan direction.

analyzed it is seen that an enlargement of 1.5to 1.0 is realized, but no longer is there a one­to-one relationship between bar and spacewidth. This change of relationship from copyto recording is due to smear. The pattern ofdensity contour of the bar is such that almostzero density exists at the ends with a flatmaximum density in the middle.

A mathematical analysis of the effect willshow that the smear value which is defined asthe ratio of the recorded bar width to the idealrecorded bar width is as follows:

h( Va)Sm = 1 + - I-m-b VF

where

h = aperture wid thVa = vel. of copy

b = resolution bar widthVF = vel. of recording

The effect that image motion will have onresolution is given in Figure 5. Along theabscissa is noted the obliquity angle. This an­gle refers to the angle of scan in a panoramiccamera. Also noted is a magnification factorwhich is used as a comparable figure in thesingle-frame and slit-type cameras. Along theordinate is plotted resolution degradation dueto image-motion.

There are several interesting facts that areapparent in this plot. It may be seen that thepercentage resolution degradation is a func­tion of the resolution in the original obliquecopy-the lower the resolution the less deg­radation of resolution. For example, in apanoramic rectification at 30° scan-angle, ifthe original resolution is 25 lines/mm., the

reproduction will have a resolution of 23.5lines/mm., or a degradation of 6%. At thesame scan-angle 100 lines/mm. copy will re­produce at 78Iines/mm., or 22% degradation.

I t is to be noted that this is resol ution deg­radation in one dimension only. The result­ant resolution degradation in the rectifiedprint will be appreciably less than the valuesindicated in Figure 5. Once the machine hasbeen completed it will be interesting to de­termine the effect of resolution degradation inone dimension on the resultant resolution.

At the nadir of the panoramic or isoline ofthe oblique photograph, there is no degrada­tion due to image motion and the degradationincreases slowly with the obliquity angle.

DIMENSIONAL CHANGE ALONG THE SLIT

Change of dimension along the slit is shownschematically in Figure 6. The image record­ing position remains fixed. The object to lensand lens to image positions are changed toobtain the required magnification and focus.

As previously noted the change in dimen-

RECORDINGDRUM

SCANNINGSLIT

~LENS

·----ffi----VARIABLE

VARIABLE

MOTOR

Fig. 6_ Dimensional change along slit.

A SLIT-SCAN ELECTRO OPTICAL RECTIFIER 745

sion along the slit is independent of the changeof dimension perpendicular to the slit. Con­sideration of these two techniques for inde­pendent change of dimension in two mutuallyperpendicular directions makes it apparentthat this scheme can be used for rectification.This applies to all cases where one of thedimensions is linear for all line elements.Types of oblique photography falling into thiscategory are single frame, vertical panoramic,and strip photography. Other cases can beaccommodated by special adaptations.

PROGRAMMED SERVO CONTROLS

As indicated in Figure 1, operation of therectifier is controlled by servomechanismdrives programmed through a digital decoderreading punched tape. The recording cylinderis rotated at constant speed by a very precisepuck drive and synchronous motor. A tachom­eter produces a timing signal for synchro­nizing the operation of the servos. The tachom­eter timing signal controls the rate at whichthe punched paper tape is read. This arrange­ment eliminates the possible error due tovariation in electrical frequency of the powerto the recording drum synchronous motor.

Punched tape is produced by a digital com­puter which solves the transformation equa­tions required for rectification. At many loca­tions where the rectifier may be used, a digitalcomputer facility is available, and the tapescan be produced at any convenient time. It isalmost always possible to rent computer timefor preparing the tape if one is not availableat the facility. Should there be no possibilityof producing punched tape on a digital com­puter it is a relatively straightforward prob­lem to design a built-in analog computerto solve the transformation equations. Forthe machine now being fabricated, the pre­punched tape approach was taken because ofthe availability of a digital computer, im­proved dimensional accuracy gained by usinga digital computer, and economy.

A digital decoder accepts seven level binarycode from the punched tape at the rate of sixblocks-per-second and produces analog signalsfor operating the four servos. These drive thelens, the copy to lens distance, the copy platenpast the projected scanning slit, and the vari­able density filter. This optical filter controlsthe light exposure level so that a uniformdensity reproduction may be obtained.

The digital decoder and servomechanismcontrol has the capability of giving a highorder of accuracy in positioning the variouselements during rectification. It is expectedthat copy-distance positioning, lens-distance

positioning, and copy-scan positioning will beaccurate to ± 0.0005 inch of the theoreti­cally required positions. This will insure ex­cellent dimensional accuracy in the rectifica­tion as well as the exact optical focusing re­qured for high resolution reproduction.

SCANNING OPTICS AND LIGHT SOURCE

In order to maintain the high resolution re­quired in present day rectification a very thinslit of the order of 10 microns width must beused for scanning. This slit is fabricated bystretching a uniform thin wire of 10 micronsdiameter across and in contract with a glassplate. The assembly is placed in a vacuumevaporator where a metal film is deposited onthe plate and the wire acts as a mask. An ex­cellent slit is obtained when the wire is re­moved.

This slit is illuminated by a high-pressuremercury-vapor tube and a set of cylindricalcondensers. An elliptical-cylindrical mirror inan off-axis position projects the illuminatedslit-image in the plane of the copy beingscanned.

In this machine the copy is mounted on aflat disk of glass which may be rotated abou tthe optical axis of the machine. Such rotationis required for swing-angle adj ustment wherethe principal line of the oblique photographyis not parallel to the edge of the format. Facil­ities are available on the copy carriage tocarry roll film so that the copy need not becut for mounting on the copy platen. Theslit-length is 13 inches and the diameter of thecopy platen is 15 inches, making it possibleto scan a 9 inch by 9 inch photo rotated inone pass. Sections of panoramic photographyup to 13 inches in length can be scanned inone pass where the format widths are up to 5inches. When it is required to rectify pano­ramic formats longer than 13 inches, the recti­fication can be performed in sections up to 13inches in length, and then assembling theresulting rectified sections.

In order to keep the exposure time to aminimum a high-intensity mercury lamp isused. High resolution recording requires a finegrain relatively slow photosensitive emulsion.By using fine-grain film and the mercury tube,it is expected that 13 inches of copy can bescanned in less than fifteen minutes. Wherehigh recording resolution is not required andfaster film may be used, the machine can beoperated at three times the speed and 13inches of copy can be rectified in 5 minutes.

CHANGE OF SIZE CAPABILITY

Because of the basic machine design it is

746 PHOTOGRAMMETRIC ENGINEERING

possible to obtain a change of size of the re­productions during rectification. This enlarge­ment or reduction refers to the isoscale sizechange, and is in addition to the change ofsize inherent in rectification. It was founddesirable to include in the machine the capa­bility of obtaining an enlargement range ofone-half to three. In order to cover this rangeit is necessary to use two lenses for projectionpurposes. It is never required that a lenschange be made during any single rectifica­tion.

SINGLE FRAME PHOTOGRAPHY

The mathematics for single frame photo­graphic rectification has been given in a previ­ous paper by Ross and Levine (1958 Vol.XXIV, No.5, pp. 789-793). Informationthat is required for producing the punchedtape includes tilt-angle, swing-angle, altitude,camera focal-length, and desired scale-factor.

Copy is mounted on the glass platen withthe edge of the format paral1el to a referenceline, and the principal point coincident with areference on the platen. The swing-angle isadjusted by rotating the platen the requiredamount. Scanning takes place with the prin­cipal-line traveling perpendicular to the slit.A positive rectified recording up to 36 inchesby 36 inches on film or paper may be obtained.

As presently designed the machine willrectify copy from 5 inches to 100 inches focal­length and resultant tilt-angles to 60 degreeswith provision for increasing these parametersif required. Corrections for earth curvature,

atmospheric refraction, and film shrinkagemay be made during the computation of theprogram tape. It is expected that a resolutionof 80 lines/mm. will be obtained at the isolinein single frame oblique and at the nadir inpanoramic photography.

PANORAMIC PHOTOGRAPHY

The panoramic photography rectificationproblem may be separated into cases-verti­cal type where the plane described by theoptical axis during scan is perpendicular tothe earth's surface, and the tipped panoramicwhere this plane is at an angle to the earth'ssurface.

Rectification of vertical photography isthe simpler case and can be accomplished byscanning up to 13 inch lengths of copy in oneoperation. If the copy format is longer than13 inches then the photography is rectified insuccessive stages.

Corrections for earth curvature, atmos­pheric refraction, and film shrinkage are madeduring the computation stage. Correctionfor the typical "S" curve of the principal lineequivalent of the panoramic picture can bemade by moving the copy platen, the imaginglens, or the recording cylinder in a directionparallel to the scanning slit length. This cor­rection would also be computed and pro­grammed in to the machine on punch tape.

In order to correct for the change of expo­sure that may occur at high angles in thepanoramic photograph, a variable densitymask is programmed to change exposure

SCANNINGSLIT

It,I I \

I I ,I "I ~ ,

I I ,I I \

I I '

/ I " xI I , tI I ,

I I \--I~

/ : ~x, :I I I

I I/ I

I II

FIG. 7. Tipped panoramic adaptation.

y'

A SLIT-SCAN ELECTRO OPTlCAL RECTlFIER 747

light level during scanning. The exposurecould not be corrected by an iris adjustmentat the lens because the resolution would thenbecome diffraction limited.

Tipped panoramic photography producesgeometry in the photograph in which there isno set of lines that are linear and perpendicu­lar to a principal line. As previously explainedthis was a requirement for rectification bythis machine. However, there are specialadaptations for handling this type of photog­raphy. An analysis of the transformationequations indicates that one method of recti­fication which may be used is a two-stageprocess. The copy is first recti fied as though itwere a vertical panoramic photograph. Theresul t of this transformation is an obliquesingle-frame photographic equivalent havinga tilt-angle equal to the tip-angle of the pano­ramic photograph. The single-frame obliqueequivalent photo which results is then recti­fied in the machine as previously described.If the size of the rectified photograph in thefirst stage is too large, it must be reduced insize during the first rectification and thenenlarged in the second stage to the requiredsize.

By modification of the machine it is pos­sible to rectify the tipped panoramic photo­graph in one step. This is indicated in Figure7. The scanning slit and copy, the lens, andthe recording plane are angled with respectto each other so that the Schei mpflug condi­tion is satisfied. That is, lines through thescan-slit, the plane of the lens, and a linethrough the plane of recording intersect at a

Panoramic Progress-Part I

THE ADVANTAGES OF PANORAMIC

PHOTOGRAPHY

GENERAL

T HE development and increasing use ofpanoramic photography in the field of

aerial reconnaissance has resulted primarilyfrom the need to cover in greater detail more

common point. In addition the recordingplane is rotated through the angle p. duringrectification. These additional motions wouldall be obtained by servos programmed frompunched tape. A mathematical developmentand proof of this method of operation hasbeen accomplished but is much too lengthyto present here.

OBLIQUE SLIT PHOTOGRAPHY

Oblique slit photography rectification isessentially a simplification of the obliquesingle-frame case. Obviously the scale re­mains constant for all lines in the photographparallel to the line of flight. It is only neces­sary to rectify in the direction perpendicularto the flight vector. This is accomplished byhaving a fixed lens to copy and lens to re­cording distance for the required scale changeand obtaining the transformation in theperpendicular direction by relative motionbetween scanning speed and recording speed.

CONCLUSION

As may be seen the machine as described inthis paper can be used to rectify the varioustypes of oblique photography discussed. Byusing optical projection, high-resolution andrelatively high operating speed is obtained.By using high-precision servomechanismdrives driven by digital computer derivedpunched tape, high-precision in metric trans­formation will be obtained. For special re­quirements of rectification, the machine maybe modified for optimum operation at thecost of introducing additional complexity.

ITEK LABORATORIES,

Lexington 73, ~Mass.

and more areas of our world and-soon,perhaps-of other worlds as well. In order tocover the large areas involved, and to resolvethe desired ground detail, present-day recon­naissance systems must operate at extremelyhigh-resolution levels. Unfortunately, high­resolution levels and wide angular coverage

EDITOR'S NOTE: This paper which is to be supplemented by Part 2 in the March 1962 issue includesthe contents of a brochure by [tek Laboratories entitled "Panoramic Progress." A reading was convincingthat the contents were so valuable and helpful to photogrammetry and photogrammetrists that repeatingin this JOURNAL was highly advisable. For permission to take this action, thanks are given to Itek Lab­oratories.