TECHNICAL FACTORS IN AEROSPACE PHOTOGRAPHY

Created: 9/1/1962

OCR scan of the original document, errors are possible

A^iiDVID FORIA HISTORICAL REVIEW PROGRAM

TITLE: Technical FactorsPhotography

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AUTHOR: John H. Cain

VOLUME: 6 ISSUE: 2

STUDIES IN

INTELLIGENCE

A collectionarticles on the historical, operational, doelrlnal, ond theorelrcal aspects ol intelligence. -

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All statements of fact, opinion or analysis expressed in Studies in Intelligence axe ihose of

ihe aulhors They do noi necessarily rcfleci official positions or views of ihe Central Intelligence Agency or any other US Govemmeni eniity. pasi or present. Nothing in the conienis should be construed as asserting or implying US Government endorsement of an article's faclual statements and interpretations.

Camera system research andkeep pace withdemands upped from wartime low obliques to spy-in-the-sky reconnaissance.

FACTORS LN AEROSPACE PHOTOGRAPHY John W.

photography has been recognized since World War IIrime means for acquiring intelligence; detailed analysis of the camera's faithful and permanent record of what falls within its view produces Information of unusually highIn recent years Its product has been particularly valuable, and an Insight into some of the technical factors involved in getting high-quality aerial photographs may be helpful to those in the intelligence community who make use of the resulting information.

In many respects intelligence aerial photography, for all its sophistication, is dependent on techniques and equipment which basically approximate those used by an amateurThe same fundamental concepts and elements of procedure are present at both of these extremes ofacquisition. The amateur photographer takingfrom the windowommercial airplane to 'somesupplies by human Judgment and manipulation many of the devices for Improving picture quality which areIncorporatedomplex aerial photographic system Heens of proper focal length to getimages at the plane's height and distance from theHehaze-penetration" niter to sharpen the image by blocking off diffuse, non-lmage-forming light He selects the right film for the results he wants and for the Oilers he uses. He Insulates the camera from vibration by keeping It and his own body off vibration-transmitting elements of the airplane. He compensates for Image motion by using anshutter speed and by "panning" the camera In the direction of the motion.

All of the factors which the amateur photographer,subconsciously, thus takes Into account plus manybe analyzed and provided for by mechanical orIn an aerial camera system designed forThe system Is particularly complicated If Itbe operated remotely, whether Just out of thereach or many miles away. The value of thedepend both on the quality of Individual elements ofand on bow well opposing considerations arcputting them Into combination. In evaluating differentbeen raade%^arrive at standard unitsfor the quality of theiro'

ystem's Potential

Each major componentamerabody, film, film-advancing mechanism, motion-compensationIts own separate Image degradation factors, depending upon Its design. The way In which each is combined with the others, considering their Individual and joint performance under dynamic conditions, determines the quality capability of the system. Many of the elementsthe ultimate Image quality ofamera system under design will be capable are subject to objectivebut some aspects of performance can be learned only by trial. The most important criteria include the following.

Photographic resolution. The most generally usedof photographic resolution is the number of lines perdistinguishable on thehotographic line Isair of lines, one black and one white or, moreoneiven photographic density and one formed by the space between it and the next line. The number of these line-pairs that can be separately Identified and counted within one millimeter, under any amount of magnification, on the exposed and processed photographic material constitutes one measure of the end quality of the photographic system used. Until quite recently these counts were for the most part made subjectively, and wide ranges of resolution have consequently been reported for the same type of material. Unless the test was made byreparedtarget" whose smallest line separations were ato the system's resolving capability, accurate

t'on figures for high-quality photography were difficult toElectronic devices to measure resolution morehave recently been designed and are now beingby. National Bureau of Standards.

Acutance. photographic acutance. or Image sharpness, is measured by the linear distanceiece of exposed and processed photographic material between the end of an area of one density and the beginning of an area ofombination of film, exposure, and chemical processing which permits transitkmensity.over..anshort distance Is one which will produce sharp or high-acuity photographic Imagery. This tea qualitywhich can be made objectively with the aidlcrodensitometer. Density ia defined in terms of the light transmission (or reflection) characteristics of the chemically developed photographic material. The transparency index Is the ratio of the amount of light passed through the material to the amount or light falling on It. Opacity Is theof the transparency, and density is the logarithm of the opacity. For example, if one-tenth of the Incident light is transmittediece of material. Its opacity Is ten and Its density Is one.

Granularity. The light-sensitive emulsions used In allof camera systems are generally, even today, of thesilver hallde variety. After exposure to light and chemical processing, silver Is deposited In granules to form the opaque areas of the negative. The emulsions which are the most sensitive to lightfastestthe ones which form the coarsest silver granules whenprocessed, and those which are slowest In imageon exposure to light form the smallest granules when processed. Obviously, It is desirableigh-quality camera system toilm which forms the finest possible silver grains consistent with the amount of light available for proper exposure; the compromise that has to be made between speed of exposure and granularity Is governed by the ambient light conditions expected during the exposure period. Granularityactor which can be objectively determinedeasure of film quality.

Tonal range. All photographic materials can be measured to determine their tonal range, the number o( discrete image densities or gray tones they can record. Since normalscenery consistsariety of colors, as well as textures and tones, it Is Important that black-and-whitematerial separate these In asange ofdifferent shades of gray asigh-quality photographic film Is expected toufficient variety ot gray shades tbat the human eye. which can distinguish

scene.

Scale. Though scale Isuality factor determinedsystem components alone. It is certainly one which

< affects tbe Information potential of aerial photography. It can be objectively measured, being nominally the quotient of I lens focal length by distance to the subject The design of j modern reconnaissance cameras, however, precludes straightforward determination of scale. In the panoramic aerial cameras now used In order to increase lateral ground coverage the picture Is projected onto the filmotating lens or prism, and the scale changes constantlyomplex i geometric pattern. It can be calculated at any point only | by experienced photogrammetrists using sophisticated proce-i dures.

Cround detection size. During the past several years this term hasommon means of expressing the quality capability of an aerial camera system, but oUffering definitions are given to it Although the minimum size of object whose image can be detectedilm can be determined accurately with relative ease, the question is whether detection oris the criterion. On purely physicaletect-J able Image must be at least as large as the width of one llne-I pair, the limit of the material's resolution capability, but an | Image of rnl"lr"lim size would be formless and unrecognizable This physical minimum has been usedtarting point however. In attempts to arriveigure for the number of I line-pairs defining the minimum size required forof an image through Its shape and dimensions. photographic scientists have set various figures for this.

ranging from 2Vi toine-pairs. Ifamera system canesolutionines perensity change covering one-hundredthillimeter could bebut the image would have to have dimensionsillimeter before its shape could be determined. Its size measured, and the object it representedour-Inch object photographed0 feetocal lengthoot could just be detected; estimates of thethat would be required for recognition range from ^lOto^che^.,

Volume. The square footage of film which can be earnedhotographic mission Is an important determinant of the total Information potential of the camera system. The film width usedarticular camera system is generally determined by tbe type of camera and the characteristics of the lens. The length of film required for the amount of ground coverage desired is subject to the weight and space limitations of the camera-carrying vehicle.

Weather. Clouds and atmospheric hazeactor to be reckoned with inamera system's netpotential. The scattering of light by haze lowers photographic resolution by reducing Imageamera systemesolutionines per millimeter In high-contrast imagery (density ratios on the order,1 contrast In opacity) will giveraction of that, say as few asines, In low-contrast imagery (densities oo the order. Timing of aerial photography missions for tbe best seasonal and daylight hours with the help of the best weather forecasts to be had is the only means available to counter this factor.

7"ii>enfp Years' Refinements

The quality of the aerial photography done for Intelligence purposes today is at another order of magnitude than that found acceptable twenty years ago. In the measurement most often used for Image quality, the resolution given by the camera systems of Worldveraged aboutines per millimeter, and this was good enough to meet wartimewith the camera-vehicle techniques employed. Today camera systems are producing photographicon the orderines per millimeter.

lng analysis Indicates that an operational capabilityines per millimeter Is quite feasible for the near future.from this extremely rapid improvement, which would have been unbelievable if predicted twenty years ago. leads photographic scientists to believe that resolutions on the orderines per millimeter will be achievable within the next five years. With the imposition of seriousrequirements for more and better aerial photography, tbe photographic industry's scientists, given sufficient time and reMUvehT.uarestjicted financial support, haveajwayjuced theTecessary technology. There'ta noon why this progress should not continue.

Thus the tremendous gains In photographic quality are the result of requirements for Intelligence extremely difficult, if not impossible, to collect by other means. During wartime operations intelligence mostlyactical nature wasin large amounts through comparatively close-range aerial photography. In peacetime, and especially In theyears of nuclear equipoise, then strategic intelligence requiring much broader photographic coverage, and at the same time the camera vehicles are excluded from close range to their target areas. Substitutes for short-range photography bad to be found.

One might think that new techniques to compensate for the forced Increase In range would center on Increasing the focal length of lenses. As the focal length Is Increased, however, the lenses become extremely huge, and because of weight and space limitations In the vehicles researchers had to seekIn other areas. While the camera designers were Improving their film-handling mechanisms, as describedthe film Industry was researching light-sensitiveof ever higher quality. With significant technological advances In this area, the crux of the problem shifted back again to the kens designers, who now had to devote theirto making lenses which would transmit the larger amounts of light required by the inherently slower emulsions of extremely high quality.

Since the basic means of getting more light transmittedens Is to Increase its diameter, here again was the problem of Increased size and weight. One way of alleviating

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was found to be lens coatings. Though theumber describes the nominal transmission characteristicsarticular lens, In aerialore definitivetop" figure Is used, which takes into account the light losses by absorption In the glass and reflections at the air-glass surfaces.ens, for example, would have with surfacestop value, but this couldy treating the glass surfaces with an anU-reflectlve coating. The effect of the coating is equivalent to

lens, and film producers thus worked together quite closely to Insure that technological breakthroughs In one area would bo matched by parallel improvements In others. Especially Important has been the fulfillment of theto provide large volumes of photographic coverage while maintaining camera and film weightsinimum The use of thinner, yet stronger support material for photographic emulsions has in recent years about doubled film footage per unit of weight At first, however, this innovation created serious problems for the tracking and movement of the film within the camera, and novel techniques had to be found for guiding the very thin films through the maze-like paths of sophisticated reconnaissance cameras.

For example, thin-base films cannot be edge-guided by the use ot flanges on rollers. Precise alignment of all of the lengthy film paths Is therefore required, and this now can be accomplished by self-leveling rollers which sense any lateral movement of the film and provide constant corrections.ollers used for right-angle turns of film hypersensl-tised It by their pressure and produced fogging. It was found that air forced through the porous material of sintered metal rollers would permit the film to "float" around the corners without contact Space requirements for film have beenreduced by space sharing between supply and take-up storage. Normallyupply spool is full the take-up spool is empty, but empty or full they require the same amount of space. New spooling techniques have now beenwhich permit film to be rolledlangeleas supply

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core, which can be placed close to the take-up spool with the full roll protruding between its empty flanges.

As each separate problem was encountered and surmounted, additional Improvements ln other areas have necessitated continuing research and development. Even as this Is written, laboratory experiments are being conducted with film bases comparable ln thickness to the cellophaneigaretteabout one-half as thick as those currently considered thin. Indeed, the coatings of light-sensitive emulsions may soon be thicker than their supporting base. The advantage ofsuch films In increasing area coverage potential is covEus, but the problems they create for camera drive mechanisms and automatic chemical processing equipmenteriousto their designhe evolution of radical techniques In lens design as aof the newly produced high-resolution film emulsionseen greatly helped by the advent of electronicapabilities. Where weight and space have Imposed severe limitations on the use of large refractingwitch to optical reflectors has been given serious consideration. In the midst of this, the dimensional stability of the new thin film bases Is challenging the lens manufacturers to provide optics that match them in preserving the geometric fidelity of the imagery.

Such has been the story of progressive improvements in aerial camera systems' components, all made In an attempt tolose-up look from distant camera vantage points. As operational limitations are continually Increased, pictures will be taken from even farther away. And since the same or even higher quality will be required, research and development on aerial Intelligence camera systems will continue to be sponsoredommensurate pace.

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