Color and False-Color Films for Aerial Photography

RATFE G. TARKINGTON and ALLAN L. SOREM

Research Laboratories, Eastman Kodak CompanyRochester, N. Y.

ABSTRACT: Color reproduction by the photographic process using three primarycolors is discussed, and the 11se of these photographic and optical principlesfor false-color reproduction is explained. The characteristics of two new aerialfilms-Kodak Ektachrome Aero Film (Process E-3) and a false-color type,Kodak Ektachrome Infrared Aero Film (Process E-3)-are compared withthose of the older products they replace. The new films have higher speed, im­proved definition, and less granularity.

KODAK EKTACHROME AERO FILM(PROCESS E-3)

FIG. 1. Schematic representation of amultilayer color film.

MAGENTA POSITIVE IMAGE

CYAN POSITIVE IMAGEREO SENSITIVE

BLUE SENSITIVE YELLOW POSITIVE IMAGE

=====::::==l=====~=~=~~[M~GREEN SENSITIVE

in a scene, but the results obtained withmodern color photographic materials are re­markably realistic representations of theoriginal scene. While a good representationof the original scene is completely satisfactoryfor practically all applications, an aerial colorphotograph of the ground taken from a highaltitude without any modification in its char­acteristics would be unsuitable for most pur­poses.

The difficulty in obtaining satisfactoryaerial photographs is due to atmospherichaze which drastically lowers the luminanceratios of grou nd detail? In color photography,in addition to lowering of the contrast in theimage as in a black-and-white photograph,the color saturation is also markedly reduced.I t has been established that photographicmaterials having a higher gradient character­istic curve will restore some of the loss in con­trast and, in addition, with color materialscolor saturation is increased. ThereforeKodak Ektachrome Aero Film (High Con~trast),* which Eastman Kodak Companysupplied to the Allied Armed Forces duringWorld War II,3 had a characteristic curvegradient about 100 per cent higher than thatof comparable pictorial materials.

* Known as Kodacolor Aero Reversal Filmwhen it was first introduced in 1942.

POPULAR processes of color photography arebased upon the facts that (1) the colors

perceived by the human eye can be producedby mixtures of only three suitably chosencolors called primaries; (2) photographicemulsions can be made to respond selectivelyto each of these three colors; and (3) chemicalreactions exist which can produce three in­dividual colorants, each capable of absorbingessentially only one of the chosen primarycolors. Although theory imposes no singleunique set of three primary colors, in prac­tice the colors chosen are those produced bylight from successive thirds of the visiblespectrum: red, green, and blue. When thesechoices have been made for the primarycolors, the colorants, each of which will absorbonly one of these primaries, are uniquelyspecified, and are cyan which absorbs red,magenta which absorbs green, and yellowwhich absorbs blue. These latter colors arereferred to as the complementary colors or asthe su btractive primaries. l

A color photographic material based on theuse of these pri maries consists of three sensi­tive layers coated on a support, each layersensitized to a different primary color andcapable of producing the appropriate com­plementary color in situ (Figure 1). In addi­tion, the amount of colorant produced in anyarea is related to the amoun t of silver de­veloped; this is, in turn, related to the in­tensity of light in this area of the originalscene. Thus each layer is a separate record ofthe brightnesses of a single primary color and,with the three layers of the film superimposed,the colors and brightnesses of the originalscene are reproduced within the limits im­posed by the process.

No process of color photography is avail­able which can accurately reproduce all colors

88

OLD PRODUCTS

KODAK EKTACHROME AERO FILM(High Contrast)

Exposure 1/500 sec. at]/4.5

KODAK EKTACHROME AERO FILM(Camouflage Detection)

Exposure 1/500 sec. at]/3.5

NEW PRODUCTS

KODAK EKTACHROME AERO FILM(Process E- 3)

Exposure 1/500 sec. a(j/8.0

KODAK EKTACHROME INFRARED AERO FILM(Process E-3)

Exposure 1/500 sec. a(j/5.6

Figure 8 - Comparison of photographs on old and new Kodak color aerial films, taken from 8,000 feetaltitude on days of average haze.

COLOR AND FALSE-COLOR FILMS 93

KODAK EKTACHROME INFRARED AERO FILM(PROCESS E-3)

INFRARED SENSITIVE CYAN POSITIVE IMAGE

GREEN SENSITIVE YELLOW POSITIVE IMAGE

RED SENSITIVE MAGENTA POSITIVE IMAGE

FIG. 2. Schematic represen ta tion of the structure(Process E-3).

Under some conditions the degree of atten­uation is dependent upon \\"ave-Iength; for a"pure" atmosphere, according to Rayleigh,the scattering is inversely proportional to thefourth power of the wave-length. This meansthat blue light is scattered more than greenlight which, in turn, is scattered more thanred light. It appears, therefore, that a colorfilm should have a higher gradient in the blue­sensitive layer than in the green-sensitivelayer which, in turn, should be higher thanthat in the red-sensitive layer. In fact, alimited investigation of this effect duringV,Torld \Var II indicated that thet"e was somewave-length-dependence, but the effect isextemely small except where the atmosphereis almost completely devoid of all aerosols.This later condition so seldom exists that formost practical aerial photographic condi tions,the contrast-degrading effect of the atmos­pheric haze can be reduced by simply increas­ing the gradient of all three sensitive layers.

The principles just discussed apply onlywhen a color photograph is desired whichrepresen ts a scene as it appears visuallyor as it would appear if certain unwanted dis­turbing elements could be minimized. It is tobe emphasized that while the colors in thescene can be approximated by photographybased on these principles, a colorimetricmatch does not imply a spectral match.4

Therefore, the recording of three separateregions of the visible spectrum and the choiceof dyes are uniquely interrelated only whena natural-appearing color picture is desired.Technically, any portions of the spectru mto which photographic materials are sensitive(within or outside the visible spectrum) canbe recorded, and the colorant developed bythis record can be any that are available,without regard to the spectral region initiallychosen. Such processes have been proposedfor specialized purposes and are known asfalse-color systems.

In aerial photography, particularly for in­terpretation purposes, spectral regions canbe chosen to emphasize differences between

particular objects which are visually quitesimilar. These principles of false-color sys­tems were used during World \Var II, andKodak Ektachrome Aero Film (CamouflageDetection)* which, beginning in 1942, wassupplied by the Eastman Kodak Company tothe Allied Armed Forces5 ; this film empha­sized the difference in infrared reflection be­tlyeen green vegetation and most visually si m­ilar green paints. The structure of this filmmaterial, showing the sensitivities and thecolors developed in the different layers, isshown in Figure 2. The material was alwaysused wi th a yellow fil ter over the camera lens\\"hich absorbed blue light to \\"hich all photo­graphic emulsions are sensitive. The chloro­phyll of green vegetation, which reflectsstrongly in the infrared region of the spec­trum, would appear red in the photographs,\\"hereas other objects visually si milar tovegetation would be rendered as dull blue.

Any spectral differences that exist in thephotographic sensitivity range can be simi­larly differentiated and emphasized by asuitable choice of emulsion sensitivities, filterabsorptions, and developed dye in the layers.For this application of the techniques of false­color photography, only two different layersand colorants are needed for differentiation

* Then known as KodacoJor Aero Film, Camou­Aage Detection.

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FIG. 3. Sensitivity curves for Kodak EktachromeAero Film (Process E-3).

94 PHOTOGRAMMETRIC ENGINEERING

FIG. 6. Sensitivity curves for Kodak EktachromeInfrared Aero Film (Process £-3).

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KODAK EKTACHROME--AERO FILM

(HIGH CONTRAST)

550 650 750

Wove length (m!")

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"Camou flage Detection" material; or thesame original picture can be used by additiveprojectio~ techniques to produce approxi-

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FIG. 5. Typical sensitometric curves for KodakEktachrome Aero Film (Process E-3) and for theolder product.

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between the two spectral regions of interest.However, two-color systems cannot produceas wide a gamut of colors, or approximate theappearance of colors, as well as can be pro­duced by a three-color system; also if neutralsmust be reproduced from neutrals in the orig­inal scene, the colors produced in the two­color system will be still less satisfactory. Inthe experimental work conducted by theEastman Kodak Company which led to themanufacture of this material, two-color filmswere actually produced and tested; but froma side-by-side comparison between photo­graphs taken by the two-color and the three­color types of film, the military and scientificpeople concerned chose the three-color ma­terial.

In a similar manner, any two (or more)different portions of the spectrum, whetherin the visual, ultraviolet, or infrared regions,can be selected to emphasize any spectraldifferences occurring in objects of interest. Ingeneral, each different portion of the spectrumused will require a particular combination offilter absorptions and emulsion sensitivities ifthe false-color technique is to be employed.Special multiple emulsion-coated materials,as was the case with the "Camouflage Detec­tion" type of film, are not required, of course,because readily available conventional ma­terials can be used with suitable techniques toproduce the same visual end result. For in­stance, two pictures of a scene taken simul­taneously-one on infrared-sensitive filmwith the visual region absorbed by a filter, andthe other taken on a panchromatic film-canbe used to prepare a color print (or trans­parency) exhibiting approximately the samedifferential between green foliage and visuallysimilar green paint as is obtained with the

FIG. 4. Spectral densities of the dyes of KodakEktachrome Aero Film (Process E-3) and KodakEktachrome Infrared Aero Film (Process E-3).

COLOR AND FALSE-COLOR FILMS 9S

FIG. 7. Typical sensitometric curves for KodakEktachrome Infrared Aero Film (Process E-3) andfor the older product.

times as fast as the older product, and hasimproved definition and less granularity.Typical sensitivity curves of the three layersare displayed in Figure 6, spectral character­istics of the dyes produced are plotted inFigure 4, and sensi tometric characteristics ofthe old and new product are compared inFigure 7.

Typical exposure conditions for the twonew materials under normal daylight illumi­nation are: Kodak Ektachrome Aero Film,1/250 second at f/ll; and Kodak EktachromeInfrared Aero Film with a Kodak \\'rattenFilter No. 15 over the lens, 1/250 second atf/8. The aerial indexes, which are for use withthe Kodak Aerial Exposure Computer, are:25 for the Ektachrome product and 10 forthe Ektachrome Infrared product.

Figure 8-the insert-demonstrates thedifference in color reproduction between thetwo types of film and the improved definitionand color quality of the two new products.The area reproduced is a portion of Durand­Eastman Park, Rochester, New York. Thephotographs were made from 8,000 feet alti­tude on days of average haze using a 35 1111ll

camera equipped with a 50mm focal-lengthlens and with the recommended filters in­stalled. The same section of the frames hasbeen enlarged to illustrate the improvementin definition obtained \\"ith the new products.The Ektachrome Infrared Aero Film and theEktachrome Aero Film (CamouAage Detec­tion) were exposed September 24, 1962. Thetwo Ektachrome Aero Films were exposedOctober 19, 1962, after some leaves had be­gun to change color.

REFERENCES

1. Evans, R. !VI., "V. T. Hanson, J r., and \V. L.Brewer, "Principles of Color Photography,"J. Wiley & Sons, Inc., New York, N. Y., 1953.

2. Tupper, J. L. and C. N. Nelson, "The Effect ofAtmospheric Haze," PHOTOGRAPHIC ENGINEER­ING, Vol. VI, No.2, 1955, pp. 116-126.

3. Clark, \V., "Photographic Materials for AerialPhotography," PHOTOGRAMMETRIC ENGINEER­ING, Vol. X, 1944, pp. 132-135.

4. Tarkington, R. G., "An Aspect of Color Photog­raphy and Interpretation," PHOTOGRAMMETRICENGINEERING, Vol. XIX, June 1953, pp. 418­420.

5. " 'False Color' Film for Aerial Photography,"International Photographer, Vol. 19, December1947, p. 18.

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mately the same result. If desired, threerecords can be made if the result would bemore satisfactory.

In the last few years, significant advanceshave been made in color photographic tech­nology which have provided improved speedand definition characteristics, color reproduc­tion characteristics, and simplicity of han­dling of the materials. These gains have beenapplied to the two aerial color films discussed;improved products, Kodak Ektachrome AeroFilm (Process E-3) and Kodak EktachromeInfrared Aero Film (Process E-3), are nowoffered by the Eastman Kodak Company asreplacements for the previous products.

The new Ektachrome Aero Film is threetimes as fast as the older product and has im­proved definition and less granularity. Typi­cal sensitivity curves of the three layers aredisplayed in Figure 3, spectral characteristicsof the dyes produced are plotted in Figure 4,and sensitometric characteristics of the oldand new product are compared in Figure 5.The Ektachrome Infrared Aero Film is three

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