Reflected infrared photography:
Films page 2

Infrared 'false' colour film

Eastman Kodak have a long history and commitment to the manufacture and supply of a false colour infrared sensitive colour transparency film. Originally developed during the Second World War for camouflage detection the idea has survived over sixty years through various configurations and changes to Ektachrome processing (although periodically going out of production or being withdrawn). Walter Clark of the Eastman Kodak Company and one of the founders of infrared photography described the history thus: "Also in these days (1940s) we researched the aerial photography and the potential of camouflage recognition. We considered the possibility of combining IR-characteristics with colour contrasts. I believe that the first thoughts about a two-colour separation camera working according to the Technicolor principles originated from D.A. Spencer and A. Marriage at Kodak in London. This idea was then transferred to Kodak Research Laboratories in the US, the persons involved were Mannes, Jelly, Wilder, Copstaff and me. We then considered the use of the multiple layer colour film, in which one layer should be infrared-sensitive. Many combinations were tested, in particular a two-layer/two-colour system and two three-layer/three-colour films. One of these three-layer films was then selected, and released to the US Air Forces as Kodacolor Aero-Reversal-Film for camouflage recognition." The modern version of this is the Ektachrome Professional Infrared EIR film.

Some workers believe that colour infrared film is more useful than black-and-white, since colour contrast is easier detected than grey-scale contrast and many publications testify to the usefulness of this film (see References and Biomedical applications). Others find the false colour representation difficult to interpret - infrared yields a red image, green records as blue, and red as green. Chlorophyll in living plants reflects both green and infrared, and therefore appears magenta on this film. Many other subjects appear with characteristic colours such as melanin that records red-brown, arterial blood that reproduces green, cholesterol and collagen that reproduce blue and so on (Figure 41).

Figure 41 (left). The characteristic appearance of Kodak's false colour infrared film, with chlorophyll appearing magenta and blood green.

Normal colour films have three light sensitive layers sensitized to record blue, green and red; during processing each layer produces a dye of complementary colour - yellow, magenta and cyan. When white light is passed through the three layers of the processed transparency the viewer sees a close visual reproduction of the colours of the original scene. In the case of infrared Ektachrome the three layers are sensitized to green, red and infrared (all three layers are inherently sensitive to blue and ultraviolet and therefore a deep yellow barrier filter must always be used with this film. Infrared reproduces as red, green as blue and red as green. Numerous other colours are formed of course depending on the proportions of green, red and infrared reflected by the subject. Figure 42 illustrates diagrammatically how this film works.

Figure 42 (above). A demonstration of how the layers in the infrared Ektachrome 'False-colour' film respond to different frequencies of light. The three layers are sensitized to green, red and infrared (all three layers are inherently sensitive to blue and ultraviolet and therefore a deep yellow barrier filter must always be used with this film. Infrared reproduces as red, green as blue and red as green. Numerous other colours are formed of course depending on the proportions of green, red and infrared reflected by the subject.

Kodak EIR is a true infrared film with a peak spectral sensitivity at 730nm and good sensitivity extending to 900nm (Figure 43). Like its black-and-white counterparts - Kodak HIE and Maco 820c - it must therefore be handled in total darkness, including loading and unloading the camera. It is coated onto a 4mil (0.101mm) Estar base with anti-halation backing. It is relatively fast - 200 ISO in daylight with the Wratten 12 filter - has fine grain and medium resolution characteristics. Storage conditions for this film are exacting - Kodak recommends storage in original packaging at -18° C to -23° C, and the film must be processed as soon as is practical after exposure. Unexposed film will not tolerate more than one week at room temperature. Exposed film must not be kept longer than two days at room temperature - it should be returned to refrigeration. Adverse storage conditions affect the three layers of this film differently, and cause a significant change in colour balance. A loss of sensitivity and a colour shift towards cyan typically indicate poor storage conditions.

Figure 43. The spectral sensitivity curve for Kodak EIR infrared Ektachrome false colour reversal film.

At exposure times of between 1/100th and 1/1000th of a second no filter or exposure correction is required for reciprocity effects but outside of this range the film starts to require considerable correction for reciprocity failure. The three layers of the film all behave differently. As a guide at 1/10th of a second the exposure needs to be increased by one stop and CC20B added to the Wratten 12 yellow filtration.

The choice of film will therefore depend on the application and the need for extended spectral response. Where the furthest penetration into the infrared spectrum is required (850 - 950nm) Kodak HIE is the only choice. Where the spectral range can be limited to less than 850nm the improved granularity and handling of the Maco 820c makes it the film of preference. In the far red range (700 - 800nm ) the Konica product comes into its own, whilst in the extended red region 700-750nm (which is actually where most amateur photographers operate), one has the additional choice of the Ilford and Agfa products. For false colour imaging there is only one option - the Kodak EIR (although there are electronic ways of achieving the same effect - see electronic recording of the infrared image).

Storage of infrared films is critical; their relative speed and contrast will deteriorate rapidly if they are not kept properly. Unexposed film, in the original sealed containers, must be kept in a refrigerator at 13° C or colder. Ideally they should be bulk stored in a freezer, removing them to a refrigerator a day or so before use and then to cool room temperatures for 6 hours before use. Exposed film should be kept cool and dry and processed as soon as possible. If exposed film must be kept for some time before processing it should be returned to the refrigerator and stored at a temperature lower than four degrees Centigrade. Even when refrigerated, the shelf life of infrared emulsions is limited, so care must be taken to ensure that only fresh stock is used.

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