False-Colour Photography: Sensitivity and Cross Section of a False Colour Film (Especially for GATE-Geospatial 2022)

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  • It has been mentioned previously that the maintenance of colour fidelity is one of the major requirements of good colour photography, and indeed in some applications, the ‘realistic’ colour rendering is deemed a major advantage. But there is one type of colour photography which deliberately distorts the true colour of the objects in order to achieve better interpretability through even greater colour contrast.
  • This is false-colour photography, which may be regarded as a type of colour photography using a special type of colour film. In essence, this colour film achieves the false-colour effect by means of a ‘colour shift’ . Instead of being sensitized to the three primary colours of blue, green, and red three layers of emulsion of the false-colour film are made sensitized to green, red, and infra-red separately. A yellow (or minus-blue) filter is used at the time of photography to cut out blue light.
  • Upon development, positive images of yellow, magenta and cyan record in the green-, red- and infra-red-sensitive layers respectively. This arrangement is best illustrated by an example of Kodak false-colour film which is also known as the false-colour infra-red: Kodak Ektachrome Infra-red aero film type 8443.
Sensitivity and Cross Section of a False Colour Film

Uses and Advantages of False- Colour Photography

  • This type of film was originally designed for camouflage detection purposes and hence was known as camouflage detection film; a Wratten filter number 12 had to be used. Depending on the proportions of green, red and infra-red radiations reflected or transmitted by the objects, numerous colours which are ‘unrealistic’ of the objects are formed. It should be noted that the film does not react to radiation emitted by terrestrial features. Therefore, the false-colour film may be regarded as a colour counterpart of black-and-violate infra-red film. Kodak has recently replaced Ektachrome Infra-red Aero Film Type 8443 with Aero chrome Infra-red film Type 2443 on Estar base - a reversal film with much higher dimensional stability and better keeping qualities. Another type is aero chrome Infra-red Film Type 3443 on Estar thin base which is a reversal film for forest survey and camouflage detection and replaces Ektachrome Infra-red Aerial Film Type SO-180. It is noteworthy that no negative system for the false-colour film is available from Kodak because it cannot be satisfactorily developed as a negative.
  • The advantage of the false-colour system lies in its modified colour rendition of the subject; because the visible light component is added to the infra-red record the film produces characteristics colours in photographs of many physiological and botanical substances. It is the infra-red component that produces the modified colour. A list of the subjects and their associated colours are shown in below Table.

False-Colour Renditions of Certain Subjects as Produced by Kodak Colour Infra-Red Film

False-Colour Renditions of Certain Subjects as Produced by Kodak Colour Infra-Red Film Table
SubjectColour
1 Healthy, deciduous, green foliagered, magenta
2 Diseased or deficient foliagegreenish, bluish
3 Badly stressed foliageyellow
4 Conifersdark purple
5 Evergreensred-brown
6 Red roseyellow
7 Blue flowersyellow
8 Blue skysky blue
9 Some green dyesmagenta
10 Some green pigmentspurple
11 Some black clothdark red
12 Dolomite-limestonegrey-brown
13 Khaki clothorange-red
  • It is obvious that false-colour photography is best applied to practical forestry and agricultural land-use studies. A detailed discussion of its advantages in forestry is given by Stellingwerf, who applied it to study trees in the temperate latitudes. One of these advantages is the fact that healthy trees can be separated from diseased trees through colour differentiation, even though crown symptoms of the disease may not be visible.
  • This is distinctly superior to the true-colour film which can differentiate between healthy and diseased trees only where crown symptoms of the disease are visible. Healthy broadleaved species, which reflect mainly in the infra-red range and to a small extent in the green range, are, with transmitted light, photographed as red. On the other hand, conifers, which have less infra-red and more green re-mission, give, with transmitted light, an image containing less red and bluer. However, Benson and Sims, working in Australia, strongly queried the ability of false-colour film to detect early or ‘previsual’ symptoms of the disease of trees and tended to support the use of true-colour photography for damage and mortality determination.

Case Study of False- Colour Photography

  • Colwell did point out that previsual symptoms of many but not all plant diseases were discernible on photography sensitized to the infra-red wavelength of about 700 to 900nm. 62 Hildebrandt and Kennweg, working in Germany, confirmed that different colour renditions occurred in accordance with the different condition of the trees. Thus, they discovered that (a) defoliated trees always appeared in a blue or green colour; (b) dead but not defoliated trees did not always appear blue or green, but maybe greyish straw colour or greenish grey, depending on the time when the tree was killed; and (c) injured trees which still have a completely and dominantly live foliage never appear blue or green, but will vary ranging from yellow through grey and brown to pink inclusive of all possible transition colours.
  • This difference in idea may be due to the fact that in Australia the dominant vegetation is the eucalypt which has little or no spongy mesophyll tissue in its leaves and is therefore not sensitive to infra-red radiation as are other vegetation types in temperate areas. More recently, Wiegand et al have found that the optical behaviour of leaves in the 0.75 - 1.3511m near-infrared band is one of high reflectance and low absorptance affected by leaf mesophyll structure, whilst the J. 35- the 2.5pm band is influenced more by the optical properties of water in the tissue than by the leaf structure.
  • They therefore concluded that vegetation discrimination and stress detection were most effective in spectral hands centred at 0.57,0.65,0.68,0.85,1.65,2.0, and 2.1 or 2.20µm. Another recent piece of work in the use of aerial photographs for detection of bark beetle infestations of spruce in Sweden has shown the importance of the time factor. ″ Its authors found that one could detect attacks 7 - 14 days old and that the changes in the trees showed up faster during the later periods of attacks. The general conclusion, however, was that colour firms were superior to black-and-white and that the colour infra-red film was better than normal colour film for the detection of infested trees.

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