Black-And-White Infra-Red Photography, Sensitivity and Cross Section of a True Colure Film (Especially for GATE-Geospatial 2022)

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Title: Black-And-White Infra-Red Photography

  • This utilises the conventional camera lens to focus an infra-red ( ‘below the red’ ) image on to a photographic emulsion sensitised to infra-red radiation so as to produce a black-and-white negative record and subsequently a positive print. This infrared radiation is therefore photographically actinic and lies in the range of wavelengths from 700nm to about 1,350nm in the electromagnetic spectrum. There is another type of infra-red radiation with wavelengths longer than about 1,350nm which exists as heat patterns and can only be imaged by non-photographic means.
  • Because of the wide range of wavelengths covered, the infra-red emulsions are sensitive to violet, blue and red light of the visible spectrum in addition to infrared. A minus-blue (i.e.. yellow) or a red filter of 600nm (eg Wratten 25A) or of 680nm (eg Wratten 89A) has to be employed over the camera lens at the time of photography to eliminate these undesirable light rays so that only the infra-red radiation is recorded. The energy so recorded may be transmitted, reflected and/or emitted from the object according to its atomic and molecular structure.
  • It should be noted that owing to the longer wavelength of infra-red compared with the visible light, it focuses further from the film plane, thus requiring the visible focus to be adjusted. Modern aerial camera lenses such as the Wild Universal Aviogon ′ lens and the Zeiss (Oberkochen) Pleogon A lens have been so well designed that they are chromatically corrected for the infra-red part as well as the visible part of the spectrum and are therefore free of these troubles.
  • Certain features of infra-red films are particularly outstanding, including the following:
    • Special reflection properties of vegetation caused by the fact that the epidermis and pigments of the leaf are very transparent to infra-red radiation, thus allowing it free access to the leaf mesophyll (including both palisade and spongy parenchyma cells) from which the near infra-red radiation is reflected and scattered.
    • Better haze penetration ability which is made possible by the film՚s sensitivity to radiation of much longer wavelengths so that the scattering effect of the dust particles in the atmosphere is very much reduced.
    • A high degree of absorption of infra-red light by water bodies rendering water features pitch-black on infra-red film.
    • Black shadow effect which is due to the insensitivity of the infra-red emulsion to diffuse or polarised light.
  • All these characteristics can be usefully exploited either singly or in combination in the following fields of application:
    • Plant ecology,
    • Soil and hydrology,
    • Geology, and
    • Glacial geomorphology.
  • All photography using monochrome films fails to give a true picture of our terrestrial environment in the sense that it cannot depict colour as we see it. Strandberg pointed out that many features in the environment have unique colours associated with them such as green grass yellow corn, blue water, golden wheat and scarlet maples. The addition of a new dimension of hue to photography, therefore, eliminates much of the feeling of unreality that occurs when one attempts to search for the significance of the various shades of grey during interpretation. It has been pointed out that as the human eye can separate 20,000 hues and chromas, but only 200 shades of grey, much more information can be obtained from colour photography than from black-and-white photography.
  • The colour that we see is really light of different wavelengths inside the visible spectrum (i.e.. 400 - 750nm) which has been modified by reflectance from or transmittance through some substance and reaches the retina, the light-sensitive surface of the eye. The retina consists of a fine array of light-sensitive cells which appear in two forms: rod-shaped and cone-shaped. The rods, which are much more sensitive to light, are not sensitive to colour and can only discern objects in terms of light and shade. On the other hand, the cones can operate at higher brightness levels to sense colour. It is interesting to note that colour sensation is produced by the response of only three types of receptor in the cones, one sensitive to blue light, one to green, and one to red. These receptors are affected in different proportions in different colours; and the blue, green, and red are called primary colours because they cannot be made from the other colours.
  • This is the principle utilised for the production of colour films which makes colour photography possible. In general, one can distinguish two basic techniques of colour sensation by our eyes:
    • The additive method, and
    • The subtractive.
  • The additive method simply mixes the three primary colours of blue, green and red in appropriate proportions. It would be possible to photograph an object three times on panchromatic emulsions through a blue, green and red filter separately. Each of the negatives so produced would be a record of one primary colour in the original object; and by changing each into positive transparency and projecting each through the appropriate filter in superimposition on a screen, the colour of the original object photographed would be reproduced. In this process, it is obvious that there is a large loss of light involved and the white colour cannot be so easily reproduced.
  • The subtractive method may be regarded as the reverse of the additive method. Instead of starting with the three primary colours, it starts with the white light, and by taking out varying proportions of the primary colours from the white light independently, a wide range of colours can be reproduced. This is achieved by using absorbing dyes having colours which are complementary to the primary colours. Thus, it was discovered that a yellow dye absorbs blue, the magenta (purple) dye absorbs green and a cyan (blue-green) dye absorbs red.
  • The subtractive method is superior to the additive method as it largely overcomes the difficulties experienced in the latter. Modern colour films are therefore constructed on this principle. A true colour film is made up of three layers of emulsion, one for each of the three primary colours, blue, green and red, in the order from top to base.
Sensitivity and Cross Section of a True Colure Film
  • Since the green and red layers are also sensitive to blue light, a yellow filter is inserted between the blue and green layers to eliminate the undesirable blue light. After exposure, each layer is developed out in the three complementary colours of yellow, magenta and cyan corresponding to the colour sensitivity of each layer.

Two Systems of Colour Film Processing

  • There are two systems of colour film processing: the colour positive or reversal system and the colour negative system. The positive or reversal process involves first of all black-and-white development, followed by re-exposure of the residual silver halide in each layer with diffuse light, and then colour development with the production of subtractive colour dyes. All the remaining silver halides and metallic silver are removed in a bleach bath followed by fixation of the image.
The Method of Reversal Olour Processing
  • The negative process does not require the black-and-white development and re-exposure.
  • The colour forming compounds have already been included in the respective light-sensitive layers during manufacture and a single stage of colour development produces the complementary colour in each layer.
The Method of Negative Colour Processing
  • There are colour films which can be processed to positives or negatives as desired, eg Kodak Ektachrome which is normally intended as a reversal film but can be processed to a negative by the Aero-Neg process.
  • The subtractive method also has its limitations in practice. The major one is the difficulty in obtaining subtractive dyes having the required colour purity, and the method called colour correction masking has to be used. Usually, the magenta dye is the one which most needs colour correction.

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