2.9 Film for Remote Sensing

Various type of films are used in cameras for remote sensing. Film can record the electromagnetic energy reflected from objects in the form of optical density in an emulsion placed on a film base of polyester. There are panchromatic (black and white film), infrared film, color film, color infrared film, etc.

The spectral sensitivity of film is different depending on the film type. Black and white infrared film has wider sensitivity up to near infrared as compared with panchromatic film. Color film has three different spectral sensitivities according to three layers of primary color emulsion (B,G, R). Color infrared film has sensitivity up to 900 nm. Kodak aerial color film SO-242 has high resolution and is specially ordered for high altitude photography.

Generally film is composed of a photographic emulsion which records various gray levels from white to black according to the reflectivity of objects.

A curve which shows the relationship between the exposure E (meter caudera second) and the photographic density is called the "characteristic curve". Usually the horizontal axis of the curve is log E, while the vertical axis is D (density) which is given as follow.

D = log (1 / T ) where T : transparency of film

The characteristic curve is composed of three parts of toe, straight line and shoulder. Gamma is defined as the gradient of the straight line part, which is an index of contrast. If is given by D / logE which gives high contrast in the case of gamma larger than 1.0, and low contrast in the case of gamma smaller than 1.0.

The sensitivity of a photographic emulsion is defined as the minimum exposure to give the minimum recognizable density. In the definition of JIS (Japan Industrial Standard), the sensitivity is given as log (1 / EA ) under the conditions of exposure and development density as denoted ( EA, EB ) and ( A, B ) respectively,

where A : the gross fog + 0.1 EB : EA + 1.5 0.75 < B - A < 0.9

The spectral sensitivity of photographic emulsion (S) represents the sensitivity with respect to each wavelength, which is usually given in the form of a spectral sensitivity curve. In the spectral sensitivity curve log S is used instead of S as the vertical axis, while sometimes relative sensitivity is used.

Figure 2.9.3 (a) - (d) show the spectral sensitivity curves corresponding to panchromatic, infrared,color and color infrared films respectively.


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