Digital Camera Color of Light and Error of the Reciprocity Law

Color film has a color sensibility similar to that of the eye. The common reference is white light: this produces a simultaneous stimulus of the three types of color sensors in the retina, and therefore, should give equal magnitude densities in the three layers of a color emulsion. The problem is that our eyes color sensitivity is variable, while the operation of a color emulsion is not. For this reason, for our eye, white light will be any light whose spectrum includes red, green and blue in more or less balances proportions: Natural light, which is produced by the incandescence of a tungsten filament, or even generated by fluorescent tubes. Unless the color of the light is excessive, the acclimatization mechanism of sight to available light tries, according to memory data, balance the color perception by adapting the group of stimuli received, to the supposedly "constant" tone of familiar objects. For example, if we see a white sheet of paper in daylight, in a tungsten or fluorescent light, the acclimatization mechanism lets us see the sheet as white in the three cases. A digital camera imitates this visual mechanism, since the color sensitivity can be adapted to the color of existing light (5.2.3, 6.4.2).

On the other hand, color film cant take in account the kind of light available. If the exposure of an originally gray tone produces equal densities in the three layers, it's evident that it can be fulfilled for a determined type of white light . For any other composition, a neutral surface reflects the new spectrum without altering it, which give place to different densities in the emulsion; in the final image, gray appears dyed with the predominant color.

The variation between natural white light and artificial is perfectly tolerable for the eye, but can produce a total distortion of the tones in photography. Parting from two basic definitions of white light ( daylight and tungsten ), balanced emulsions are made for one or the other: the tonal reproduction will be balanced as long as we respect the color of light that it has been designed for (6.4.1).

Error of the Reciprocity Law

If the error in the reciprocity law for abnormally low or high intensities is common in all emulsions (4.1.3.2), in color film it is represented also as a tonal unbalance . It is not likely that for very short or very long exposure, the three layers of the emulsion fail in the same amount. The necessary compensation for each layer is usually different which can cause unbalanced densities of the corresponding pigments.