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Acolour cast is a tint of a particular colour, usually unwanted, that evenly affects a photographic image in whole or in part.[1]
Certain types of light can causefilm anddigital cameras to render a colour cast. Illuminating a subject with light sources of differentcolour temperatures will usually cause colour cast problems in the shadows. Thehuman eye generally does not notice the unnatural colour, because our eyes and brains adjust and compensate for different types of light in ways that cameras cannot.
In film, colour casts can also be caused by problems inphoto development. Improper timing or imbalanced chemical mixtures can cause unwanted casts.
Colour casts can also occur in old photographs due to fading of dyes, particularly under the action ofultraviolet light. These may be correctable on a scanned version of the photograph with image editing techniques.[2]
Most digital cameras try to automatically detect and compensate colour cast and usually have a selection of manually setwhite balance settings to choose from. Otherwise, photo editing programs, such asPhotoshop, often have built incolour correction facilities. For film, bluefilters and amber filters are used to counter casts. Amber filters are used to reduce the blueish tint caused by daylight. Blue filters reduce the orange colour caused by incandescent light.
A variety of coloured filters in varying degrees of intensity are available.Kodak's amber filters, for example, vary from palest yellow ("81C") to deepest amber ("85B"). A photographer chooses which filter to use based on the quality of the ambient light.Colour temperature meters can read the temperature of the existing lighting conditions and guide the selection of the filter. Clouded sky, for example, requires a paler amber than clear blue sky. If a filter is unavailable, using aflash is an alternative solution which usually provides enough neutral white light to counter the cast.
In the case of film, if photographs all contain the same cast, it is usually indicative of improper chemical development. If the film itself does not contain any cast, it can be reused to create another set of photographs in proper chemical conditions. If the film contains a cast, filters can be used duringphoto processing to correct it.
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