ISOCHROMATIC PHOTOGRAPHY Photography in which colours are rendered in a monochrome picture according to their true visual brightness. Known also as orthochro matic photography. It is well known that yellow or red objects appear very dark in an ordinary photograph. The ordinary plate is chiefly affected by violet and blue rays of light, and is comparatively insensitive to green, yellow, orange, and red rays. Hence, blue objects impress the plate far too strongly, comparatively, the result being that they appear much too light in the photographic print. This serious fault is very evident in photographs of vividly coloured objects, and isochromatic methods are adopted to overcome it. The plates used (see " Isochro matic Plates ") have been treated in a way that renders them colour sensitive," but as plates made sensitive to green, yellow, and orange-red rays are still too sensitive to blue-violet, a com pensating screen is placed in front of the lens, so that the blue-violet rays are lessened in intensity. A further function of the screen is that it entirely absorbs the ultra-violet rays, which otherwise affect the plate ; and as they are invisible, it is of course necessary that they play no part in the formation of the photographic image.
Vogel and others discovered that by mixing certain aniline dyes with the silver bromide emulsion of a plate, the latter becomes sensitive to the colours absorbed by the dye (or the dyed silver bromide particles). It is hence possible to adapt a dyed or " isochromatic " plate and a screen so that the combination will result in objects of all colours being represented in the print in tones of visual brightness corre sponding. Yellow-green being the brightest colour visually, objects of this colour will appear lightest in the print, and so on. Isochromatic photography as usually practised is more par ticularly a rough compromise, as to give accur ate colour records too long exposures would be necessary, owing to the slowing effect of the yellow screen. (See also under the headings Isochromatic Plates," " Isochromatic Screens," and " Monochrome, Rendering Colours in.")