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Color Filters or Color Screens
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becomes redder and less luminous, as shown in Fig. 9, in which curve I represents the absorption of a gelatin film containing fast red of a density of 0.1, whilst curve II shows two such films, and III that of four films. It will be seen that there is a slight shift in the red absorption from a to a1 and a2 ; but with increasing concentration, or what comes to the same thing, increasing thicknesses of dyed film, this shift becomes less and less till a point is reached at which there is no longer any further absorption. This point is known as the limiting absorption, and the resultant
Fig. 9.
color as the limiting color. The importance of this is well seen in the making of dark-room screens. Suppose for instance, it is desired to make a screen with a limit of transmission of wave-length 6500. It will at once be seen that it is useless to use such a dye as rose Bengal, which has the absorption limit of 6000; for no matter how much the concentration of the dye be increased this limit will not be overstepped. One must then have recourse to some other dye with an absorption limit further in the red, like one of the violets. In the case of the yellow filter mentioned above, while there is practically no shift of the absorption, the color does change and becomes with increasing depth of filter less greenish, till a pure yellow is obtained.
One of the disadvantages of unduly increasing the thicknesses of the dye is the chance of making the color blackish, that is to say, there are very few dyes of such purity of color that they transmit absolutely unweakened their own particular color. This is specially noticeable with the greens and blues, with which even double the thickness will cause an increase of blackness that is noticeable visually. This naturally means that there is loss of light, which in most cases is that which should not be reduced.
It may also happen that a dye possesses absorption bands that are not contiguous, then the increase of the concentration may so reduce the light at one point more than another, that there is distinct change of color. This occurs with many green and blue dyes, which in somewhat dilute solutions appear blue or green ; but in greater concentrations are distinctly red. This is also due to the fact that the absorptions of nearly all dyes increase much more rapidly on the blue side than on the red, therefore, a somewhat weak blue or green transmission band will be much more rapidly