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Light Filters — Jones 147
is great, or conversely the condition described as "sharp cut" appHes to the case where a relatively small change in wave-length is accompanied by a large change in absorption.
Curve B applies to a piece of amber glass and to such a filter the descriptive term "gradual cut" is applied. It will be noted that the wave-length band over which the change from its minimum to maximum density occurs is very broad, extending from 600 m/x to 300 m/x. The slope of the absorption curve in this region of variable absorption is low and hence the filter is described as one having a "gradual cut." The transmission of this filter for the wave-length it transmits most freely is very low, being approximately 50 per cent (density = 0.3) in the region from 600 to 800 m/i. Filter A has a bright yellow color, while B has a hue slightly more orange and exhibits a dull "muddy" appearance. This term "muddy" is also used frequently as descriptive of light filters and indicates a relatively high general absorption for all wave-lengths in the visible region. The muddy appearance may be considered as due to an admixture of black in the filter. For instance let the dotted curve B' represent a filter having an absorption curve similar in shape to that of B but for which the density at all wave-lengths is .24 less than that of B. The maximum transmission of 5', in the wave-length band from 600 to 690 m/x, is 90 per cent and such a filter has a clean brilliant appearance although the dominant wave-length is somewhat longer than in the case of filter A thus giving filter B' a hue which is more orange. Now suppose that to this filter (curve B') is added a black dye, represented by curve C of such concentration as to give a density of 0.24 at all wave-lengths. The addition of C to B' gives B, and the B' filter is changed thereby from a clear brilliant yellow-orange to a dull "muddy" amber. "Muddiness" in a filter is therefore due to something equivalent to the addition of a black component and is an indication of high absorption in the wave-length region of maximum transmission and hence of low optical efficiency.
Filter Factor
When a filter which absorbs some of the radiation to which the photographic material is sensitive is placed over the lens of the camera, it is evident that an increase either in exposure time, in the lens aperture, or in the illumination incident on the object, must be made in order to obtain the same exposure on the negative as when no filter is used. If any two of these factors are constant then the ratio