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146
Transactions of S.M.P.E., August 1927
spectral region by superposing two or more separate filters. This is due in part to the increasing loss in surface reflections as the number of separate filters is increased. Furthermore each filter is designed by the manufacturer to give some specific spectral absorption with maximum efficiency and to this end the best possible available dyes are selected. If some entirely different spectral absorption is required it is probable that dyes can be selected which will function
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Fig. 6. Spectrophotometric curves illustrating "sharp cut" A, gradual cut "inefficient" filter B, and gradual cut efficient filter B'.
with greater efficiency than can be obtained by combining two filters designed specifically to meet other requirements.
The terms "sharp cut" and "gradual cut" are frequently applied as descriptive of light filters. The significance of these terms may be illustrated by reference to Fig. 6. Curve A is the spectrophotometric curve of a brilHant yellow gelatine filter. Its density at all wavelengths greater than 480 mju is 0.1 (transmission = 86%). The absorption at wave-lengths less than 480 m/>t increases rapidly so that at 460 mjLt its density is 1.5 (transmission = 3.1%). Such a filter is described as a "sharp cut" filter. It is evident therefore that the term "sharp cut" applies to a filter of which the absorption curve is steep, that is the rate of change of absorption with variation in wave-length