The basis of this proposal is that the complete definition of the chromatic properties of filters used in illumination is transmission as a function of wave-length; an approximate definition suitable for commercial specification should, therefore, take the form of an abbreviated notation for the curve of spectral transmission. It is proposed to specify filters for theatrical lighting by a seven-digit number, the first digit of which refers to spectral transmission at the short wave extreme of the spectrum, the second, that at some longer wave portion, and so on throughout to the long wave extreme. The present paper discusses choices of wave-lengths to which the digits should refer and also what range of transmission should be indicated by digit, 0, what by digit, 1, what by digit, 2, and so on for maximum usefulness of the specification. Recommended choices are presented based upon a spectrophotometric examination by William F. Little and Allan E. Parker of the Electrical Testing Laboratories made upon two groups of theatrical gelatines, one group of 11 pink filters and one group of 11 blue filters.
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