regularly transmitting device used to modify the radiant flux or luminous flux, the relative spectral distribution, or both, of the radiation passing through it

NOTE A distinction is made between selective filters and non-selective filters or neutral filters according to whether they do or do not alter the relative spectral distribution of the radiation. A selective filter that makes a significant change in the chromaticity of the radiation is called a colour filter; one that alters the spectral distribution but, because of metamerism, transmits radiation of nearly the same chromaticity as that of the incident radiation, may be called a grey filter.

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