Definition: | regularly transmitting device used to modify the radiant 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 or neutral grey filters according as 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 coloured 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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