| Definition: | coordinate system commonly used in the photometry of luminaires in which the C-planes are half-planes that rotate round the vertical line through the photometric centre of the luminaire, and the γ-angles are measured up to 180° from the direction of the perpendicular to the rotation axis of the C-planes Note 1 to entry: Other coordinate systems [(A, α) coordinate system and (B, β) coordinate system] exist as well. See CIE 121, The Photometry and Goniophotometry of Luminaires.
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