vertical axis through the photometric centre of a luminaire when it is at its tilt during measurement Note 1 to entry: The poles of the (C, γ) coordinate system lie in this axis. Note 2 to entry: The first axis is tilted when the attitude of the luminaire is changed from its attitude during measurement. Note 3 to entry: See CIE 121, The Photometry and Goniophotometry of Luminaires for more information on the first axis of a luminaire.
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