half-peak divergence (of a projector, in a specified plane)

angular extent of all the radius vectors of the polar curve of luminous intensity in the specified plane having lengths greater than 50 % of the maximum

Equivalent term used in the US: "one-half-peak spread"

NOTE Outside US "beam spread" relates to the total angle within which the illuminance on a plane normal to the axis of the beam exceeds 10 % of the maximum.

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