nurk läbi valgusvihu telge läbiva tasandi kahe kujuteldava joone vahel, mis läbivad lambi esipinna keskpunkti ja punkte, milles valgustugevus on 50 % valgusvihu keskkiire valgustugevusest, angle between two imaginary lines in a plane through the optical beam axis, such that these lines passthrough the centre of the front face of the lamp and through points at which the luminous intensity is50 % of the centre beam intensity
