On its arrival at the test surface the incident beam must have a diameter of at least 19 mm. The reflected beam must not be wider than the sensitive surface of the photoelectric cell, must not cover less than 50 % of that surface and must, if possible, cover the same portion of the surface as the beam used to calibrate the instrument.

The photoelectric cell must be positioned in such a way as not directly to receive the light from the incident or reflected beams.

Whereas it is desirable to take into account the technical requirements adopted by the UN Economic Commission for Europe in its Regulations No 1 ("Uniform provisions for the approval of motor vehicle headlights emitting an asymmetrical passing beam or a driving beam or both") (5) and No 2 ("Uniform provisions concerning approval of incandescent electrical lamps for headlights emitting an asymmetrical passing beam or a driving beam or both") (5), which are annexed to the Agreement of 20 March 1958 concerning the adoption of uniform conditions for approval and reciprocal recognition of approval for motor vehicles equipment and parts;