Document ID: chunk:federal_register_of_legislation:F2015C00542:reg:254:p11
Version: federal_register_of_legislation:F2015C00542
Segment Type: reg
Provision Reference: reg 254 (pt 11/18)
Character Range: 128071–130894

appropriate accuracy.

              9.1.1.2. Optical system consisting of a lens with a focal length f of at least 500 mm and corrected for chromatic aberrations. The full aperture of the lens shall not exceed f/20. The distance between the lens and the light source shall be adjusted in order to obtain a light beam which is substantially parallel. A diaphragm shall be inserted to limit the diameter of the light beam to
              7 mm ± 1 mm. This diaphragm shall be situated at a distance of
              100 mm ± 50 mm from the lens on the side remote from the light source. The point of measurement shall be taken at the centre of the light beam.

              9.1.1.3. Measuring equipment

               The receiver shall have a relative spectral sensitivity in substantial agreement with the relative spectral luminous efficiency for the ICI[15] standard photometric observer for photopic vision. The sensitive surface of the receiver shall be covered with a diffusing medium and shall have at least twice the cross-section of the light beam emitted by the optical system. If an integrating sphere is used, the aperture of the sphere shall have a cross-sectional area at least twice that of the parallel portion of the beam.

               The linearity of the receiver and the associated indicating instrument shall be better than 2 per cent of the effective part of the scale.

               The receiver shall be centred on the axis of the light beam.

              9.1.2. Procedure

               The sensitivity of the measuring system shall be adjusted in such a way that the instrument indicating the response of the receiver indicates 100 divisions when the test piece is not inserted in the light path. When no light is falling on the receiver, the instrument shall read zero.

               Place the test piece at a distance from the receiver equal to approximately five times the diameter of the receiver. Insert the safety-glass pane between the diaphragm and the receiver and adjust its orientation in such a way that the angle of incidence of the light beam is equal to 0 ± 5°. The regular transmittance shall be measured on the test piece, and for every point measured the number of divisions, n, shown on the indicating instrument, shall be read. The regular transmittance r is equal to n/100.

              9.1.2.1. In the case of windscreens, alternative test methods may be applied using either a test piece cut from the flattest part of a windscreen or a specifically-prepared flat square with material and thickness characteristics identical to those of the actual windscreen, the measurements being taken normal (perpendicular) to the glass pane.

              9.1.2.2. For the windscreens of M1 vehicles[16]  the test shall be carried out in test area B defined in Annex 18,