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at the reflecting surface. The reflecting surface may lie within or at either face of the mirror sample, depending on whether it is a first surface, second surface or prismatic "flip" type mirror.

       3. Procedure

       3.1. Direct calibration method

        In the direct calibration method, air is used as the reference standard. This method is applicable for those instruments, which are so constructed as to permit calibration at the 100 per cent point by swinging the receiver to a position directly on the axis of the light source (see Figure 1).

        It may be desired in some cases (such as when measuring low-reflectivity surfaces) to use an intermediate calibration point (between 0 and 100  per cent on the scale) with this method. In these cases, a neutral density filter of known transmittance shall be inserted in the optical path, and the calibration control shall then be adjusted until the meter reads the percentage transmission of the neutral density filter. This filter shall be removed before reflectivity measurements are performed.

       3.2. Indirect calibration method

        The indirect calibration method is applicable in the case of instruments with fixed source and receiver geometry. A properly calibrated and maintained reflectance standard is required. This reference standard should preferably be a flat mirror with a reflectance value as near as possible to that of the test samples.

       3.3. Flat mirror measurement

        The reflectance of flat mirror samples can be measured on instruments employing either the direct or the indirect calibration method. The reflectance value is read directly from the indicating meter.

       3.4. Non-flat (convex) mirror measurement

        Measurement of the reflectance of non-flat (convex) mirrors requires the use of instruments which incorporate an integrating sphere in the receiver unit (see Figure 2). If the instrument-indicating meter indicates ne divisions with a standard mirror of E per cent reflectance, then, with a mirror of unknown reflectance, nx divisions will correspond to a reflectance of X per cent, in accordance with the formula:

Figure 1
Generalised reflectometer showing experimental set-ups for the two calibration methods

Figure 2
Generalised reflectometer, incorporating an integrating sphere in the receiver

              4. Spectral tristimulus values for the CIE 1931 standard
              colormetric observer[11]

              This table is taken from CIE publication 50 (45) (1970)

Explanatory figure
    Example of device for measuring the reflection factor of spherical mirrors

C    =  Receiver
D    =  Diaphragm
E    =  Window of entry
F    =  Window of measurement
L    =  Lens
M    =  Object window
S    =  Light source
(S)  =  Integrating sphere

Annex 7

Procedure for determining the radius of curvature "r" of the reflecting surface of a mirror

       1. Measurement

       1.1. Equipment

        A "spherometer" similar to the one described in Figure 1 of this annex having the indicated distances