Document ID: chunk:federal_register_of_legislation:F2023L01006:front:0:p26
Version: federal_register_of_legislation:F2023L01006
Segment Type: other
Provision Reference: 
Character Range: 72533–75527

photodetector output as read on the indicating meter shall be a linear function of the light intensity of the photosensitive area. Means (electrical and/or optical) shall be provided to facilitate zeroing and calibration adjustments. Such means shall not affect the linearity or the spectral characteristics of the instrument. The accuracy of the receptor indicator unit shall be within ±2 per cent of full scale, or ±10 per cent of the magnitude of the reading, whichever is the smaller.
         2.5. Sample holder
         The mechanism shall be capable of locating the test sample so that the axes of the source arm and receptor intersect 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.