Document ID: chunk:federal_register_of_legislation:F2024C00147:body:0:p63
Version: federal_register_of_legislation:F2024C00147
Segment Type: other
Provision Reference: 
Character Range: 207541–210529

of 127 mm.  The sample and incident beam apertures in the sphere wall shall be of such a size as to admit the entire incident and reflected light beams.  The photodetector shall be located so as not to receive direct light from either the incident or the reflected beams.

       2.4. Electrical characteristics of the photodetector‑indicator unit

        The photodetector output as read on the indicating meter shall be a linear function of the light intensity on 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 with + 2% of full scale, or +10% 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 are 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

       3.1.1. 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% point by swinging the receiver to a position directly on the axis of the light source (see figure/1).

       3.1.2. It may be desired in some cases (such as when measuring low‑reflectivity surfaces) to use an intermediate calibration point (between 0 and 100% 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 making reflectivity measurements.

       3.2. Indirect calibration method

        The indirect calibration method is applicable for those 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. Non‑flat (convex) mirror measurement

        The 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 reference 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, given by the formula:

          Figure 2 - Generalized reflectometer, incorporating an integrating sphere in