It is often claimed that silicon photodiodes are linear over many decades, but very little evidence on the experimental verification can be found in the literature. Therefore, an effort was made to investigate the linearity of Si photodiodes over as many decades as possible. Optically, the experiments are based on a cascaded use of the linearity tester described by Sanders, which utilizes the double-aperture method. The electronics include an operational amplifier, a voltage-to-frequency converter, and a counter. Details of the optical and electronic instrumentation will be presented. The measurements are extended to cover a range of about 9 decades, from 10 mA to about 10 pA. Measurements were made on Si photodiodes from various manufacturers, and the results are given.
M easurements of the diffuse spectral reflectance are usually not made as direct measurement of the incident and the reflected r adi ant flux but r ather as measurements relative to a standard of known reflectance value.For the calibration of such standards, different m ethods have been described in the literature :1. Goniophotometric methods, also called Indicatrix methods or point-by-point m ethods. 2. Methods based on the Kubelka-Munk theory. 3. Integr ating sphere methods according to T aylor, Benford, Sharp-Little, van den Akker, Korte. Various materials such as magnesium oxide, barium sulfate or opal glass ar e being used as standards. Their suitability as transfer or as working standards will be discu sed.The r esults of comparative m easurements between some of these methods will be given.
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