Materials having a very low reflectance over a large spectral region are necessary for many scientific application, especially in aerospace, spectrophotometry, spectroradiometry, and photography. This article surveys black absorbing materials and devices being used throughout the industry. Also described is a new black backing device with reflectances of less than 0.1% i.e., 99.9% absorptance, that can be used as a sample backing and for determining the zero photometric scale in spectrophotometry and photometry. The reflectance of the device is nearly perfectly flat between 0.200 and 2.5 μm.
The determination of optical absorption in turbid media requires optical integrators to collect all the light scattered by the sample. If under these measurement conditions the sample fluoresces, its emission is collected as scattered light by the integrator and produces a signal of the opposite sign to that of an absorption. The magnitude of this effect is a function of the luminescent quantum efficiency of the sample and the spectral response of the detector. The implications of the effects of fluorescence on the absorption measurements of turbid media are discussed, along with a method for direct measurement of absorptance in a 4 π geometry. In the presence of turbidity, the measurement of regular transmittance is not valid if obtained with very small angles of detection. It is perfectly valid, however, if a detection acceptance angle of 4 π steradians is used.
This article describes the techniques for measuring specular (regular) reflectance of mirrors on an absolute basis. Two independent methods of measuring mirrors are described, and data are presented for the two methods of measurement in the range 400–1500 nm.
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