Conic mirror reflectometers

REFLECTANCE MEASUREMENTS OF DIFFUSING SURFACES USING CONIC MIRROR REFLECTOMETERS... 

Throughput (ratio of output flux to input flux) of conic mirror reflectometer for sample... 

A fundamental difference between the conic mirror and integrating sphere devices is in how absolute reflectance values are determined. Absolute measurements in conic mirror reflectometers are direct and simple. Sources of error, as discussed in Section V, need to be accounted for. However, absolute reflectance values from measurements made with integrating sphere reflectometers are based on integrating sphere theory. Also, integrating sphere theory is based on a variety of assumptions including that of a perfect Lambertian inner wall coating. The effects of deviations from the assumptions of the theory are difficult to quantify (43). This dependence of absolute results on the sphere theory restricts the use of most integrating sphere reflectometers in the infrared primarily to relative measurements. 

Conical mirror reflectometers may exhibit systematic errors due to a variety of sources. As shown in detail below, the size of the resulting errors in measurement can be substantial. Hence, a thorough list of the sources of error are first provided, and then in Sections V,A-V,H a detailed analysis of each error source is given as well as a summary of the approaches taken to minimizing the size of the errors. 

Conical mirror reflectometers may exhibit systematic errors due to the following phenomena ... 

If a conic mirror reflectometer is not designed to minimize systematic errors, the magnitude of the associated corrections and uncertainties can be so large as to render it unsuitable for calibrating reflectance standards. 

We are not aware of any published data on the uniformity of sources used in 2 r/0 mode conic mirror reflectometers. Estimating the size of measurement errors due to spatial nonuniformities may require a raytracing analysis. 

The mirror reflectance can be independently measured relative to a reflectance standard with an absolute V-W apparatus (60) or with the conic mirror reflectometer itself. The measurement of was discussed in Section V,D. An estimate of the hemispherical/hemispherical reflectance of the detector can be obtained by performing a series of measurements of the directional/hemispherical reflectance at various incidence angles. Beckett (15) did not suggest using the technique with 2n/d instruments however, the application is straightforward. 

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