Validation of the gonioreflectometer.

Technical report PCG-03-02, Program of Computer Graphics, Cornell University, May 2003.

This report describes a series of experiments conducted in the Light Measurement Laboratory of the Program of Computer Graphics in the summer of 1999 to validate the Gonioreflectometer. The Gonioreflectometer is an automated device to measure the BRDF (Bidirectional Reflectance Distribution Function) of a flat test sample. The sample is illuminated and reflected light collected at multiple preset angles of illumination and/or reflection. Light detection is at 1024 spectral bands over the visible wavelength range.

The validation experiments described herein include stray light, polarization, and detector noise and linearity tests. Further tests determined the light source and detector footprints, the instrument solid angles, and the instrument signature (i.e., instrument response when scanning the incident beam without a test sample). All parts of the instrument (including the light source, the positioning mechanism, and the detector) were carefully studied.

Many of the experiments were to explain an unexpected difficulty that was discovered: the spectral bias of recent BRDF measurements (i.e., after Foo's thesis [1] was completed). Extensive work showed that the error comes from the chromatic aberrations of the light source, the nonlinear response of the detector, and the background noise measurements.

Chapter 7 recommends a new method to obtain reflectance measurements with better accuracy, and which avoids the spectral bias of the light source. BRDFs are measured relative to a white reference material (Spectralon). Such a procedure is known as a relative reflectance method, and is the preferred method for reflectance samples that are strongly diffusing.

Absolute reflectance measurements are still possible with the Gonioreflectometer, however, and may be preferred for reflectance samples with strong specular, or mirrorlike, reflection behavior. For such materials, the peak reflected signal and the incident source signal might be close in magnitude. A special measurement procedure is recommended for such surfaces, and is discussed in Chapter 6.

Several samples were measured. Strongly specular reflecting materials were measured with the absolute reflectance technique (Chapter 6). The materials were a smooth gold mirror, a smooth black plastic, and a smooth blue plastic. Materials with strong directional-diffuse reflection were measured using the new relative reflectance method (Chapter 7). The materials included metals (rough steel Q-panel; two aluminum coated ground glass surfaces), nonmetals (white Spectralon), and paints (Ford metallic gray; Krylon blue; Bristol light gray).

The appendix of the report presents an alignment procedure for the Gonioreflectometer.

This paper is available as a PDF file LT03.pdf (7.0M).