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Design, Calibration and Evaluation of a Camera-based Go-
nioreflectometer

Dipl.-Ing. Dr. Andreas Winkler

The bidirectional reflectance-distribution function (BRDF) is a characteristic property of a reflecting surface and describes the reflectance in terms of the directions of the incident and the reflected light. Among other fields of application, BRDF measurements are used for automated optical inspection in production processes or in computer graphics where virtual surfaces are visualized under different artificial lighting conditions.

A device to measure the BRDF is referred to as gonioreflectometer. The camera-based gonioreflectometer (CBGR) presented in this thesis uses one of nine available LEDs to illuminate the specimen and a conventional CCD-camera to measure the radiance of the reflected light. Four degrees of freedom are required to adjust the directions of the incident and the reflected light. These are provided by a construction with four mechanical axes, allowing flat specimens with weights of up to 3 kg. Despite the readily available and quite inexpensive components in use, sophisticated calibration procedures lead to measurements with low uncertainty. A custom integrating sphere is applied for the radiometric calibration of the camera. The spectra and the radiant intensity distributions of the LEDs are measured. Moreover, the geometric parameters of the complete setup are estimated by means of acquisition of two specially designed calibration targets. The exact knowledge of the geometry enables, among other things, the measurement of the spatially varying BRDF (SVBRDF) without the need for dedicated registration marks on the specimen.

The performance of the CBGR is evaluated by measurements of a diffuse reflection standard and a mirror. The measurements of the reflection standard are also compared to measurement results provided by the manufacturer of the standard. Finally, the measured SVBRDF of a banknote is shown. Keywords: Digital image processing, Bidirectional reflectance distribution function, Reflectometer, Calibration

March 19, 2018

Figure 1: The main components of the CBGR are the spherical wrist (used to move the specimen), an array of LEDs in order to illuminate the specimen, the light arm (used to move the LED-array) and a CCD-camera in order to measure the amount of light reflected into the direction of its optics. Figure 2: The measured BRDF of the diffuse reflection standard. The flatter the angle of the incident light (here we have a polar angle of 60◦), the more the BRDF of the standard differs from that of an ideal Lambertian reflector.