An Image Based Bidirectional Reflectivity Distribution Function Experiment

The temperature dependence of Bidirectional Reflectivity Distribution Functions (BRDFs) is not well documented. For the sake of time and customer demand, current measurements are taken at room temperature, but the reflectivity of a material changes as a function of temperature. The assumption is that this change is uniform, and as such, the BRDF will retain its relative angular shape but perhaps be scaled in magnitude as a function of temperature. When BRDFs are to be used at elevated temperatures, a scaling factor is then applied. In addition, BRDF as a function of rate of change of temperature has been identified by the Directed Energy community as being of interest in optical determination of power on target. BRDF acquisition is often taken with very expensive pieces of equipment known as gonioreflectometers. The process of collecting a BRDF for a sample can take on the order of a day. To speed the collection of BRDF data, the computer graphics industry developed a technique using a series of images that contain enough information to extract the BRDF of the sample in question. This experiment begins the journey down the path towards understanding temperature dependence of BRDF through image-based BRDF measurement. With the eventual goal of temperature coupled samples, the size of the optics, used here had to be large to accommodate the size of thermally controlled samples. Much of this initial experimental set up was creating custom components to hold the large optics, the sample or the camera

High-Speed Measurement of BRDF using an Ellipsoidal Mirror and a Projector

Measuring BRDF (Bi-directional Reflectance Distribution Function) requires huge amounts of time because a target object must be illuminated from all incident angles and the reflected lights must be measured from all reflected angles. In this paper, we present a high-speed method to measure BRDFs using an ellipsoidal mirror and a projector. Our method makes it possible to change incident angles without a mechanical drive. Moreover, the omni-directional reflected lights from the object can be measured by one static camera at once. Our prototype requires only fifty minutes to measure anisotropic BRDFs, even if the lighting interval is one degree. 1