Increasing the Spatial Resolution of BTF Measurement with Scheimpflug Imaging

Abstract

We present an improved way of acquiring spatially varying surface reflectance represented by a bidirectional texture function (BTF). Planar BTF samples are measured as images at several inclination angles which puts constraints on the minimum depth of field of cameras used in the measurement instrument. For standard perspective imaging, we show that the size of a sample measured and the achievable spatial resolution are strongly interdependent and limited by diffraction at the lens' aperture. We provide a formula for this relationship. We overcome the issue of the required depth of field by using Scheimpflug imaging further enhanced by an anamorphic attachment. The proposed optics doubles the spatial resolution of images compared to standard perspective imaging optics. We built an instrument prototype with the proposed optics that is portable and allows for measurement on site. We show rendered images using the visual appearance measured by the instrument prototype.