We present a method for transparent watermarking using a custom bidirectional imaging device. The two innovative concepts of our approach are reflectance coding and multiview imaging. In reflectance coding, information is embedded in the angular space of the bidirectional reflectance distribution function (BRDF) and this information can vary at each surface point. In order to achieve a transparent watermark, reflectance coding is implemented using a spatial variation of the Brewster angle. The novel multiview imaging method measures the reflectance over a range of viewing and illumination angles in order to instantly reveal the unknown Brewster angle. Unlike typical in-lab measurements of the Brewster angle or the refractive index, this method does not require accurate prior knowledge of the surface normal so that imaging in non-lab conditions is feasible. Furthermore, a range of incident angles are examined simultaneously, eliminating the need for scanning incidence angles. The approach is well-suited for transparent watermarking where the observer cannot see the watermark because it is comprised of spatial variations of refractive index. The transparency and angular coding of the watermark has great utility in deterring counterfeit attempts. In this paper, we present the imaging device and demonstrate it's effectiveness in detecting and measuring changes in refractive index. This device acts as the decoder in a transparent watermark system.