The disclosure provides a method of determining reflected irradiance for a surface point on a surface whose reflectance properties are represented by a measured BSDF. Additionally, the disclosure provides a renderer and a computer program product. In one embodiment, the method includes: (1) determining u, v and w coordinates in the measured BSDF for the surface point based on an incoming and an outgoing ray direction, (2) selecting a triangle for barycentric interpolation based on values of the v and the w coordinates at the surface point and (3) performing the barycentric interpolation for evaluating the measured BSDF for the surface point.