An illuminator/collector assembly can deliver incident light to a sample and collect return light returning from the sample. A sensor can measure ray intensities as a function of ray position and ray angle for the collected return light. A ray selector can select a first subset of rays from the collected return light at the sensor that meet a first selection criterion. In some examples, the ray selector can aggregate ray intensities into bins, each bin corresponding to rays in the collected return light that traverse within the sample an estimated optical path length within a respective range of optical path lengths. A characterizer can determine a physical property of the sample, such as absorptivity, based on the ray intensities, ray positions, and ray angles for the first subset of rays. Accounting for variations in optical path length traversed within the sample can improve accuracy.
