Effects of a measurement floor on Mueller matrix measurements in a dual rotating retarder polarimeter bidirectional...

Since a measurement of the bidirectional scatter distribution function (BSDF) of a material is proportional to the intensity of the scattered light, a BSDF measurement system with the addition of a dual rotating retarder polarimeter can be used to calculate the Mueller matrix of a scatterer. One advantage of a BSDF system using a laser source is its large dynamic range, which allows the measurement of scattered light both near to and away from the specular region. As BSDF measurements move away from the specular region and into a more diffuse-scatter region, the measured signal decreases and may approach the system's measurement floor. Therefore, BSDF and Mueller-matrix measurements are dependent not only on the scatter from the sample but also on the noise floor of the system. By analyzing numerically created bidirectional reflectance distribution function data, we show that since the noise floor of a system is typically constant, the Mueller-matrix measurement at the noise floor appears to be that of a perfect depolarizer. Therefore, as the BSDF measurement space moves away from the high-signal region and the noise floor is approached, the Mueller matrix assigned to the sample artificially approaches that of a perfect depolarizer. The rate and location in scatter-angle space of this shift is dependent on the BSDF of the material and on the signal-to-noise ratio in the system. Therefore, caution must be taken when drawing conclusions about measured Mueller matrices for scattered light, particularly in measurement regions where the measured signal approaches the system floor.