Origin and nature of measurement bias in catadioptric parallel goniophotometers

We briefly categorize and compare parallel goniophotometers, which are instruments capable of simultaneously measuring the far-field distribution of light scattered by a surface or emitted by a source over a large solid angle. Little is known about the accuracy and reliability of an appealing category, the catadioptric parallel goniophotometers (CPGs), which exploit a curved reflector and a lens system. We analyzed the working principle common to all the different design configurations of a CPG and established the specifications implicitly imposed on the lens system. Based on heuristic considerations, we show that the properties of a real (thick) lens system are not fully compatible with these specifications. This causes a bias to the measurements that increases with the acceptance angle of the lens system. Depending on the angular field, the measured sample area can be drastically reduced and shifted relative to the center of the sample. To gain insights into the nature and importance of the measurement bias, it was calculated with our model implemented in MATLAB for the CPG configuration incorporating a lens system with a very large acceptance angle (fisheye lens). Our results demonstrate that, due to the spatio-angular-filtering properties of the fisheye lens, this category of CPGs is so severely biased as to give unusable measurements. In addition, our findings raise the question of the importance of the bias in the other types of CPGs that rely on a lens system with a lower acceptance angle.