Optical correction for multiple back reflections in an automated spectroradiometric measurement system

The University of Arizona's Remote Sensing Group depends heavily upon automated solar radiometers and transfer radiometers for calibration of sensors. Interference filters are essential for these devices and accuracy in determining filter transmittance characteristics is crucial. The Remote Sensing Group uses a commercially available automated spectroradiometric measurement system equipped with a dual monochromator and a filter transmittance accessory for measuring filter transmittance. Examination of the design of transmittance attachment and the detector assembly indicated the possibility of multiple back reflections between an interference filter and the detector and that higher than expected transmittance values were likely. To reduce this, a fine annealed BK7 wedge with an 8-degree deviation angle was placed in the optical path between the transmittance accessory focusing lens and the detector. The purpose of this paper is to evaluate the performance of the system with the BK7 wedge. The effect of the wedge will be negligible for an absorption filter and possibly significant for interference filters in the band-pass region. Two interference filters were analyzed via three repeats for each of the following scenarios: broadband with and without the wedge and band-pass with and without the wedge. The broadband at low spectral resolution and band-pass at high spectral resolution trials had comparable results while the greatest percent difference in transmittance occurred in the out-of-band region due to the extremely small transmittance values associated with the noise level for the instrument in general and for each interference filter specifically. For the band-pass region, the trials yielded a 0.015 to 0.062 difference in transmittance with the greatest difference occurring in the large gradient zone between the band-pass and the out-of-band region. The wedge makes a significant difference in transmittance measurements.