Calibration of sunphotometer for measurements of turbidity

Summary

This study investigates an alternative method to the Langley plot, a widely used but complex calibration method for sunphotometers. A sunphotometer has been calibrated using two different methods: the Langley method, a calibration to the extraterrestrial irradiance, and second by comparison to a standard instrument. The standard instrument used for these studies is spectrophotometer. The relative difference between the calibration factors obtained by the two methods is between 0.13% for the channel with the greatest sensitivity (500 nm) and 2% for the channel with the lowest signal (368 nm). The accuracy of both calibrations is of the same order of magnitude with relative errors between 1.2 and 7% for the Langley method and 2.9 to 5.3% for the standard instrument method. Analyses of the origin of possible errors show the sensitivity of the Langley method to less than ideal weather conditions, which could cause an error in calibration of up to 45% under extreme conditions and when too few measurements are made. This studies are made only for the UV and the visible range, investigations about the application of this technique in the near IR have still to be done and would also require spectrometers with a wider sensitivity range.

These investigations do not alter the fact that frequent calibrations are still needed due to sensitivity changes like filter degradation.