Calibration and error analysis of multiwavelength telephotometers

Abstract In preparation for the EPA-VISTTA summer 1979 field program, 12 multiwavelength telephotometers were tested and calibrated. A standard light source was used in the calibration process, and the linearity of the instruments was checked using neutral density filters. Calibration of the telephotometers was performed before and after this experiment to determine individual instrument response and drift. Results of the calibration indicated that the instruments had an average inherent error of approximately 1 per cent when operating in the contrast (or ratio) mode. The degree of error is less if a correction is made. In the absolute radiance mode, the instrument was estimated to have an average inherent error of approximately 10 per cent. Results of two of the telephotometer comparisons performed during the VISTTA program are shown to illustrate the problems of target selection and inhomogeneity. A comparison of the extinction coefficients determined from the telephotometers and the scattering coefficients measured by nephelometers showed reasonable agreement for the first example. A more comprehensive, side-by-side comparison showed that results from the two telephotometers agreed within 2 per cent in the ratio mode. However, the uncertainty in the extinction coefficient is between 10 and 15 per cent. This uncertainty can be reduced by eliminating the low contrast cases.