Calibration of a sunphotometer by simultaneous measurements of direct-solar and circumsolar radiations.

A new method is proposed for the calibration of the sunphotometer. Well-known difficulties of the usual Langley-plot method when applied to unsteady turbidity conditions can be avoided by monitoring the circumsolar radiation. To realize this idea, an alternate of the Langley-plot method is developed, in which the logarithm of the sunphotometer reading is plotted against the ratio of intensity of singly scattered circumsolar radiation to that of direct solar radiation instead of the optical air mass in the usual Langley-plot method . Results of numerical simulations and field tests with a newly developed instrument show that the rms error of the calibration constant could be reduced to 1/5-1/10 of the usual method for wavelengths larger than 500 nm .