Calculating direct normal irradiance from sun photometer measurements

Abstract. An alternative method to modeling is proposed for determining direct normal irradiance from sun photometer measurements. Direct normal irradiance is calculated by integration of the sun photometer spectral irradiance measurements. Using pre and post calibration spectral coefficients and the extraterrestrial spectral irradiances at each filter effective wavelength, digital counts are converted into direct spectral irradiances data. Then the trapezoidal integration procedure is applied to calculate the direct normal beam. The error associated to the integration method is estimated between 6 and 2 % depending if the infrared filter is for the 1240 or 1640 nm respectively. Validation with collocated quasi-simultaneous manually measured and with modeled direct normal irradiance shows encouraging results. Although the results are not categorically conclusive, they evidence the capabilities of the proposed method for deriving direct normal irradiance under clear sky and also for cloudy conditions. Better results are expected for validations with simultaneous automatic direct normal irradiance measurements.