Estimating the irradiance spectrum from measurements in a limited number of spectral bands

Accurate measurement and characterisation of fluctuations in the irradiance environment is important for many areas of optical remote sensing. This paper describes a method of estimating spectral irradiance over the region 400–1000 nm from the radiance of a calibrated reference panel, measured in four narrow spectral bands (FWHM approx. 10 nm). The reproducibility of the method was found to have an average root-mean-squared error of approximately 30 mW m− 2 nm− 1 over the region 400–1000 nm when applied to spectra covering a range of clear-sky conditions typical of mid-latitude temperate regions. This was approximately twice as precise as the sequential method, even when the interval between target and reference panel measurements was very short (median interval 23 s). The method provides an alternative to linear interpolation between successive reference panel measurements and is particularly appropriate for conditions when irradiance is varying in a non-systematic way, for example, during the passage of sub-visual clouds.