Stability of narrow-band filter radiometers in the solar-reflective range

We show that the calibration, with respect to a continuous-spectrum source, and the stability of radiometers using filters of about 10 nm full width, half maximum (FWHM) in the wavelength interval 0.4 to 1.0 μm, can change by several percent if the filters change in position by only a few nanometres. The cause is the shifts of the passbands of the filters into or out of Fraunhofer lines in the solar spectrum or water vapor or oxygen absorption bands in the Earth's atmosphere. These shifts can be due to ageing accompanied by the absorption of water vapor into the filter of temperature changes for field radiometers, or to outgassing and possibly high energy solar irradiation for space instruments such as the MODerate resolution Imaging Spectrometer ― Nadir (MODIS-N) proposed for the Earth Observing System