A method by which to calibrate a spectral radiometer using the sun as the illumination source is discussed. The procedure is simple, requiring only a calibrated reflectance panel, relatively low aerosol optical depth, and accurate measurements of atmospheric transmittance. Solar-based calibrations eliminate several uncertainties associated with applying a lamp-based calibration to field measurements and may be performed with a comparable uncertainty. A solar-reflectance-based calibration (SRBC), by removing the need for extraterrestrial irradiance spectra, further reduces calibration uncertainty to approximately 2.2%. This calibration may be applied to radiometric measurements of any system illuminated by the sun, such as the atmosphere or ocean, with associated reduction of uncertainty in properties derived from such measurements (e.g., aerosol single_scattering albedo). The procedure is ideal for on_site calibration of long_term field instruments, mitigating the logistics and costs associated with transport to a calibration facility.