Characterization of a multipoint measuring system for photosynthetically active radiation

The principles for characterization of a multipoint measuring system of pho tosynthetically active radiation are described. The absolute irradiance res ponsivity, at a wavelength of 550 nm, and the relative spectral responsivit y of the device are determined so that a calibration factor can be provided to convert the readings of the device to the units of photosynthetically a ctive radiation. A high-accuracy spectrometer has been utilized for measuring the relative s pectral responsivity of the system sensor. A reflection trap detector whose spectral responsivity is traceable to a cryogenic radiometer has been used as the reference detector. The absolute irradiance responsivity of the dev ice has been determined by using a lamp with known spectral irradiance. The combined standard uncertainty of this set of calibrations is 3.6% at l sig ma level. The uncertainty is caused almost completely by the multipoint mea suring system.