The principles for characterization of a multipoint measuring system of photosynthetically active radiation are described. The absolute irradiance responsivity, at a wavelength of 550 nm, and the relative spectral responsivity 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 active radiation. A high-accuracy spectrometer has been utilized for measuring the relative spectral 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 device has been determined by using a lamp with known spectral irradiance. The combined standard uncertainty of this set of calibrations is 3.6% at 1σ level. The uncertainty is caused almost completely by the multipoint measuring system.