Utilization of the parametric down—conversion provides a unique possibility of absolute standardless brightness calibration ofhigh—temperature visible, infrared and near UV radiation. The calibration method has been suggested in Moscow State University in 1977 and then was applied there to experimental measurements of radiation spectral brightness for laser and luminiscence sources of visible range and filament lamps radiating in IR range. The method is based on the comparison between two signals: the signal p, corresponding to the measured radiation parametric up-conversion and the "noise" signal P corresponding to parametric scattering of the pump; this "noise" signal is observed both in the presence of the measured radiation at the converter's idler channel entrance and in the absence of it. The noise signal intensity is determined by the effective brightness of zero vacuum fluctuations at the idler frequency which equals to one photon per mode, or, in the energetic units, Bvac = hc2/A, where h is the Plank constant, c —the light velocity, )2 —the idler wavelength. Measuring the ratio of signals P" P' + P and P at the exit of the crystal—converter at frequency w' = wo — w (w0 being the pump frequency, and w —the frequency of the flux being calibrated), one can calculate the absolute value N of energetic brightness spectral density for the radiation which fills the idler channel of the converter: N = k[(P"/P )— 1] in the units "photon/mode" or B = BvacN in the energetic units (here icis a correction coefficient describing the non-ideal transparency of the converting crystal). No additional standard sources or detectors are required here; the method uses as a reference the effective value of zero vacuum fluctuations brightness. The reference effective brightness temperature depends on the radiation wavelegth: T0 = hc/Ak in 2, k being the Boltzmann constant, so that for A2 10 p Tvac 2000 K, and for ' 0.5 Tvac 40 000 K. In this work the accuracy of the method is analysed and the resolution for different schemes of the parametric photometer is calculated.