In this paper we present schemes of experimental setups for the radiation spectral brightness measurements in the range of 0.6 - 5 (mu) for N varying from 10-1 to 102 (if, for example, (lambda) equals 1 (mu) , this range of N corresponds to the brightness temperature range from 6 (DOT) 103 to 106 K), and for photomultipliers quantum efficiency measurements in the range of 0.4 - 5 (mu) with a dynamical range 10 - 1012 photon/sec and accuracy not worse than 1%. The new measurement methods are based on the utilization of the parametric light scattering phenomenon which is a spontaneous decay of laser pump photons in correlated photon pairs in crystals with quadratic nonlinear susceptibility. The first of two methods allows measurement of the radiation spectral brightness N in absolute units ('photons per mode') in visible and infrared range. The quantity N is related to the energetic brightness spectral density B through the equation B equals (hc2/(lambda) 5)N, where h is the Plank constant, c - the light velocity, (lambda) - the wavelength. The method is absolute and does not require any reference source or detector of radiation. Quantum noise of a parametric down-convertor, caused by the zero vacuum fluctuations with an effective brightness Nvac equals 1 photon per mode, is the reference in this case. The second method concerns the quantum efficiency of photodetectors determination, and it is based on the connection between the statistics of photocurrent and the radiation which causes it. The parametric scattering is a unique source of rather intensive and directed radiation flow consisting of photon pairs. Such a flow can be used to determine the absolute quantum efficiency of photodetectors.