We consider in this paper a numerical model of the radiation transfer and the method of field calibration of the instrumentation operated in the UV spectral region. Under favorable atmospheric conditions the distortions introduced by the atmospheric layer into the Sun brightness may be small enough thus enabling one to exactly take those into account. Using that type of calibration procedure the instrumentation readouts are referred to the Sun brightness as if it is observed at the upper boundary of the atmosphere. As the estimates made show the accuracy of absolute power calibration of spectrophotometers that are operated in the 310 nm region is better than 5% at the spectral irradiance values above 65 W/(m2•mm). We also present in the paper a description of the software complex for calculating the UV radiation field in the atmosphere – underlying surface system that allows for multiple aerosol and Rayleigh light scattering, absorption of light by aerosol as well as the absorption at lines of atmospheric gases, while simultaneously taking into account the reflecting properties of the underlying surface. Some results of a comparison made between the calculated and experimentally measured values are being discussed in the paper as well.