Classically-designed X-ray image intensifiers, in which the primary converter of X-ray image (X-ray luminescent screen) is placed inside the vacuum space of the X-ray image intensifier, and the photocathode converting light image into electronic one is in optical contact with the primary converter, have decisively displaced a combined systems -- external X-ray luminescent screen, light-transmission optics, image intensifier -- from medical and industrial X-ray diagnostic devices since they possess essentially higher luminance amplification (higher X-ray-to-light image conversion coefficient) and higher signal-to-noise ratio. We have assumed that this conversion coefficient is determined as a ratio of luminance of light image at the output screen of the X-ray image intensifier (measured in cd/sq.m.) to intensity of the X-ray radiation at the input of the X-ray image intensifier, derived from the power of the exposure dose (in R/sec). When we analyzed an X-ray-to-light image conversion dependence, we found out that there is an inconsistency between a method of measuring the efficiency of this conversion and this dependence such that the power of the exposure dose determines absorption of X-ray radiation only in the air and can not be recalculated into absorption in components of the X-ray image intensifier. In order to solve these contradictions, in this paper we have considered spectral X-ray characteristics. We further developed assumptions that the spectral density of the power flow of bremsstrahlung X-ray radiation depends linearly on the quantum power of this radiation. We succeeded in relating this dependence with the power of the radiation exposure dose and in obtaining a family of spectral characteristics of bremsstrahlung X-ray radiation at different anode voltages of the X-ray tube for a certain exposure dose. The paper describes calculations of the total power flow of the bremsstrahlung X-ray radiation at the input of the X-ray image intensifier, calculations of absorption of this flow in the functional assemblies of the X-ray image intensifier -- in the input window made of different materials (glass, aluminum, beryllium, steel, and glass-carbon) and in the X-ray luminescent screen made of iodine cesium. We have calculated values for the conversion coefficient of the X-ray image intensifier on the whole.