The authors argue that the use of pyrometers can be recommended for a limited number of tasks. The method of spectral brightness conversion proposed by the authors serves as the founding principle of a universal measuring device that has a low rate of errors. This method is effective at different wavelengths. The number of errors is reduced two-fold: first, the number of instrument-related errors is reduced through increasing the sensitivity of the pyrometer and identifi cation of the finest pyrometry ratio. Second, the authors suggest using, at least, three effective wavelengths to register the spectral brightness. Moreover, the authors believe that the reduction of the systematic error of the measured optical transition temperature designated for the derivation of its genuine values requires the right choice of an effective wavelength so that the equivalent wavelength of the double spectral ratio became infinitely big. This methodology makes it possible to derive an independent pyrometry equation to identify unknown emissivity factors within three wavelengths. Thus, the pyrometric system does not only take different temperature measurements, but also serves for the simultaneous determination of monochromatic emissive power in the online mode, which is essential for the purposes of taking measurements in the course of production of construction materials.