The true surface temperature for an object can be determined by measuring emitted radiation. Primary spectrum pyrometry should be generally carried out by radiation measurement within an infinitesimal solid angle or within a finite solid angle in the case of diffuse emission, so that the radiation thermometry equations become a closed system for temperature and other undetermined parameters. The radiation thermometry equation with linear emissivity model is obtained, which can be used to measure the temperature of the object with non-diffuse emission within a finite solid angle. This equation is universal in radiation measurement, based on which equations for primary spectrum pyrometry are deduced. These equations are also universal in radiation measurement, in which the emissivity scales under different measurement conditions are limited to the same range, but have different physical meanings.