Radio Engineering and Electronics, t. XXVI, No. 1, 1981. C. 27-36. A function of mutual intensity of electromagnetic (in particular, light) field is introduced to describe the properties of partially coherent quasi-monochromatic radiation in the case when the polarization effects of the radiation field are insignificant (for example, when the radiation receiver does not react to them). With the help of this function and its spectral decomposition in flat waves (in a two-dimensional Fourier integral), the coherent properties of optical emitters with low spatial coherence are investigated in a quasi-radiation approximation ,. These , as multimode lasers (solid state,) and semiconductors, LEDs, multimode fiber and planar light-guide emitters. The relationship between the function of the mutual intensity of the radiating surface and its brightness. Formulas for the efficiency of excitation of low-mode fiber light guide by a emitter with a given function of mutual intensity and multimode gradient fiber with a power profile of the refractive index of the emitter with a given brightness are given