The methodical approach to an estimation of efficiency of the masking means is developed, allowing strictly enough to consider distributions of intensity of radiation on surfaces of object and a spreading surface. By an investment peak distribution of radiation of object on an external surface of masking means in total distribution of a total area, is received compound distribution in a subject plane with the further carrying over of the image to a focal plane of system of investigation. It is shown, that at equal values of noise in images of the disguised object and a spreading surface distinction in signal levels in 5-10 % does not render essential influence on likelihood indicators of distinction.