An express critical flicker fusion frequency (CFFF) measuring method, used to assess the functional state (FS) of a person, is of great practical importance in biophysics and applied physiology. To implement this method, an express-diagnostic device for assessing FS of a human was used in the work. Testing was carried out at different levels of intensity of light exposure for three wavelengths. The comparison of reactions after various intellectual workloads for the subjects was investigated: test assignments and a computer puzzle game. The research results demonstrated an increase in the sensitivity of the technique with the addition of blue light, as well as the different nature of the frequency curves of the subjects, depending on the type of workload used. Keywords: functional state, critical flicker fusion frequency, express-diagnostics, biophysics, psychophysiology.