The relative coefficient of harmfulness (lambda) of light sources (lamps) produced in the USSR and used in museums has been evaluated. A method of determining the quality of visual perception (QVP) of plane polychromatic exhibits has been developed. The choice of optimal lighting conditions is made by a group of experts; Each should be informed about the harmfulness coefficients of those kinds of lighting conditions which he is to observe. Guided by intuitive ideas about the desired and acceptable QVP of the particular exhibit, the expert chooses such lighting conditions that correspond to the normal ones by definition. The same method is also used in selecting the optimal conditions when lighting devices (or windows) are supplied with light-protective filters removing UV and IR regions of the spectrum.