We consider a method of measurement of apparent illumination using the perceptual phenomenon of fluorescence to which we give a psychophysical explanation based on the hypothesis of direct relation of perception of self-luminosity to the apparent illumination of the scene observed by the viewer. We report results of three experimental studies: (1) measurements of apparent illumination when a fiat mondrian in the darkness is observed; (2) measurements of apparent illumination in a "rich" 3D scene; (3) testing linearity of the subjective illumination space. Based on the obtained data, we confirm the basic hypothesis of a relation between the self-luminosity phenomenon and the perceived illumination, and show that the suggested method of measurement of apparent illumination gives reliable and accurate (errors were in the range 4-10%) values of its chromatic components. Apparent illumination in the case of a mondrian observed in the darkness is found unstable, which can be expected given that the context of such a scene is insufficient for obtaining a unique solution for all the components of the photometric problem. In the second experiment, we show that the measurement results remain constant when the context of a "rich" 3D scene is changed and that they follow the changes in illumination conditions. In the third experiment, we study the composition of different illuminations and show that the simple relation f = f(1) + f(2) holds for the measurement of the illumination (f) when two other illuminations (f(1), f(2)) are applied simultaneously.