A Method for Measuring Apparent Illumination

Knowledge of perceived illumination is very important for almost all experimental investigations in colour science. We consider here a method for measuring apparent illumination based on the perceptual phenomenon of fluorescence. The underlying hypothesis is that the minimal illumination level at which a patch begins to look fluorescent exactly corresponds to the respective chromatic component of the apparent illumination in the scene. We report results of three experimental studies: (1) measurements of apparent illumination for a flat mondrian presented on a screen and mondrians made of paper presented in a specially designed box with controlled illumination observed monocularly; (2) measurements of apparent illumination in a ‘rich’ 3-D scene; (3) measurements to test the linearity of the subjective illumination space.

The results confirm the basic hypothesis of the relation between fluorescence phenomena and perceived illumination and demonstrate that the proposed method gives reliable and accurate values for each of its chromatic components (errors were in the range 4% – 10%), eg for paper mondrians there was a strong correlation between the level and colour of illumination and the fluorescence. In the case of mondrians presented on a screen in darkness the measured apparent illumination was found unstable, however, as would be expected given that the context of such a scene is insufficient for a unique solution of the photometric problem. In the second experiment we found that the measurement results remain constant when the context of a ‘rich’ 3-D scene is changed and that they follow the changes in illumination conditions. In the third experiment we found that the linear relation f = f₁+ f₂ holds for the measured apparent illumination (f) produced by combining of two other apparent illuminations (f₁, f₂).