light-induced change in a chromophore, resulting in the loss of its absorption of light of a particular wave length. The photon energy causes a conformational change in the photoreceptor proteins affecting PHOTOTRANSDUCTION. This occurs naturally in the retina (adaptation, ocular) on long exposure to bright light. Photobleaching presents problems when occurring in PHOTODYNAMIC THERAPY, and in FLUORESCENCE microscopy. On the other hand, this phenomenon is exploited in the technique, fluorescence recovery after photobleaching, allowing measurement of the movements of proteins and lipids in the cell membrane.