The visual threshold of the fovea is raised by a surrounding ‘glare’ ring of bright light. If this is due entirely to scattered light, then the equivalent uniform background that raises the threshold equally will bleach the cone pigments equally. The equivalence of bleaching was measured by two different techniques: retinal densitometry, and the effect on subsequent dark adaptation. Both agree that the background which matches the glare in raising the threshold for foveal cones also matches it (correct to 0.1 log unit) in bleaching the pigment in those same cones.
W. A. H. RUSHTON and R. W. GUBISCH, "Glare: Its Measurement by Cone Thresholds and by the Bleaching of Cone Pigments," J. Opt. Soc. Am. 56, 104-110 (1966)