A Test of the S/P ratio as a correlate for brightness perception using rapid-sequential and side-by-side experimental protocols

An experiment was performed to: 1 Test the hypothesis that the ratio P*(S/P)^0.5 can be used to predict the perception of spatial brightness, and 2) directly compare the rapid-sequential and side-byside evaluation modes for assessing spatial brightness. The two evaluation modes produced comparable results. Though the side-byside method has in the past been questioned by others, the data herein illustrates that it is neither faulty nor invalid. Red, green, and blue LEDs were employed to create four light settings that were permutations of two S/P ratios (1.7 and 2.6) and two luminance levels (24 and 30 cd/m²). The S/P ratios corresponded to the practical extremities of CCT (2900 and 7200 K) and were structured to have their chromaticity on the blackbody locus. At equal luminance there was no difference in the perception of brightness, irrespective of CCT. At unequal luminance, but when the ratio of P*(S/P)^0.5 was set to 1:1, brightness perception was predicted by luminance. These data suggest that spatial brightness perceptions at photopic light levels are unrelated to the S/P ratio of the illumination. These results are consistent with the "prime color" theory of vision that is rooted in the trichromatic nature of human vision.