Brightness perception under photopic conditions - experiments and modeling with contributions of S-cone and ipRGC

Abstract In 1931, the CIE published and standardised the photopic luminous efficiency function. Based on these standardized curves,luminous flux in lumens, luminance in cd/m2, and illuminance in lux are determined by an integral of these curves and theincident light spectra in photometers and are considered as a physical brightness. However, human brightness perception isnot only weighted by this simple determination, but is a more complicated combination of all L-cones, M-cones, S-cones, rods,and later ipRGCs, which was partly illustrated by the equivalent brightness of Fotios et al. with the correction factor (S/V)0.24.Recently, new researches have mentioned the role of ipRGCs in the human brightness perception. However, it is still unclearhow these signal components of the human visual system are involved in overall human brightness perception. In this work, thehuman brightness perception under photopic conditions was studied by experimenting with 28 subjects under 25 different lightspectra. These spectra were varied not only in brightness but also in spectral geometry. In this way, the contributions of thesignal components can be investigated. Subsequently, an optimization process was performed with the obtained database. Theresults show that not only the photopic component, but also the S-cones and ipRGC play their role, although it is smaller. Thus,the visually scaled brightness model based on the database optimization was constructed with not only illuminance but alsoS-cones and ipRGC with R2 of 0.9554 and RMSE of 4.7802. These results are much better than the Fotios-based brightnessmodel with only S-cones (R2 = 0.8161, RMSE = 9.7123) and the traditional model without S-cones and ipRGC (R2 = 0.8121,RMSE = 9.8171). It also suggests that a "blue-sensitive" signal (S or G=ipRGC or their combination) should be given seriousenough attention in the human brightness perception, and a more comprehensive study is needed to investigate it more deeply.