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

Abstract

In 1924, the CIE published and standardized the photopic luminous efficiency function. Based on the standardized curve, luminous flux in lumens, luminance in cd/m${}^2$, and illuminance in lux are determined by an integral of the curve and the incident light spectra in photometers and are considered physical brightness. However, human brightness perception is not 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 described by the equivalent brightness of Fotios et al. with the correction factor ${(S/V)}^{0.24}$. Recently, new research has demonstrated the role of ipRGCs in human light perception. However, it is still unclear how these signal components of the human visual system are involved in the overall human brightness perception. In this work, human brightness perception under photopic conditions was investigated by visual experiments with 28 subjects under 25 different light spectra. In this way, the contributions of the signal components can be investigated. An optimization process was then performed on the resulting database. The results show that not only the $L+M$ component, but also the S-cones and ipRGC play a role, although it is smaller. Thus, the visually scaled brightness model based on the database optimization was constructed using not only illuminance but also S-cones and ipRGC with $R^2$ of 0.9554 and RMSE of 4.7802. These results are much better than the brightness model after Fotios et al. using only S-cones ($R^2$ = 0.8161, RMSE = 9.7123) and the traditional model without S-cones and ipRGC ($R^2$ = 0.8121, RMSE = 9.8171).