Perception of brightness when the eyes are closed

Abstract To describe the lighting environment during sleep accurately, it is necessary to know the extent to which light is still perceived when the eyelids are closed. In this study, we measured the perception of equivalent brightness under closed‐eye and open‐eye conditions in binocular photopic vision. The light sources used were monochromatic red (peak wavelength 630 nm), yellow (593 nm), green (515 nm), and blue (460 nm) LEDs, as well as a white LED ( T cp = 4188 K, R a = 93). A total of 33 subjects (Asian adults aged 22 years on average) with eyelids of approximately the same color were recruited. The average effective transmittances of their closed eyes were 52.4% ± 31.5% for red, 26.2% ± 18.2% for yellow, 21.6% ± 16.7% for green, 4.5% ± 3.9% for blue, and 42.7% ± 24.8% for white light. These values were up to 10 times higher than eyelid transmittances reported previously, so eyelid transmittance cannot by itself explain perceived closed‐eye brightness. There were also significant individual differences. As it is unlikely that the physical transmittances of the eyelids differed from subject to subject, psychological factors may be responsible. This phenomenon should be further investigated in the future.