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Light of around 555 nanometres is accepted as the most efficient level of light for daytime vision. But recent research has shown that we also have biological receptors for non-visual response peaking in the blue wavelength range of 446-477 nanometres, a range abundant in clear daylight. Researchers at Brown University in 2002 discovered that non-visual ganglion cells in the eye detect sky-blue light to set our internal clock.
Daylight has an abundance of wavelengths at 446-477 and in the 555 nanometre range, satisfying both perceptual and biological and perceptual demands. The challenge is to develop lighting solutions that will perform like daylight.
The choice of colour is controversial, in part because many colours we perceive are not interpolated but are ‘invented’ by the brain. The theory is that some colours enhance low-light vision provided by the cones in the eye. The eye also has rods, used for normal intensity light. Originally, it was believed that the cones, occupying a narrow slice in the centre of the retina, were red sensitive, so using red lighting would enhance night vision. But the cones are blue-green (507 nm) sensitive; although the fovea, an even more narrow slice at the centre of the cones is very red sensitive.