C53625: Light and the visual cycle

The electromagnetic spectrum covers a continuum of electromagnetic waves from radio waves, microwaves, infrared, visible and ultraviolet (UV) radiations, through to X-rays and gamma-rays, the photon energy increasing with decreasing wavelength [figure 1]. Sunlight comprises 5-10% UV (100-380nm), ~40% visible radiation (380-780nm), and 50-55% infrared radiation. These are either absorbed or transmitted by the successive layers of the eye, modulating the light reaching the retina.¹

Figure 1: Visible light (380-780nm) in the electromagnetic specturm. HEV – high energy visible; LEV – low energy visible

UV waves are harmful to the anterior part of the human eye. In a healthy adult’s eye no UV radiation actually reaches the retina. UVC (100-280nm) from sunlight are filtered by the atmosphere, while most UVB (280-315nm) are absorbed by the cornea. Residual UVB and most UVA (315-380nm) are then absorbed by the crystalline lens. In contrast, visible light reaches the retina in high proportions.²