Light is made up of energy bunches, known as photons, moving in space as electromagnetic waves (or radiation), whose length determines the quantity of photons that can be used by the body: the positive biological effect of radiation thus only depends on its wavelength. According to a classification based on wavelengths, sunlight is made up (Fig. 10) of ultraviolet rays (UV), visible rays (violet, indigo, blue, green, yellow, orange, red), and infrared rays (IR). Ultraviolet (UV) rays are responsible for the greatest benefits of light; based on wave frequency in nanometres (nm), they include in turn UVC rays (200-260 nm), UVB rays (261-310 nm), and UVA rays (311-400 nm). The hyperpigmenting action of sunlight has been known for a very long time: tanning is obtained through exposure to the intense natural light of summer months or of the Tropics. However sunlight has no impact on vitiligo, because melanin production in melanocytes is only restored by UVB rays, which are contained in sunlight in very small quantities (0,2% and as little as 0,02% in industrialised regions). Such quantities, however, may perform a positive supporting role, provided that exposure to sunlight occurs after a number of  Ratok®therapy sessions. IR and UVC rays are not significant for the treatment of vitiligo. UVA rays, accounting for 4,8% of sunlight, cause a quick but transitional tanning of the normally pigmented skin by increasing the quantity of the existing melanin; on the other hand they do not affect melanocytes and are therefore unable to promote the production of new melanin required for repigmentation of vitiligo patches. In addition, UVA rays may reach the second skin layer, the dermis, thus causing a light-related skin damage known as photoaging.