What is considered the safest radiation for increasing pigmentation in the skin?

UV-A radiation is considered the safest for increasing pigmentation in the skin because it penetrates deeply into the skin and is primarily responsible for the tanning effect. Unlike UV-B, which can cause more immediate and intense sunburns due to its higher energy level and shorter wavelengths, UV-A is generally less intense and causes a slower and more gradual increase in melanin production, which contributes to a more even tan.

On the other hand, X-rays are not utilized for pigmentation purposes due to their potential to cause significant damage to skin cells and increase the risk of skin cancers. Infra-red radiation, while not as harmful as X-rays, does not effectively stimulate melanin production in the same way that UV radiation does. Thus, considering safety, efficiency, and skin response, UV-A is the preferred choice for increasing pigmentation safely.