UV rays are the wave-lengths between X rays and visible light. Because of their physical properties they belong to the group of the optical rays, even though we can’t see them.

UV rays are sub-divided into three categories:

• UVA (long waved) 400 - 315 nm
• UVB (middle waved) 315 - 280 nm
• UVC (short waved) 280 - l00 nm and below

1 nm = 1 Nanometre = 0,000 000 001 m