Light rays invisible to the human eye, of wavelengths from about 4 × 10-4 to 5 × 10-6 millimeters (where the X-ray range begins). Physiologically, they are important but also dangerous, causing the formation of vitamin D in the skin and producing sunburn in excess. They are strongly germicidal and may be produced artificially by mercury vapor and arc lamps for therapeutic use.
Much of ultraviolet radiation may be detected with ordinary photographic plates or films. It can also be studied by its fluorescent effect on certain materials. The desert iguana, Disposaurus dorsalis, uses it to locate the boundaries of its territory and to find food.