Ultraviolet (UV) light is energy within the electromagnetic spectrum that has shorter wavelengths than that which are visible to the human eye. UV light is a range of electromagnetic waves from 100 to 400 nanometre (between x-ray and visible light). The division of UV light is classified as Vacuum UV (100-200 nm), UV-C (200-280 nm), UV-B (280-315 nm) and UV-A (315-400 nm). The energy waves provided in the UV-C spectrum demonstrate the germicidal deficiencies that provide highly effective disinfection.
NOTE: The violet visible light is not the UV light which disinfect the water, so this light is not indicator that the lamp is working well. Most of the manufacturers recommend to change the lamp every 9000 hours as there is several studies that the lamp performance reduces to 60% after 9000 hours.