405nm

405 nanometers is a wavelength of light in the visible spectrum that falls within the violet portion of that spectrum. It lies just below the ultraviolet region, with wavelengths shorter than 405 nm being classified as ultraviolet-B. In terms of frequency, 405 nm corresponds to about 741 terahertz. The light at this wavelength is perceivable to the human eye as a strongly blue–violet hue and has a relatively high photon energy compared to longer visible wavelengths.

Because of its position just below the ultraviolet range, 405‑nm light can be used in applications that benefit from a balance between visibility and the ability to break down certain molecules. For example, many laser-based sterilization systems employ 405‑nm wavelengths to inactivate bacteria and viruses on surfaces or in air, as the light can generate reactive oxygen species without reaching the more harmful levels of ultraviolet radiation. The same wavelength is common in some solid-state laser diodes and in the green phosphors of certain LED displays. In communications, 405‑nm lasers have been explored for free‑space optical links where visibility to human observers is a safety requirement.

Health considerations differ from those of ultraviolet lasers. While 405‑nm photons carry less energy than UV, prolonged exposure can still cause eye damage, particularly at high power levels, by heating retinal tissue. Eye protection such as UV‑blocking lenses is therefore advised when working with high‑intensity 405‑nm sources. Regulations in many countries classify high‑power 405‑nm lasers as hazardous and require that operators be trained and that safety measures be in place.