Near ultraviolet refers to the band of electromagnetic radiation occupying the region of the spectrum immediately adjacent to and just beyond the short wavelength limit of human visible light, spanning wavelengths from approximately 400 nanometres - the violet boundary of the visible spectrum - down to around 250 nanometres. This region sits between the visible light spectrum above it and the far or extreme ultraviolet below it, and while it is invisible to the human eye, it has significant relevance to photography due to the natural sensitivity of most photographic emulsions and many digital image sensors to radiation within this wavelength range.
The sensitivity of conventional silver halide photographic emulsions to near ultraviolet radiation is a direct consequence of the fundamental photochemical properties of silver halide crystals, which are naturally responsive to short wavelength radiation including the near ultraviolet band without requiring any special sensitising treatment. This inherent ultraviolet sensitivity, while useful in certain scientific and technical applications, can be a source of unwanted effects in conventional photography - particularly in outdoor situations where the atmosphere contains significant levels of near ultraviolet radiation scattered from the sky, which can cause a bluish haze in distant landscapes and slightly degraded colour rendition in colour photography. Ultraviolet absorbing filters, commonly known as UV filters, are used on camera lenses to block near ultraviolet radiation before it reaches the film or sensor and thereby prevent these effects.
Near ultraviolet photography - the deliberate use of near ultraviolet radiation as the primary imaging wavelength - is a specialised technique used in scientific, forensic, medical, and fine art photographic applications to reveal information about subjects that is invisible under normal visible light illumination. Many materials fluoresce when illuminated with near ultraviolet radiation, absorbing the ultraviolet photons and re-emitting the energy as visible light of longer wavelengths - a phenomenon exploited in ultraviolet fluorescence photography to reveal hidden details in documents, artworks, biological specimens, and crime scene evidence. Other materials reflect or absorb near ultraviolet radiation differently from visible light, allowing ultraviolet reflectance photography to reveal surface details, chemical compositions, and physical characteristics that are indistinguishable under visible illumination.
The atmosphere absorbs increasingly strongly at wavelengths below approximately 300 nanometres, with ozone in the stratosphere providing substantial absorption of ultraviolet radiation below around 280 to 300 nanometres. This atmospheric absorption limits the near ultraviolet radiation available at ground level primarily to the longer wavelength portion of the near ultraviolet band, roughly from 300 to 400 nanometres, with shorter wavelengths in the near ultraviolet range becoming progressively less available at the Earth's surface due to atmospheric absorption. Scientific ultraviolet photography conducted at shorter wavelengths within the near ultraviolet range therefore typically requires controlled artificial ultraviolet light sources rather than relying on ambient ultraviolet radiation.