Ultraviolet (UV) filters are optical filters which can be used to filter light in the ultraviolet part of the spectrum. These filters reduce haziness that can be created by the presence of ultraviolet light. UV filters appear clear and neutral to the naked eye, and can be left on for nearly all applications. They are commonly used for reflection, transmission, absorption, and blocking UV light or wavelengths, which they do without affecting light within the visible region.
UV filters are available in a variety of absorption levels; they are measured by their transmission percentage at 400 nanometers (nm), the visible-UV wavelength boundary. In addition to UV protection filters, other types include UV neutral density filters, UV rejection filters, UV bandpass filters, UV interference filters, UV cold mirrors, hot mirrors, and UV Dichroic filters.
UV neutral density filters commonly consist of thin alloy films applied to a UV fused silica substrate. These flat spectral response filters work down UV wavelengths to 200 nm. Neutral density UV filters are widely available in a range of optical densities, including 0.04, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.3, 1.5, 2.0, 2.5, 3.0, and 4.0.
UV rejection filters block UV radiation while transmitting high quality, visible light. These filters may be applied for heat resistance, a color rendering index of nearly 95%, and significant longevity compared to plastics or gels.
Other types of UV filters include UV bandpass filters and UV interference filters. Ultraviolet bandpass filters are designed to isolate UVA, UVB, and UBC spectral regions. This type of filter is used with UV spectral lamps to simulate specific ultraviolet regions, while blocking other wavelengths. Interference filters for the ultraviolet region are precision tools used to provide a steep cut-on or cut-off at a designated wavelength. UV interference filters are commonly available for 200 nm.
Hot or Cold Mirrors
UV filters may also be hot or cold mirrors. UV hot mirrors feature a multi-layer coating that reflects infrared radiation heat, while allowing transmission of UV and visible light. UV cold mirrors reflect shorter wavelengths and transmit heat. Ultraviolet mirror coatings can often be modified to include additional wavelengths and cut-on or cut-off points.
Dichronic UV Filters
Dichronic UV filters are also available in two types: short wave pass or long wave pass. Short wave passes apply to the left of the optical transition slope, while long wave passes apply to the right side. These filters provide advantages over using colored glass filters, including that the rejection band is reflected, not absorbed; dirchronic filters have steeper transition slopes between the pass band and the rejection band for sharper cutoff. Other, unlisted types of UV filters may also be available.