Ultraviolet radiation is electromagnetic radiation or light having a wavelength greater than 10 nm but less than 400 nm. Ultraviolet radiation has a wavelength longer than that of x-rays but shorter than that of visible light. Ultraviolet is energetic enough to break some chemical bonds.
Advanced Image Quality Space-Qualified Ultraviolet Interference Filter (Courtesy-NASA/JPL/Caltech)
(UV)filters block ultraviolet radiation, but let visible light through. Because
photographic film and
digital sensors are sensitive to ultraviolet (which is abundant in skylight) but the human eye is not, such light would, if not filtered out, make photographs look different from the scene that the photographer saw. This causes images of distant mountains to appear hazy. By attaching a filter to remove ultraviolet,
photographers can produce pictures that more closely resemble the scene as seen by a human eye.
Neutral density (ND) filters have a constant attenuation across the range of visible wavelengths, and are used to reduce the intensity of light by reflecting or absorbing a portion of it. They are specified by the optical density (OD) of the filter, which is the negative of the common logarithm of the transmission coefficient. They are useful for making photographic exposures longer. A practical example is making a waterfall look blurry when it's photographed in bright light. Alternatively, the photographer might want to use a larger aperture (so as to limit the depth of field); adding an ND filter permits this. ND filters can be reflective (in which case they look like partially-reflective mirrors) or absorptive (appearing grey or black).