Neutral density filters are light filters that decrease the intensity of light without altering the relative spectral distribution of the energy. They are used to filter the entire visible spectrum evenly, allowing light reduction, without influencing color or contrast. For this reason, neutral density filters are often referred to as gray filters or ND filters. Attenuation is accomplished by using either a light-absorbing glass or a thin-film metal coating that combines absorption and reflection. Metallic type neutral density filters obtain their optical density by depositing a metal alloy coating onto a specific type of substrate, which is determined by the wavelength region of interest. They are slightly sensitive to angles but they are much more forgiving than interference filters. Neutral density (ND) filters attenuate spectral regions selected from 250 to 2500 nm. The level of attenuation can be specified from optical density 0.04 to 4.0.
Neutral density filters reduce transmission either by reflection or absorption. Reflective neutral density filters use partial reflection to reduce light transmission evenly, while absorptive neutral density filters do the something using partial absorption. Variable neutral density filters, which change the transmission linearly over time, are also available. Neutral density filters are used with all types of film, but are especially valuable when working with high-speed films or long-exposure motion applications. When there is too much light, or a wide aperture is required, neutral density filters may be used. They diminish distracting backgrounds in a scene by permitting the use of wider apertures to reduce depth of field as required to throw the background out of focus while maintaining focus on the subject. This allows for the emphasis of the subject by controlling depth of field or creating motion effect with subject blur. Neutral density filters come with different levels of tint for fine-tuning exposures. Neutral density filters of higher values are required for observation of the sun or when photographing smelters, arc welding, electrical generators, and other intense light sources.