In photography and optics, a neutral density filter or ND filter is a "grey" filter. An ideal neutral density filter reduces light of all wavelengths or colors equally. The purpose of standard photographic neutral density filters is to allow the photographer greater flexibility to change the aperture or exposure time, allowing for more control, particularly in extreme circumstances.
For example, on a very bright day, one might wish to photograph a waterfall at a slow shutter speed to create a deliberate motion blur effect. In order to do this, one would need a shutter speed on the order of tenths of a second. There might be so much light that even at maximum film speed and a minimum aperture such as f/32, the corresponding shutter speed would still be too high. In this situation, by applying an appropriate netrual density filter one or more stops can be taken out of the exposure, allowing a slow shutter speed and more pleasing effect. Alternatively one can use a wider aperature to narrow the plane of focus and make the subject stand out from a blurred background.
Another use of neutral density filters is in controlling exposure with mirror lenses, which do not have traditionally adjusted aperture settings.
A graduated ND filter is similar except the intensity varies across the surface of the filter. This is useful when one region of the image is bright and the rest is not, as in a picture of a sunset.
Practical ND filters are not perfect, and do not reduce the intensity all wavelengths equally. This can sometimes create color casts in recorded images, particularly with inexpensive filters. More significantly, most ND filters are only specified over the visible region of the spectrum, and do not proportionally block all wavelengths of ultraviolet or infrared radiation. This can be dangerous if using ND filters to view sources (such as the sun or white-hot metal or glass) which emit intense non-visible radiation, since the eye may be damaged even though the source does not look bright when viewed through the filter. Special filters must be used if such sources are to be safely viewed.
Neutral density filters find applications in several high-precision laser experiments. This is because the power of a laser cannot be adjusted without changing other properties of the laser light (e.g parallelicity of the beam). Moreover, most lasers have a minimum power setting at which they can be operated. Therefore, one or more neutral density filters can be placed in the path of the beam to achieve the desired light attenuation.
ND filters are quantified by their optical density, for example a filter of OD 0.3 transmits 50% of the light falling on it.
The content of this page is retrieved from http://en.wikipedia.org/wiki/Neutral_density_filter under GFDL
