Neutral Density Filter

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A neutral density filter or ND filter is a "grey" filter. An ideal neutral density filter reduces light of all wavelengths or colors equally. ND filters that are created by being partially reflective are often used in holography as a Beam Splitter.

For an ND filter with 'OD' the amount of optical power transmitted through the filter is given by:

Fractional Transmittance = 10-OD

In a graduated ND filter the intensity varies across the surface of the filter.

Practical ND filters are not perfect as they do not reduce the intensity of all wavelengths equally. Most ND filters are only specified over the Visible spectrum region of the spectrum, and do not proportionally block all wavelengths of ultraviolet or infrared radiation.

ND 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 collimation of the beam). Moreover, most lasers have a minimum power setting at which they can be operated. To achieve the desired light attenuation, one or more neutral density filters can be placed in the path of the beam.

ND filters are quantified by their optical density or equivalently their f-Stop reduction as follows:

Attenuation Factor Filter Optical Density f-Stop Reduction % transmittance
2 0.3 1 50%
4 0.6 2 25%
8 0.9 3 12.5%
64 1.8 6 1.5625%
1,000 3.0 10 <0.1%
10,000 4.0 13 <.01%
1,000,000 6.0 20 <.0001%

Another practical way of determining what type of ND filter to use is by the percent of light that the filter allows to pass (transmittance). This parameter is typically applied to microscopy applications versus photography applications. Here is a more complete list of Filter Optical Density versus percent light transmitted: [1]