Neutral density (ie,
ND) filters reduce the amount of light that can pass through a lens.
These filters have the effect of reducing the exposure over part of the
image being taken in either a graduated or abruptly graduated (split)
They are used to filter the entire visible
spectrum evenly, allowing light reduction, without influencing color or
Neutral Density Filters
They are sometimes called ND filters or gray
filters. Attenuation is accomplished using light-absorbing glass or a
thin-film metal coating that combines both absorption and reflection.
Metallic type neutral density filters deposit a metal alloy coating
onto a specific type of substrate, to obtain their optical density
which is determined by the wavelength region of interest. Neutral
density filters are sensitive to angles but are more forgiving than
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.
Transmission is reduced by the Neutral Density Filter by either
reflection or absorption. A reflective neutral density filter uses
partial reflection to reduce light transmission evenly, while
absorptive neutral density filters reduce light transmission using
Variable neutral density filters change transmission linearly over distance.
Neutral density filters are available with different levels of tint for
fine-tuning exposures. Neutral density filters of higher values
are required for observation of intense light sources.
More about Neutral Density Filters