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.
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.
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.