Neutral Density Filters
A neutral density filter is one which reduces light intensity, modifying the intensity of all wave lengths or colours, but with no change in hue.
This allows a photographer to be able to change aperture, shatter speed and or motion blur under varying conditions and circumstances.
Application: ND filter is primarily used for photography to make use of a larger aperture. A ND filter can also be used within large telescopes to reduce brightness/contrast of the moon and planets.
ND filters are also found in high precision laser experiments where they are used to reduce light levels without disrupting the settings of a laser.