ND Filter (Neutral Density Filter)
ND filter reduces light intensity at a certain rate in arbitrary wavelength range. Two types of ND filters, a metal type that uses surface reflection by metal thin film and a low-reflection type of which reflection is suppressed, are available according to the application.
Transmittance spectrum is so flat in the range of visible light that optical density (OD) is precisely achieved in the range from 0.1 to 3.0.
ND filter is used for control the light intensity that comes in lens or image sensor.