Neutral density (ND) filters are often used in imaging or laser applications to avoid damage to camera sensors or other optics caused by bright light.

ND filters absorb or reflect the untransmitted part of the light to uniformly reduce the transmittance of a part of the spectrum. Reflective neutral density filter, using the principle of thin film interference, will part of the light through, and the other part of the light reflected. Absorption neutral density filters generally refer to the material itself or after some elements are mixed in the material, the absorption of some specific wavelengths of light, while the other wavelengths of light do not occur or have little impact. Usually the damage threshold of the absorption neutral density filter is lower, and after a long time of use, there may be fever.