1. An optical filter that reduces or diminishes the intensity of transmitted light without significanly changing its color.

It is used on a camera to decrease the amount of light entering a camera when the lens cannot be stopped down sufficiently for use with a given film.

2. A light-absorbing filter whose absorption spectrum is moderately flat.

Depending on the type, the absorption curve is flat primarily in the visible spectral range, or it may extend to varying degrees beyond the visible range.

For video microscopy, this is an important procedure since the absorbance may or may not extend into the near-infrared region where the sensitivity of many video image pickup devices is very high.