Neutral density (ND) filters are designed to reduce transmission evenly across a portion of the spectrum. There are two types: absorptive and reflective. The absorptive type absorbs light that is not transmitted, while reflective filters reflect it away. ND filters are often used to prevent blooming or overexposure of cameras and other detectors.
Note: Specifications for experimental grade optics may vary as much as 10% from those listed. Also, no additional information is available other than what is provided.