neutral-density filter
ND filter

He explains, for example, how neutral-density filters can be used to photograph moving subjects at very slow shutter speed with the camera mounted on a tripod.

Singh-Ray sold them as "The Singh-Ray set of 4 Galen Rowell Graduated Neutral-Density Filters."

In the applications of Ginger et al. a neutral-density filter is added to increase laser attenuation and the precision of laser alignment is enhanced by the addition of the inverted microscope.

A graduated neutral-density filter, also known as a graduated ND filter, split neutral-density filter, or just a graduated filter, is an optical filter that has a variable light transmission.