A neutral density or ND optical filter reduces or modifies the intensity of all wavelengths or colors of light equally, therefore providing for no changes in the hue of the color rendition. Neutral density filters can be colorless or clear, or they can be grey in tone.
For photography, the purpose of a ND filter is to reduce the amount of light entering via the camera lens, preventing overexposure of images. The filter allows for a longer exposure times than would otherwise be possible. The longer exposure time may allow the photographer to emphasize motion in the image taken. ND filters also enable larger apertures, producing a shallow depth of field.