What is an ND Filter?

An ND, or neutral density filter, is a type of filter designed to decrease the intensity of the input light without affecting the color (or spectral) distribution. Due to this neutral characteristic, the filters appear gray in color. ND filters are typically designated by their optical density (OD). Transmission for an ND filter can be calculated by using the equation T=10^-OD x 100 = Percent transmission Where OD is the optical density of the ND filter. Typical applications for ND filters include decreasing light consistently over the visible or near infrared spectrum, protecting sensing devices from excessive intensity, such as blooming/overexposure of CCD cameras or testing the linearity of a photodetector or photodiode. ND filters are preferred over polarizers for reducing light in the case of extreme light intensities.