Neutral Density Filters, usually called ND Filters, is a type of grey or dark grey glass filter, which function is to reduce the transmission rate of the optical intensity in full spectrum, with no change to color balance. ND filter is a very important kit in photography. By using different OD value filters, photographer can catch the expected perfect images in bright circumstances, while the existed aperture of the camera is fixed.

ND filters, reducing the light intensity to image sensor, is very helpful for various applications, such as surveillance or monitor optical systems in bright environments, optical measuring equipments or some optical devices with bright positive light sources.