Neutral Density Filters can be absorptive or reflective.  These filters evenly attenuate the transmission across a certain spectrum by absorbing or reflecting non-transmitted light.  Hence, they are used in imaging applications to prevent damage or inaccuracy from excessive light for instance in-camera, photometer, and light control applications. These filters are available with us in UV, Visible, NIR, and IR spectral regions and in a range of optical densities (0.1–3).