Reflective Neutral Density filters (ND Filters) provide a linear spectral transmission across the visible region. Reflective Neutral density filters control light intensity without selectively absorbing energies at specific wavelengths. They are, therefore, generally used to attenuate the intensity of a beam of light over a broad spectral region.
This property is often required to prevent saturation or damage to a wide variety of detectors and even the human eye.
Reflective ND filters are available in standard densities of 0.3, 0.5, 0.6, 0.9, 1.0, 1.3, 1.5, 2.0, 2.5 and 3.0. Stack them and the total density is the sum of the density of each filter in the stack, permitting the creation of intermediate neutral density values.