The purpose of the neutral density (ND) coating is to attenuate the incident flux. In its simplest form, the thickness of a single metallic layer is adjusted so that a prescribed transmittance is attained.
The transmission is dependent on the wavelength region. This graph shows the transmission of 2 different metallic neutral density filter coatings over visible wavelengths.
Adhesion, moderate abrasion as per the following (where applicable): MIL-C-675, MIL-M-13508, MIL-M-14806, MIL-C-48497, MIL-F-48616, MIL-STD-810F