ND filter (Neutral-density) sets make possible the attenuation, comparison, splitting, or combining of beams in a wide range of irradiances ratios with no significant dependence on wave length. These carefully prepared sets find wide application in the precise attenuation of control of light. For example, beams can be attenuated to levels at which photometers or radiometers are most accurate and linear, thereby extending the useful range of such instruments. Initial beam irradiances can then be calculated from accurately known filter densities.
Definition of neutral density filter (ND filter)
The definition of optical density is analogous to the definition of decibel as used in electronics. That is, densities of neutral density filters used in combination are additive provided multiple reflections between filters do not occur in the direction of interest.
The reciprocal of the remittance 1/T, is called the opacity. Also in widespread use is the relative optical density DR ,
defined as the difference between the density
D of a coated substrate and the density DO
of an uncoated region of the same substrate:
= D – DO
, or D = DR
In terms of the substrate refractive index n,
DO = 2 log
Which at 550nm is typically about 0.0376 for BK7, and about 0.0309 for synthetic fused silica. Relative density DR
, and not the absolute density D, is the quantity with appears on the individual microdensitometer traces supplied with our circular neutral density wedge filters.
Transmittance and density values may, like reflectance values, refer to either small angular fields (specular or un deviated values) or very large angular fields (diffuse or hemispherical values).