Neutral density filters uniformly attenuate the intensity of light over a broad spectral range. Attenuation is accomplished by either absorption using light absorbing glass or by a combination of absorption and reflection using a thin-film metal coating. Neutral density filters can attenuate spectral regions selected from 250 to 2500nm. The level of attenuation can be specified from optical density (OD) 0.04 to 4.0 where OD is defined as -log10 (T).
Overall attenuation may be increased by stacking neutral density filters serially in the light path. For example, a 0.5 (OD 0.5) neutral density filter placed in series with a 0.3 (OD 0.3) neutral density filter will produce total attenuation of OD 0.8 (10-0.8) or 16% transmission. A six-filter set of 0.1, 0.3, 0.5, 1.0, 2.0, and 3.0 optical densities can achieve a graduated range of spectrally neutral attenuation from OD 0.1 to 6.9. See OD conversion table below.
- Neutral Density (metallic)
Neutral density filters are defined by:
- Optical Density—the level of neutral transmission across the wavelength region
- Optical Density—OD = -log10 (T)
Absorption Glass Neutral Density (ND)
Uncoated, gray tinted glass that attenuates entirely by absorption. The degree of attention is determined by the thickness of the glass. They are not transmissive in the UV region and are generally only effective in the visible portion of the spectrum.
Metallic-Coated Neutral Density (ND)
Attenuate by a combination of reflection and absorption. This type of neutral density filter is much more uniform/neutral than the absorption glass filter and can be manufactured for a wider spectral range. (See Figure 1 for a comparison)
Figure 1: Neutral Density Comparison
A comparison of an absorption glass and a metal coated neutral density filter.
Standard Specifications: Neutral Density Filters
The following specifications apply to all Omega Optical metal coated neutral density filters unless other custom specifications are requested.
|Transmission Tolerances (at 550nm):|
|For OD <1.0||±0.05 OD from nominal|
|For OD 1.0–3.0||±0.1 OD from nominal|
|For OD >3.0||±0.2 OD from nominal|
|Neutrality Tolerance||The variation in optical density from 400 to 700nm will be less than 10% of the OD measured at 550nm|
|Outer Dimension Tolerance||+0, -0.25mm (+0, -0.01”)|
|Clear Aperture||4mm less than nominal outside dimension|
|Substrate Material||Optical quality glass or UV-grade quartz|
|Surface Quality||80/50 scratch/dig per Mil-O-13830A|
|Operating Temperature Range||-60°C to +80°C|
|Coating||Vacuum-deposited metallic alloy, if not absorption glass type|
|Construction||Either (a) exposed, front-surface metalic coating, (b) metallic coating laminated and protected in a moisture-resistant assembly, or (c) absorption glass|
Spectrophotometric trace of the neutral region is provided.