FIR filters have a 80-50 scratch dig Germanium substrate and offer higher OD tolerance through the wavelength range. The graph shows that the curve is almost flat over the full wavelength range.

NDIR series ND filters feature a 60-40 scratch dig Germanium substrate with looser OD tolerances than the FIR series, but at a good price point.

Optical density (OD) is given by the relationship: OD = -log(T) or T = 10^(-OD) where T is transmittance (0≤T≤1). Optical Density, unlike transmittance, is additive. When multiple filters are stacked, the total density is easily found by adding up the densities of each filter in the series. For example to achieve desired transmittance of 8%, first calculate the equivalent density (OD = -log(0.08) = 1.097 ≅ 1.1), then find a combination of filters that adds to the desired number, such as a combination of an OD1 filter and an OD0.1 filter.

To prevent unwanted interference from etalon effects, filters should be angled slightly with respect to the incident beam. To minimize back reflections, the filter should be oriented with the coated metallic surface facing the incident beam. Any light reflected from the uncoated second surface will pass through the metallic coating first, which will greatly attenuate unwanted back reflections.

The FS-IR Infrared ND Filter Set includes five 25.4 mm diameter filters with optical densities of 0.3, 0.6, 1.0, 2.0 and 3.0. Filter kits provide convenience and cost savings.