Made with Inconel® metallic coatings on glass substrates, these ND filters provide spectrally uniform attenuation over the wavelength range from 0.4 to 2.0 µm. The reflective nature of the coatings helps reduce thermal effects when these filters are used with moderate-power lasers. Filters may be used individually, or combined in series, to obtain any desired density from 0.04 to 2.5.

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.

The 5247NF, 5248  and 5249NF Neutral Density (ND) Filter Sets contain the same filters that we use in our ND-filter wheels. The 5247NF set contains six metallic, borofloat filters of OD 0.04, 0.5, 1.0. 1.5. 2.0 and 2.5. The 5248 set contains six metallic, borofloat filters of OD 0.04, 0.1, 0.2, 0.3, 0.4 and 0.5. The 5249 set contains ten metallic, borofloat filters of OD 0.04, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 1.5, 2.0 and 2.5. Filter kits provide convenience and cost savings.

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.

Borofloat 33 is a high quality borosilicate glass that offers low thermal expansion and high thermal shock resistance. Its material properties and moderate cost make Borofloat 33 an ideal substrate for general purpose applications. For more information, please see our Optical Materials technical note.