Newport's absoprtive ND fitlers are precision polished, and offer uniform attenuation over lower power beams across the visible range at an economical price suitable for OEM integration.
* Parts are marked with OD 0.05 but actual optical density on these parts is 0.04 +/- 4%.
These neutral density filters are made from absoprtive glass. This results in a lower cost product that is somewhat less fragile compared to coated metallic ND filters. Absorptive ND filters also reflect far less light toward the source compared with metallic ND Filters, which may be beneficial. Nevertheless stacking a large number of ND filters can still result in excessive undesired reflection due to fresnel surface reflection. Due to the absorptive nature of the filter glass, use with high power lasers should be avoided. Power levels less than 30 W/cm2 are advised. For attenuation of high-energy lasers, our Model 935 High-Energy Variable Attenuators are recommended.
Absoprtive glass ND filters are spectrally neutral over the visible spectral region. OD05 to OD40 optical densities are shown.
Our absoprtive ND filters are available individually or in a set of 9. Filters may be used individually, or combined in series, to obtain any desired desnity from 0.04 to 4.0. The Ø25.4 mm and 50.8 mm square sizes are permanently marked with the optical density.
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.
Note: Due to the absorptive nature of the filter glass, use with high power lasers should be avoided.