The majority of neutral density filters offered are manufactured with a metallic coating which attenuates incident light by both reflection and absorption. Depending upon the spectral range of interest, these filters are manufactured on substrates of quartz, sapphire, germanium or silicon.
Neutral density filters are categorized by their optical density (D) which is defined as the logarithm to the base 10 of the reciprocal of the transmitted radiant power (T)
Optical densities can be added so several filters can be placed in series to obtain a specific value (e.g. D1 + D2 + D3...). However, care must be taken when aligning the filters to avoid the effects of multiple reflections, from parallel surfaces, which could result in a higher transmitted energy than expected.
In general, neutral density filters are manufactured on a custom basis with optical densities from 0.1 to 2.0 (79 to 1%).
Northumbria Optical Coatings Ltd (NOC) was established in 1995, specializing in the design and manufacture of Infrared Optical Filters and Coatings, covering the spectral range 1.5μm - 20.0μm. Before 1995 NOC operated as a business unit for Rolls Royce International Research & Development based in Newcastle upon Tyne. NOC offers competitively priced standard and non-standard filters which are used in many applications including environmental monitoring, analysis and control of gaseous effluent, analysis of vehicle exhaust gases, ozone layer monitoring, night vision systems and space applications. Contact EOC for more information.