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1 October 1990 Design of narrow-band-pass IR filter with a long-wavelength pass band
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Narrow-band-pass (NBP) interference filters are commonly used in the infrared (IR) spectral region. It may happen that such a filter is to be used in conjunction with an optical system which must pass thermal radiation in the 8 to 12 m wavelength range. Thus the NBP filter must be designed in such a way that it also transmits from 8 to 12 pm. This paper presents two design techniques for achieving the required performance. In one technique two reflective stacks are used, providing blocking on either side of the NBP region. Additional layers are used to provide the transparency over the region 8 to 12 m. The second approach involves using a multiple-cavity Fabry-Perot filter to achieve the NBP performance. The long-wave--pass region can be achieved by varying the refractive index of selected layers within the multilayer coating. Changing these indices has a minimal effect on the performance in the pass band. The above design techniques are applied to the NBP filter with the following specifications: Pass-band center: 4.0 m Pass-band width: m Pass-band transmittance: 80% Pass-band edge slope: ( 0.05 im (5% to 95%) Long-wave region: 8-12 m Long-wave transmission: T 90% One useful design trick that can be employed takes advantage of the fact that the long-wave transmission region covers a range of wavelengths which are approximately twice the wavelength of the NBP region. Thus layers with a halfwave optical thickness of 4 pm can be added without affecting the NBP performance. These layers will have an optical thickness close to a quarterwave over the 8 to 12 pm range and can be used to reduce the reflectance over this region.
© (1990) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Bradley J. Pond and Charles K. Carniglia "Design of narrow-band-pass IR filter with a long-wavelength pass band", Proc. SPIE 1307, Electro-Optical Materials for Switches, Coatings, Sensor Optics, and Detectors, (1 October 1990);

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