The bandwidth of an optical interference filter when used in an optical system is limited by the angular range of its illumination. The limitation has two main aspects: The central wavelength of the filter shifts to shorter wavelength and the shape of the transmission band degrades. The first effect results in filters with increased transmission width and decrease average transmission while the second results in reduction in the average transmission of the filter. For compact systems with high numerical aperture these effects can be pronounced. By minimizing the propagation angles in the filters these effects can be substantially reduced in the near infra red (NIR) to levels similar to IR filters. We have recently developed materials and design techniques that permit us to obtain low angle shift coating that maintain very high transmission in the spectral range from 800 to 1100 nm. In this paper we will demonstrate the performance, design and fabrication, of fully blocked (200 to 1200 nm) narrow band pass filters and their measured performance over a range of angles (0-30°). Filters with bandwidths from 15 to 100 nm will be shown to illustrate the versatility of these techniques.
Robert W. Sprague, "The angular dependence of optical interference filters in the Near IR can be greatly reduced through the use of novel coating materials and design strategies," Proc. SPIE 10181, Advanced Optics for Defense Applications: UV through LWIR II, 101810M (Presented at SPIE Defense + Security: April 10, 2017; Published: 1 June 2017); https://doi.org/10.1117/12.2264918.
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