Very Large Narrow IR Bandpass Filter Development

Program objective is to design an extremely narrowband filter assembly that restricts focused irradiance on focal plane arrays to 5.43 microns with a bandwidth less than 0.005 microns. This novel multi-element optical configuration uses two narrowband filters in conjunction with wedge-shaped corrector faceplates to produce the required spectral profile over an irradiance collection angle at least +/- 12.5 degrees. Technical feasibility of this approach is based on the physical principles that (1) spectral bandpass wavelengths are decreased when a narrowband filter is tilted relative to the optical axis and (2) the cone angle of focuses irradiance traversing a window material is compressed in proportion to the material's refractive index (Snell's Law). In subsequent discussions, this spectral bandpass design will be referred to as the "Dirac Composite Filter" (DCF). Proof of concept will be accomplished by designing, fabricating, assembling and testing the Dirac filter and wedge components that exhibit the required spectral band profile. Results derived from this prototype DCF program will establish a verifiable methodology for manufacturing narrowband filter designs that (1) have more precisely defined spectral profiles over large 4 inch apertures; (2) are less expensive to fabricate; and (3) can be installed between optics and focal planes without any widening of spectral bandwidth.