DIFFRACTION EFFECTS IN IR HALF-TONE TRANSPARENCIES

Halftone transparencies are a useful tool for performance characterization of IR systems. This paper describes halftone transparencies called thermoscenes, which are lithographically produced patterns of apertures in a metallic coating placed on a transparent IR substrate. To fabricate thermoscene transparencies that are radiometrically accurate, a linear dependence is desired between aperture size and transmittance. We examine the transmittance as a function of hole size for structures suitable for application in halftone transparencies in the 3-5 and 8-12 mum bands in the IR. A theoretical model is developed, based on vector diffraction theory, that agrees with measurements within +/- 2% for the 3-5 mum and +/- 3% for 8-12 mum spectral regions. Results are that the transmittance is proportional to the aperture area, for apertures larger than four times the wavelength.