Refracting Infrared Objectives For The 8-13 Micron Band

AbstractSeveral fast, three element refractive objectives have been designed for use withnarrow bandpass filters and cooled detector arrays in the 8 -13 pm infrared band. Theperformance of these designs, measured by the fraction of incoming radiation concentratedwithin the boundaries of a typical detector element, approaches the theoretical maximum.The designs also illustrate the extent to which performance can be improved through theuse of aspheric surfaces.First order propertiesThe basic requirement was for a refracting objective of 200mm focal length and f /ratioof 2 or faster. A linear detector array of 180 50x5opm elements on 100µm centers was tobe used to cover half the field, defining the gaussian image height at 18mm and the half field at 5.2 °. Narrow -pass filters were to be used to isolate spectral regions within the 8 -13 µm band. To avoid the necessity of refocusing when switching filters, it was required thatperformance at the nominal back focal length not change significantly within these limitsof Wavelength. To assure uniformity of spectral response across the field, telecentricitywas also a desirable feature.Finally, a long back focal length (bfl) was required to provide room for a dewarwindow, chopper, and filter wheel.Starting solution and materials choiceNo off - the -shelf objective was found to be available, primarily because of therelatively large focal length and bfl requirements. A three element starting design,lacking only the necessary bfl, was obtained from the patent literature' and reoptimizedusing Grey's program.The wide spectral region over which the lens must operate results in performancebeing limited by secondary chromatic aberration. Control of this aberration in theinfrared is particularly difficult because of the limited choice of materials havinggood physical properties (e.g. other than salts) and because these materials tend tohave similar partial dispersions. Combinations of germanium with zinc sulfide or zincselenide afford the possibility of reasonable correction. A thin -lens triplet ofgermanium, zinc selenide, and cadmium telluride could theoretically2 be corrected forsecondary color. This possibility was pursued briefly in a thick, separated design,but the improvement in performance did not appear significant considering the high costof CdTe.Results of optimizationAn optimized solution, lens A, employing only spherical surfaces is illustrated infigure la and described in table la. Performance is indicated by the radial energydistributions of the polychromatic point spread function (psf), figure 2a, the mono-chromatic rms opds, and the percentage of the polychromatic energy lying within adetector element boundary,table 2a. At up to 0.7 of full field (3.6°), the detectorreceives roughly 80% of the incoming energy, as opposed to 90% in the diffractionlimited case. Performance drops significantly at full field, but can be improved byreducing the bfl as much as can be tolerated mechanically.The addition of a single conic surface was next investigated. The first germaniumelement was found to be the optimum candidate for the conic due to the high index andrelatively large ray heights. The concave surface (3) was chosen to facilitate testing.The optimized lens, lens B, is described and evaluated in figures and tables 1