Use of a dual band FPA necessitates an optical system that is capable of imaging both mid wave infrared (MWIR) and long wave infrared (LWIR) spectral bands simultaneously. Such optical system can have up to 10 lenses, (20 surfaces that require antireflection (AR) coatings) which, if 95% transmitting in each band, will result in overall throughput of just under 60%1. With 99% transmitting in each band, overall throughput would be just over 90%, a relative improvement of 50%. An earlier paper presented dual band antireflection designs, as well as early fabrication attempts on plano Ge, ZnSe, ZnS, AMTIR-1, and CaF2 windows2. This paper presents results of prototype coating fabrication on ZnSe, Ge, and BaF2 lenses that comprise a 7 lens set. The measured performance of the individual elements is used to model overall system performance. The elements were incorporated into an optical assembly and measured overall imager performance is analyzed and presented.

A method of antireflection coating the interior and exterior surfaces of a deep concave optic is under development and is described. The challenges of coating such an optic include obtaining uniform performance, good mechanical and optical performance across a temperature range of ambient to 1000oC, and the transition to cost effective production. The coating process utilizes a tuned cylindrical magnetron sputtering source which sits inside the nose cone to coat the inner surface and a complementary cylindrical sputtering source to coat the outside surface. The flux from the sputtering source is tuned along the length of the cylinder by stacking an inner core of magnets in such a way as to produce a spatially variant magnetic field which allows the source distribution to approximate a uniform deposition on the surface of the optic. A deposition occulting mask provides fine tuning of source uniformity.

Dual band infrared imagers require a similar set of filters as are needed by single band infrared imagers but with the added requirement of high transmission in the mid and far infrared. The design of discrete layer filters with optimized dual band transmission is investigated for three types of filters. These are a visible-infrared beamsplitter, a long wavelength edge filter and a dual bandpass cold filter. These designs illustrate the role that harmonic reflection bands can play in the design of dual band filters. The visible reflection beamsplitter design does not have harmonics in the infrared but requires additional layers to reduce reflection at mid and long wavelengths. The long wavelength edge filter requires suppression of the second and third harmonics while the sensor band pass cold filter can use harmonics to advantage. Design techniques are discussed and the results of an initial set of fabrication runs are presented to assess the sensitivity of example designs to manufacturing errors.

Sensor performance for dual band forward looking infrared (FLIR) imagers can be substantially improved by increased simultaneous throughput of both sensor bands in the optical systems. Currently available antireflection coatings (ARs) have optimized performance for either spectral band, but not both on the same optic. Where AR coatings cover the mid and long wave infrared (LWIR) bands, or the entire broad band spectrum from visible to LWIR, performance is not sufficient for future systems. A method of designing and fabricating high performance ARs has been developed. This paper presents a discussion of the trade-off of film thickness and complexity versus transmission performance. Fabrication results for high, medium and low index lens materials are also presented.

Spectral control is an important consideration in achieving high conversion efficiency with thermophotovoltaic (TPV) energy conversion systems. TPV modules using front surface filters as the primary spectral control device have demonstrated conversion efficiencies in excess of 20% with power densities in excess of 0.4 W/cm2. The front surface filter we are developing is a short pass, long wavelength reflection filter consisting of an interference filter deposited on a plasma filter. The materials used in the interference filter must exhibit high broad band transmission and good film quality and sufficient temperature stability at the operating temperature of the TPV cells and over any potential temperature excursions that may occur. Three high refractive index materials that offer good potential for use in TPV spectral control filters are antimony selenide (Sb2Se3), antimony sulfide (Sb2S3), and gallium telluride (GaTe). The highest spectral efficiency has been demonstrated using Sb2Se3; however this material develops significant near infrared (NIR, 0.72–2.5μm) absorption at temperatures in excess of 90°C. The other two materials are being developed as high temperature alternatives to Sb2Se3. TPV filters using GaTe and Sb2S3 have been designed and fabricated, and initial results indicate that GaTe based filters are capable of operation at temperatures of 150°C or greater. Measured performance of TPV filters containing Sb2Se3, GaTe and Sb2S3 are presented, along with the impact that these have on TPV module performance.

Optical interference notch filters shift to shorter wavelengths with increasing angles of incidence. This phenomenon restricts the filter's field of view and limits the practical application of narrow reflection notch filters. The amount of shift is inversely proportional to the effective average index of the composite film. A method of designing narrow notch optical filters with very broad field of view and controllable bandwidth is demonstrated. Because this method produces a filter that is predominantly composed of the high refractive index material, it will shift on angle less than a typical quarter-wave notch filter. Increasing the effective index of the filter also reduces the separation of S and P-polarized light with angle. This paper presents modeled and measured performance for both mid and far-infrared filters developed using this technique. Narrow notch discrete and rugate filter designs are compared.

The efficiency of thermophotovoltaic (TPV) energy conversion is dependent on efficient spectral control. An edge pass filter (short pass) in series with a highly doped, epitaxially grown layer has achieved the highest performance of TPV spectral control.

The efficiency of thermophotovoltaic (TPV) energy conversion is dependent on efficient spectral control. An edge pass filter (short pass) in series with a highly doped, epitaxially grown layer has achieved the highest performance of TPV spectral control. Front surface, tandem filters have achieved the highest spectral efficiency and represent the best prospect for even higher spectral efficiency for TPV energy conversion systems. Specifically, improvements in the physical vapor deposition process, identification of other materials with a high index of refraction and a low absorption coefficient, and more efficient edge filter designs could provide higher TPV spectral performance.

An InGaAs monolithic interconnected module (MIM) using reflective spectral control has been fabricated and measured in a thermophotovoltaic radiator/module system (radiator, optical cavity, and thermophotovoltaic module). Results showed that at a radiator and module temperature of 1039/spl deg/C and 25/spl deg/C, respectively, 23.6% thermophotovoltaic radiator/module system radiant heat conversion efficiency and 0.79W/cm/sup 2/ maximum thermophotovoltaic radiator/module system power density were obtained. The use of reflective spectral control increased the spectral efficiency and thus the thermophotovoltaic radiator/module system radiant heat conversion efficiency by /spl sim/16% (relative). However, the amount of useful radiation reaching the MIM decreased by /spl sim/7% (relative) compared to using transmissive spectral control. Also, the thermophotovoltaic system radiant heat conversion efficiency and maximum power density using either transmissive or reflective spectral control decreased as the MIM temperature increased. The MIM using reflective spectral control was found to be more sensitive to changes in the MIM temperature than the MIM using transmissive spectral control.

The efficiency of thermophotovoltaic (TPV) energy conversion is dependent on efficient spectral control. An edge pass filter (short pass) in series with a highly doped, epitaxially grown layer has achieved the highest performance of TPV spectral control.