Wide spectral band imaging suggests reflective optics, but achieving a large field-of-view, low f/number, cold-stopped system requires large packaging volumes. Refractive designs are compact but material selection challenging. A design using desirable materials was achieved.
Sensors operating in the 8-12 micron long wave infrared (LWIR) portion of the electromagnetic spectrum have long
been used to extend the useful range of operating conditions beyond those of sensor systems operating in the visual
spectral band. Infrared systems must cover widely varying fields-of-view (FOV) depending on application, at fast
f/numbers compared to systems operating in the visible band. Typical FOVs for LWIR sensors run the gamut from < 1
degree to >50 degrees for large focal planes, necessitating the use of long focal lengths. When the focal length of the
optics increases, the sensitivity to defocus caused by thermal effects also increases.
Optical materials with useful transmission in the infrared region exhibit larger changes (> 400X) in refractive index
with temperature (dN/dT) than optical glass. This in turn introduces larger changes in focus over temperature for
infrared systems compared to comparable focal length visual systems. Thermal expansion and contraction of the
materials also contribute to changes in system performance and the coefficient of thermal expansion (CTE) is generally
larger for infrared materials than for visual band optical glasses.
The thermal performance problem is exacerbated with low f-numbers systems. The ability to detect targets having a
small temperature difference from ambient is proportional to the light collecting ability of the optics, especially when
uncooled detectors are used. It is typical to require f-numbers in the f/1 regime for the LWIR for uncooled applications.
Methods have been developed to find optical designs with reduced thermal sensitivity for these applications.
Refractive infrared optical designs have traditionally covered modest FOVs in one spectral band. The design of a fast, extreme fisheye lens imaging 360° azimuth by 120° elevation over the full 3-12 micron band is described. The various relevant tradeoffs that were explored are detailed and the advantages and disadvantages of each approach are discussed. In particular, the applicability of reflective and diffractive solutions is described.
The eyepiece design is one of the most challenging of all for the optical designer, since the result will be judged by the human eye, which is a very sensitive and subjective instrument. The combined effects of field curvature and astigmatism, geometric distortion, and the chromatic aberrations yield an optical system that is truly unique. No two eyepiece designs present an image that looks quite the same, and even different samples of the same design can produce different looking images due to the effects of manufacturing tolerances. In some cases the design must accommodate a large exit pupil to allow for head movement, and these dynamics introduce even more visual effects; image “swimming”, changes in distortion leading to “corner pulling”, and color fringing to name a few. When the design must be compact and weigh very little, the number of lens elements permitted is few and the design process becomes all the more difficult. The use of aspherics in the eyepiece design can compensate for the necessary limit on the number of lens elements. A case history in the design of a successful eyepiece is presented, showing the tradeoffs made in the selections of materials, aspheric complexity, fabrication concerns and packaging limitations. The performance capabilities of these designs will be discussed. The tools used to analyze optical image quality and the criteria upon which success was judged is also presented. The example used is a large exit pupil eyepiece designed to view either a miniature color CCD or LED display.
Helicopter mounted optical systems require compact packaging, good image performance (approaching the diffraction-limit), and must survive and operate in a rugged shock and thermal environment. The always-present requirement for low weight in an airborne sensor is paramount when considering the optical configuration. In addition, the usual list of optical requirements which must be satisfied within narrow tolerances, including field-of-view, vignetting, boresight, stray light rejection, and transmittance drive the optical design. It must be determined early in the engineering process which internal optical alignment adjustment provisions must be included, which may be included, and which will have to be omitted, since adding alignment features often conflicts with the requirement for optical component stability during operation and of course adds weight. When the system is to be modular and mates with another optical system, a telescope designed by different contractor in this case, additional alignment requirements between the two systems must be specified and agreed upon. Final delivered cost is certainly critical and "touch labor" assembly time must be determined and controlled. A clear plan for the alignment and assembly steps must be devised before the optical design can even begin to ensure that an arrangement of optical components amenable to adjustment is reached. The optical specification document should be written contemporaneously with the alignment plan to insure compatibility.
The optics decisions that led to the success of this project are described and the final optical design is presented. A description of some unique pupil alignment adjustments, never performed by us in the infrared, is described.