Challenges, constraints and results of lens design in 8-12micron waveband for bolometer-FPAs having a pixel pitch 12micron

In the 8-12 micron waveband Focal Plane Arrays (FPA) are available with a pixel pitch of 12 microns or less. High resolution FPAs with VGA, XGA and SXGA resolution should become available at a reasonable price. These will require new lens designs to give the required fields of view. The challenge for the Optical Designer is to design lenses when the pixel pitch of the detector is the same as the wavelength of the light imaged. The lens specification will need to give more thought to the resolution required by the system. A smaller pixel pitch detector defines a requirement for a shorter focal length to give the same field of view. This will have a number of effects upon the lens design. Geometrical aberrations decrease proportionally with the focal length. Reverse telephoto layouts will become more common, particularly when the system has a shutter. The increase in pixel count will require wide field of view lenses which present particular challenges. The impact of diffraction effects on the lens design is considerably increased. The fast F-number causes an increase in the diffraction limit of the system, but also increases geometric aberrations by a cube law. Therefore the balance between the diffraction limited and the aberration limited performance becomes more difficult. The first approach of the designer is to re-use proven designs originally intended for use with 17micron detectors. Some of these designs will have adequate performance at the Nyquist limit of the 12 micron detectors. Even smaller detector pitches, such as 10 micron, will demand new approaches to Infra Red lens design. The traditional approach will quickly increase the number of elements to 3 or even more. This could lead to the lenses with medium fields of view driving the system cost. A close cooperation between the camera developer and lens designer will become necessary in order to explore alternate approaches, such as wavefront coding, in order to reach the most cost effective solution.