Two-lens designs for modern uncooled and cooled IR imaging devices

 This text is an abstract. The full text is available at the SPIE Digital Libary.

In recent years, uncooled, or thermal, detectors with a 17µm pixel pitch have become well-established for use in various applications. Examples of applications include thermal imaging in cars for driver’s vision enhancement. This has allowed the civilian infrared market to steadily mature. The main cost driver for the manufacture of these lens designs is the number of lenses used.

The development of thermal detectors, which are less sensitive than quantum detectors, has compelled camera manufacturers to demand very fast F-numbers, often f/1.2 or faster. This has the advantage of minimizing the impact of diffraction in the 8-12mm waveband.

The freedom afforded by the choice of the stop position in these uncooled, thermal systems has been used to create high-resolution lenses that operate near the diffraction limit. Based on GASIR^®1, a chalcogenide glass, two-lens designs have been developed for all pixel counts and fields of view. Additionally, all these designs have been passively athermalized, either optically or mechanically.

Lenses for cooled quantum detectors have a defined stop position, called the cold stop, near the plane of the detector (FPA). The solid angle defined by the cold stop fixes not only the F-number (which is slower than for thermal detectors), but determines also the required resolution. The main cost driver of these designs is the lens diameter. Lenses must be sufficiently large to avoid any vignetting of ray bundles intended to reach the cooled detector.

Dr. Schuster’s paper studies the transfer of approved lens design principles for uncooled thermal detectors to lenses for cooled quantum detectors for same pixel count at three horizontal fields of view: a 28° medium field lens, an 8° narrow field lens, and a 90° wide field lens. One conclusion is that the lens arrangements found for each category have similar lens costs. 

(NS, JF)