Passive athermalization of lenses has become a key-technology for automotive and other outdoor applications using
modern uncooled 25, 17 and 12 micron pixel pitch bolometer arrays. Typical pixel counts for thermal imaging are
384x288 (qVGA), 640x480 (VGA), and 1024x768 (XGA). Two lens arrangements (called Doublets) represent a
cost effective way to satisfy resolution requirements of these detectors with F-numbers 1.4 or faster.
Thermal drift of index of refraction and the geometrical changes (in lenses and housing) versus temperature defocus
the initial image plane from the detector plane. The passive athermalization restricts this drop of spatial resolution in
a wide temperature range (typically -40°C…+80°C) to an acceptable value without any additional external refocus.
In particular, lenses with long focal lengths and high apertures claim athermalization. A careful choice of lens and
housing materials and a sophistical dimensioning lead to three different principles of passivation: The Passive
Mechanical Athermalization (PMA) shifts the complete lens cell, the Passive Optical and Mechanical
Athermalization (POMA) shifts only one lens inside the housing, the Passive Optical Athermalization (POA) works
without any mechanism.
All three principles will be demonstrated for a typical narrow-field lens (HFOV about 12°) with high aperture
(aperture based F-number 1.3) for the actual uncooled reference detector (17micron VGA). Six design examples
using different combinations of lens materials show the impact on spatial lens resolution, on overall length, and on
First order relations are discussed. They give some hints for optimization solutions.
Pros and cons of different passive athermalization principles are evaluated in regards of housing design, availability
of materials and costing. Examples with a convergent GASIR®1-lens in front distinguish by best resolution, short
overall length, and lowest weight.