Quantify Passive Athermalization in Infrared Imaging Lens Systems

Passive athermalization has become a key-technology for automotive and other outdoor applications using modern uncooled 25 and 17 micron bolometer arrays. For high volume applications, passively athermalized optical designs with a minimum of lenses reduce costs and require a careful choice of lens and housing materials. But, up to now, metrics of athermal properties of these lenses are seldom published. Metrics for athermalization are mentioned in two categories: MTF-based to describe application limits under environmental conditions, and first order relations which are helpful in the optical and mechanical design process. Correlation between both categories is analyzed on several GASIR (R)-lens designs. The allowable degradation of MTF in the Temperature Range depends on the lens application. The MTF-approach proposed to quantify passive athermalization considers different metrics: Several Through-Focus-MTF-graphs at interesting temperatures for optical design, the MTF-versus-field-graph at interesting temperatures offers the complete customer information; the On-Axis-MTF versus temperature shows the typical thermal drift. The most effective way to describe the athermalization status is the value pair of Temperature Range and of percentage in MTF-loss for on-axis point. This pair of values is applicable for all IR-imaging lenses, closely related to lens application, and independent of the camera detector. First order relations identify the most critical influences on athermalization. Different lens materials are discussed whereby the achromatic correction by diffractive structures reduces also the effective Thermal Glass Constant. GASIR (R) possesses inherent passive athermalization properties. Known first order relations are expanded to two group lens systems. This new relation gives a good overview on where the most effective place for the PMA-mechanism in the lens assembly is and how to arrange it. A narrow field of view example shows different kinds of movement: first group only, second group only and both groups together. It will be seen that the shortest compensation mechanism depends on power and distance of groups.