Precise opto-mechanical characterization of assembled infrared optics

Abstract

The imaging quality of assembled optical systems is strongly influenced by the alignment errors of the individual lenses in the assembly. Although instrumentation for characterizing centering errors for the visual spectral range existed for some time, the technology to include the LWIR (8-12µm) and the MWIR (3-5µm) spectral ranges was only recently developed. Here, we report on the development and performance of such a measurement system that is capable of fully characterizing the alignment of all individual elements of an IR lens assembly in a non-contact and non-destructive fashion. The main component of the new instrument is an autocollimator working in the LWIR that determines the position of the center of curvature of each individual IR lens surface with respect to the instruments reference axis. This position data are used to calculate the shift and tilt of the individual lenses with respect to each other or a user-defined reference axis like e.g. the assembly housing. Finally, to complete the whole picture, the thicknesses and air gaps between individual lenses are measured with a low coherence interferometer built into the instrument. In order to obtain precise data, the instrument software takes the measured real centering error into account and directs the user to optimally align the assembly with respect of the interferometer reference axis, which then determines the position of the vertex positions along the optical axis and from these the center thicknesses of each lens and the air gaps between lenses with an accuracy below one micrometer.