14 May 2015 Thickness and air gap measurement of assembled IR objectives
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Abstract
A growing number of applications like surveillance, thermography, or automotive demand for infrared imaging systems. Their imaging performance is significantly influenced by the alignment of the individual lens elements. Besides the lateral orientation of lenses, the air spacing between the lenses is a crucial parameter. Because of restricted mechanical accessibility within an assembled objective, a non-contact technique is required for the testing of these parameters. So far commercial measurement systems were not available for testing of IR objectives since many materials used for infrared imaging are non-transparent at wavelengths below 2 μm.

We herewith present a time-domain low coherent interferometer capable of measuring any kind of infrared material (e.g., Ge, Si, etc.) as well as VIS materials. The fiber-optic set-up is based on a Michelson-Interferometer in which the light from a broadband super-luminescent diode is split into a reference arm with a variable optical delay and a measurement arm where the sample is placed. On a photo detector, the reflected signals from both arms are superimposed and recorded as a function of the variable optical path. Whenever the group delay difference is zero, a coherence peak occurs and the relative lens’ surface distances are derived from the optical delay. In order to penetrate IR materials, the instrument operates at 2.2 μm.

The set-up allows the contactless determination of thicknesses and air gaps inside of assembled infrared objective lenses with accuracy in the micron range. It therefore is a tool for the precise manufacturing or quality control.
© (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
B. Lueerss, B. Lueerss, P. Langehanenberg, P. Langehanenberg, } "Thickness and air gap measurement of assembled IR objectives", Proc. SPIE 9489, Dimensional Optical Metrology and Inspection for Practical Applications IV, 94890C (14 May 2015); doi: 10.1117/12.2182505; https://doi.org/10.1117/12.2182505
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