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 lenses. 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 most 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 set-up is based on a Michelson interferometer in which the light from a
broadband superluminescent diode is split into a reference arm with a variable optical delay and a measurement arm
where the sample is placed. On a 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 distances of the lens surfaces are derived from the optical delay. In order to penetrate IR materials, the
instrument operates at 2.2 μm.
Together with an LWIR autocollimator, this technique allows for the determination of centering errors, lens thicknesses
and air spacings of assembled IR objective lenses with a micron accuracy. It is therefore a tool for precision
manufacturing and quality control.