In contrast to VIS systems, the light emitted from the focused autocollimator head cannot be seen by the naked eye which is however no problem in practice for aligning the sample. Apart from that, the operation of the infrared systems is not different to the VIS systems, so a user can be quickly trained for a new wavelength range.
Differences to VIS from a technical perspective
From a technical side, apart from using suitable optics and illumination sources in the measurement heads, the most important difference between visual and infrared range is that in the infrared range every object, including the sample, emits light in this wavelength region, so the instrument needs to compensate for the thermal background before taking a measurement. This is done automatically by the software and requires no operator intervention. Also, the contrast between background and illuminated areas is lower than in the VIS, so specialized image processing algorithms are used to reach the required high resolution.
OptiCentric® in the standard reflection mode relies on back-reflection from the lens surface, so the light intensity of the reflected reticule image strongly depends on the type of coating used. Typically, all infrared imaging lenses are AR-coated, however there is a wide variation in efficiency which the instruments compensates by adjusting illumination power and shutter times where available.
In general, the typical accuracy of the centration error measurement is approximately 1 µm, slightly higher than for the instruments in the VIS, which is due to the longer wavelength and larger pixel size of the cameras used in the autocollimators.