Low-Coherence Interferometers: Measurement of Lens Air Gaps, Center Thicknesses of Lenses, Objectives and All Other Transparent Materials

Low-coherence interferometers are used in optics to measure the center thickness and air gaps of lenses against each other, in other words the positions of optical surfaces along the optical axis can be determined. As such, low-coherence interferometry is an ideal complement to centration measurement (link to centration measurement), in which the positions of the centers of curvature are determined in the x/y direction. By combining both methods, it is possible to determine the absolute position of each surface in a mounted objective lens. TRIOPTICS products for this application: OptiCentric® 3D, OptiCentric® 3D Dual …..

At the same time, centration measurement is also a prerequisite for the highly accurate measurement of center thickness and air gaps, since they can be used to determine the alignment of the sample.

Measuring Principle of Low-Coherence Interferometers

The design of a low-coherence interferometer, as implemented by TRIOPTICS in the OptiSurf® system is described in Fig. 1. The light from a low-coherence light source is divided by a beam splitter into an object beam and a reference beam.

Fig. 1: Operating principle of a low-coherent interferometer
Fig. 1: Operating principle of a low-coherent interferometer

The object beam illuminates the lens system along its optical axis. A fraction of the incoming light beam is reflected back at each surface of the sample. This light is superimposed on a photo detector with the light from the reference arm. The light in the reference arm is varied by means of a delay section in the time span.

The length of the reference arm is varied by means of a movable mirror, and measured using an encoder. When the resulting intensity of the back-reflected reference beam and object beam are analyzed as a function of the change in length/delay of the reference arm (Fig. right) then interference patterns are always observed when the optical path lengths coincide in the two interferometer arms. There the difference of the group delay is zero. Thus, the position of each surface of the sample can be determined mathematically.

Products, that Work with a Low-Coherence Interferometer: