Image Processing Reference
In-Depth Information
Wavefront Sensing
27
Figure 3.2 Cross section through an Airy function showing the location of the first min-
ima, or dark ring.
3.3 Interferometry
One of the primary tools for converting phase into intensity information is the inter-
ferometer. Interferometers can be made in many different configurations to meet
the needs of many different applications (Malacara 1992). It is important to recog-
nize that some interferometers measure the wavefront directly, while others mea-
sure the slope or the differential wavefront.
Continuing the discussion from Sec. 3.2, the spot shape described by the Airy
function results from diffraction of the light from the aperture. Interference and dif-
fraction are the same phenomenon; however, the term interference is usually re-
served for combining separate beams of light. Interference in the focal plane of a
lens can be illustrated by placing an aperture mask with two holes near the outer
edges of the lens and viewing the two beams as they combine in the focal plane. The
resulting image will be the anticipated Airy pattern; however, the image will con-
tain distinctive vertical lines. These lines are caused by the interference of the two
beams in the focal plane and provide a measure of the optical path difference be-
tween the wavefronts entering the holes of the aperture mask. A representative
interferogram is shown in Fig. 3.3.
The conditions required to make such an interferogram requires a monochro-
matic, highly coherent light from a single source. This is most easily available from
a laser. In Fig. 3.3, the coherent laser beam was divided by the aperture mask and
combined onto a single spot, having traveled nearly equal path distances. This is
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