Image Processing Reference
In-Depth Information
Wavefront Sensing
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Figure 3.9
Ronchigram showing the effect of an unaberrated wavefront and that of
spherical aberration.
3.4 Phase Contrast
3.4.1 Foucault knife-edge test
One of the earliest tools available to optical builders was the knife-edge test, a sen-
sitive, simple test that can be used to detect longitudinal shifts in focus. A
knife-edge test involves placing a knife edge, such as a razor-blade edge, at the
paraxial focus of an optical system. If the knife edge cuts the converging beam in
front of the focus, a screen positioned behind the focus shows a bright region in the
lower half. If the beam is cut after the focus, the screen is bright in the upper half of
the screen. When the knife edge is located at the focal point, the screen shows a uni-
formly dim or dark spot. This is illustrated in Fig. 3.10.
To convert the knife-edge test into a wavefront sensor, the screen is replaced
with two or more detectors whose analog output can be compared. The focus point
Figure 3.10
A classical knife-edge test showing the position of the edge being in-
serted into the beam (lower) and the image generated on a screen. The shading of
the screens (top) is inverted for the position of the knife edges.
