Image Processing Reference
In-Depth Information
34
Chapter 3
occurs where the signals from the detectors are at minimum. There are several vari-
ations on this sensor to provide automated focus sensing and feedback.
3.4.2 Phase-diversity sensors
Phase-diversity or curvature sensors are based on the idea that a differential
measurement of intensity in two or more planes yield information on the wavefront.
Figure 3.11 illustrates how this type of sensor works. Consider an unaberrated wave-
front propagating through space. If one samples the wavefront with an intensity de-
tector in two planes, shifted by a distance Z , the difference between the two inten-
sity patterns is zero. If one then looks at an aberrated wavefront, the difference
between the two planes gives rise to excess or defect of intensity. These nonzero re-
gions of intensity difference are directly related to the second derivative, or curva-
ture, of the wavefront, and are mathematically described in Eq. 3.2 by the so-called
transport equation of intensity and phase:
I
z
I
z
)
(
2
k
=−∇⋅
I
φ
=−∇ ∇ +
I
φ
I
φ
k
,
(3.2)
Figure 3.11 Layout of a simple phase-diversity sensor. Notice that the CCD camera
shows an image and an out-of-focus spot using a beamsplitter and a right-angled
prism.
 
Search WWH ::




Custom Search