Image Processing Reference
Image-Stabilization Systems Simplified
Figure 5.5 Photograph of the PSD device and drive electronics used in the im-
age-stabilization system. (Image courtesy of M. Colleen Gino.)
One of the simplest optical sensors is a position-sensitive detector (PSD) used
in the role of the wavefront sensor. In the truest sense, the PSD is not a wavefront
sensor, but is used to track the position of the focused spot. PSDs are analog devices
that provide a direct readout of the position of a spot of light.
A growing number of suppliers of PSD devices can be found, many of them
through Web sites. Most PSDs operate in a similar fashion, though their electrical
characteristics can vary.
The PSD converts the position of the center of mass of a light spot into two out-
put voltages for each direction. These voltages change as the position of the spot
changes, so the difference between the two voltages provides a measure of position.
Many PSDs provide position monitoring in two orthogonal directions on a single
The PSD requires external drive electronics to drive and condition the output
signals. Figure 5.5 shows a photograph of a PSD and electronics used to drive the
device. Similar circuits can be constructed or purchased from commercial vendors.
This circuit was constructed initially on a breadboard, but noise performance is ex-
pected to be somewhat better on a printed circuit board.