Teledyne Princeton Instruments PI-MTE, System Manual

Page: 53 / 118

Chapter 5 Operation 53
Readout of the CCD begins with the simultaneous shifting of all pixels one row toward
the shift register, in this case the row at the top. The shift register is a single line of pixels
along one edge of the CCD, not sensitive to light and used for readout only. Typically
the shift register pixels hold twice as much charge as the pixels in the imaging area of
the CCD.
Figure 5-6: Full Frame at Full Resolution
After the first row is moved into the shift register, the charge now in the shift register is
shifted toward the output node, located at one end of the shift register. As each value is
“emptied” into the output it is digitized. Only after all pixels in the first row are digitized
is the second row moved into the shift register. The order of shifting in our example is
therefore A1, B1, C1, D1, A2, B2, C2, D2, A3, ….
After charge is shifted out of each pixel the remaining charge is zero, meaning that the
array is immediately ready for the next exposure.
A subsection of the CCD can be read out at full resolution, sometimes dramatically
increasing the readout rate while retaining the highest resolution in the region of
interest (ROI). To approximate the readout rate of an ROI, in Equation 2 substitute the x
and y dimensions of the ROI in place of the dimensions of the full CCD. Some overhead
time, however, is required to read out and discard the unwanted pixels.
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