User`s guide

ACQUISITION MODES
SDK SECTION 4
Page 44
Figure 8 takes you through the capture sequence for an FT CCD:
Figure 8: Capture sequence for a Frame Transfer CCD
Step 1: Both Image and Storage areas of the CCD are fully cleaned out. This is known as a Keep Clean
Cycle. Keep Clean Cycles occur continuously to ensure that the camera is always ready to start an
acquisition when required.
Step 2: On receipt of a start acquisition command the CCD stops the Keep Clean Cycle. This allows the
image, photoelectric charge, to build up in the Image area of the CCD. The CCD remains in this state until the
exposure time has elapsed, at which point the readout process starts.
Step 3: In this step the charge, built up in the Image area, is quickly shifted into the Storage area. The time
required to move the charge into the storage area is calculated as follows:
No. of Rows in the Image Area x Vertical Shift Rate.
Once the Image area has been shifted into the storage area the Image area stops vertically shifting and
begins to accumulate charge again, i.e. the next exposure starts.
Step 4: While the Image area is accumulating charge the Storage area is being read out. This readout phase
can take tens of milliseconds to seconds depending on the image size, readout pattern and readout speed.
Step 5 & 6: On completion of the readout, the system will wait until the exposure time has elapsed before
starting the next readout (Step 6).
As the captured image is quickly shifted into the Storage area, a Frame Transfer CCD system can be used
without a mechanical shutter.
NOTES:
• When using Frame Transfer mode, the minimum exposure time for a FT CCD operated in
frame transfer mode is the time taken to readout the image from the storage area.