User`s guide

ACQUISITION MODES
SDK SECTION 4
Page 47
NOTES:
• When using an FT CCD as a standard CCD, the Exposure Time, Accumulation Cycle Time
and Kinetic Cycle Time can be set independently.
• The minimum exposure time is not related to the time taken to read out the image.
• External trigger operates as if the CCD was a Non-FT CCD.
• As the captured image is quickly shifted into the storage area, even in non-frame transfer
mode, the system may still be used without a mechanical shutter.
• For short exposure times the image may appear streaked as the time taken to shift the
image area into the storage area may be of similar magnitude.
• Light falling on the Image area while the Storage area is being read out may contaminate
the image in the Storage area due to charge spilling vertically along a column from the
Image area. The slower the readout rate or the shorter the exposure time the greater the
possibility of corruption. To see why this is the case, consider the following situation:
“During a 100us exposure enough light has fallen on a pixel to register 10000 counts, or 100,000
electrons assuming 10e/count. The image is then shifted into the Storage area. To read out the
image, assuming 1000x1000 pixels, it would take approximately 100ms at 10MHx readout rate. This
means that during the reading out of the image 10 million counts (10000 * 1000) will have been
acquired into the pixel described above. As a pixel saturates at approximately 160,000 electrons this
means that the pixel will over saturated by 60 times. All the excess charge has to go somewhere, and
spreads vertically along the CCD column. As the clocks in the Image area are not actively shifting the
charge, the mobility of the charge will be low and you may not see any effect. However, when you
consider that more than one pixel in any given column could be exposed to 10000 counts per 100us,
the chance of corrupting data is correspondingly increased. Changing the readout rate to 1
microsecond per pixel will greatly decrease the possibility of data corruption due to the reduced time
to read out the image. Reducing the amount of light falling on the CCD and increasing the exposure
time accordingly will also reduce the possibility of data corruption.”