Filter
Top Products
Chapter 6 Advanced Topics 77
External Sync with Continuous Cleans
The third timing mode available with the MicroMAX camera is called Continuous
Cleans. In addition to the standard “cleaning” of the array, which occurs after the
controller is enabled, Continuous Cleans will remove any charge from the array until the
moment the External Sync pulse is received.
Shutter opens
Shutter remains open
for preprogrammed
exposure time
System waits while
shutter closes
Shutter opens
CCD is continuously
cleaned until External Sync
pulse is received
(shutter preopen)
(shutter normal)
CCD is continuously
cleaned until External Sync
pulse is received
Figure 32. Continuous Cleans Flowchart
Once the External Sync pulse is received, cleaning of the array stops as soon as the
current row is shifted, and frame collection begins. With Normal Shutter operation the
shutter is opened for the set exposure time. With PreOpen Shutter operation the shutter is
open during the continuous cleaning, and once the External Sync pulse is received the
shutter remains open for the set exposure time, then closes. If the vertical rows are shifted
midway when the External Sync pulse arrives, the pulse is saved until the row shifting is
completed, to prevent the CCD from getting “out of step.” As expected, the response
latency is on the order of one vertical shift time, from 1-30 μsec depending on the array.
This latency does not prevent the incoming signal from being detected, since photo
generated electrons are still collected over the entire active area. However, if the signal
arrival is coincident with the vertical shifting, image smearing of up to one pixel is
possible. The amount of smearing is a function of the signal duration compared to the
single vertical shift time.
Note: If EXT SYNC is still active at the end of the readout, the hardware will interpret
this as a second sync pulse, and so on.