CCD Cleaning During Experimental Cycles

When not acquiring actual data, CCD cameras nevertheless accumulate thermally generated ‘dark current’   (Refer to RT-TD-003 for a complete description of dark current).  Operating the camera at cold temperatures significantly reduces this dark current; however, additional removal of this unwanted charge automatically occurs in all Retriever Technology cameras by continuous cleaning of the chip.  

Retriever Technology’s rapid cleaning methodologies not only keep the chip free of dark current, but do so without compromising the camera’s ability to rapidly respond to input triggers.  When not imaging and not waiting for a trigger, the camera operates in an 'Idle' mode, which keeps the CCD clear of charge.   In Idle mode the charge is cleared out of the serial registers, which takes a few milliseconds per line.  When experiments require the camera to wait for data input while still remaining free of dark current, the mode by which charge is cleared out of the CCD is changed.  All of our cameras are set to operate in this ‘asynchronous timing mode’, in which the camera waits for an external TTL trigger to begin exposure and readout.  Now, charge is dumped out the parallel registers, which allows for very fast cleaning of the CCD.    The camera now cleans the CCD and looks for a trigger signal every 20 microseconds. Because it is rapidly dumping data, no significant dark charge accumulates.   

Because the charge is dumped out of the output drain, total cleaning cycle is only a fraction of normal CCD readout speeds.  For instance, if a 2048 x 2048 area array chip is running at 1 MHz readout rate, each line takes approximately 0.002 seconds to read.    With a delay of 20 x 10-6 seconds, only a fraction of a line of data is lost as the camera is currently configured.   If there is less than 20 microseconds between the trigger pulse being asserted and the image being flashed onto the CCD, then the most that will be lost is one line of image data, from the line closest to the readout, because it will be dumped by the clearing process. All remaining 2047 lines of data contain the image data and will be read normally because they are acquiring the image even while the CCD  is being cleared.

 For most applications, this 20 microsecond delay is sufficient.  However, the cameras are also easily configured to run in the ‘circular buffer’ mode.  In this mode, the camera is continuously reading out the chip, and saving several consecutive images in a buffer.  Upon receipt of an input signal, the camera will save whatever frame was currently being read out, as well as the subsequent frame or frames.  Using this method there will be no loss of data whatsoever, as there will always be at least two frames containing complete information before, during and after receipt of a trigger.  Moreover, cameras are available that have significantly less than one microsecond delay, opening up a variety of imaging possibilities for high speed events.   Contact us for details.