An interline CCD imager can be operated in a TDI mode, as defined, for example, in U.S. Pat. No. 4,922,337, allowing exposure of moving objects without stopping motion, as in a conventional TDI camera, while enabling conventional live video or long exposure of a still object, which is not practical in a conventional TDI camera. Thus, for example, high-speed scanning of a large area can be achieved with continuous motion and then followed by a return to a very small, highly magnified, still region of interest with the same camera and device.
When an interline CCD is operated in TDI mode for a number of TDI transfers, N, that is less than the total number of rows, TOTAL_ROWS, at completion there will be N trailing rows that are only partially exposed. During readout, the entire TOTAL_ROWS rows are transferred out, resulting in a readout time for TOTAL_ROWS rows but a usable image of only TOTAL_ROWS-N rows. Thus, the time to read out and discard the N rows is wasted time, slowing down the TDI scanning process.
In most scanning applications, more rapid image acquisition for a given exposure time is desirable.
A TDI system using an interline system was developed by DVC Company for an OEM customer. However, it did not attempt to trim the N partially exposed rows, and the entire TDI shift sequence was controlled by an internal timebase. TDI shifts are not individually responsive to an external control signal and therefore unable to be controlled precisely in response to subject motion.
Another TDI system, the Hamamatsu Orca R2 with TDI option, does not permit TDI followed by readout as described above. The product only permits a continuous TDI, where the charge is transported across the entire length of the CCD array (the entire number of rows, where TOTAL_ROWS equals approximately 1040). This method avoids the problem of the N partially exposed rows, since all rows are exposed and read in a continuous fashion. However, the scheme suffers significant limitations, including a relatively long minimum exposure of approximately 120 ms and a fixed, very large number of TDI transfers.