Time delay and integration
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A time delay and integration (TDI) charge-coupled device (CCD) is widely used for observation of high speed moving objects undetectable by classic CCD. Useful in a wide range of imaging applications that require both high speed and high sensitivity: earth observation satellites, inline monitoring and inspection, guidance, sorting, etc.
A TDI clock is used to synchronize the movement of charged packets in a CCD with that of another movement. For example, in some digital X-ray mammography systems, the X-ray tube and CCD detector move across the breast at a speed V+. At the same time, the CCD detector reads out the information in the opposite direction at speed V−. This compensatory motion essentially freezes the motion of the detected X-ray information. The TDI clock ensures that speed V+ equals V−.
TDI operation, In this section we describe the TDI operation of the x-ray detector CCD, and derive a mathematical formula for charge transfer between the detector elements.The CCD sensitive area is divided into a del matrix. Imagine the detector completely covered by an opaque sheet that has a pinhole in it, that allows a very narrow pencil beam to irradiate one del. The charge generated in the del is proportional to the x-ray intensity and the time interval between microsteps. Just before the detector steps into the next position, the charges in each del of each column are transferred to the neighboring dels of the next column, in the direction opposite to that of detector motion. The charges accumulated in the last detector column are transferred to the shift register.[1]
References
- ^ [1]
- Elbakri IA, Lakshminarayanan A, Tesic MM: Automatic exposure control for a slot scanning full field digital mammography system. Medical physics 2005, 32:2763.