■Dramatically enhanced sensitivity by TDI mode
TDI mode provides integrated exposure while synchronizing object movement with the signal charge transfer timing. The signal charge is integrated a number of times equal to the number of vertical stages (128 stages), so TDI mode yields enhanced sensitivity about 128 times higher than ordinary linear image sensors.
【TDI (Time Delay Integration) mode】
In FFT-CCD, signal charges in each line are vertically transferred during charge readout. TDI mode synchronizes this vertical transfer timing with the movement of the object irradiating on the CCD, so that signal charges are integrated a number of times equal to the number of vertical stages of the CCD pixels. In TDI mode, the signal charges must be transferred in the same direction at the same speed as those of the object to be imaged. In the figure below, when the first stage charge is transferred to the second stage, an additional charge is produced in the second stage by photoelectric conversion
and accumulated. When this operation is continuously repeated until reaching the last stage M, a signal charge which is M times greater than the initial charge is accumulated. This shows that TDI mode can enhance sensitivity up to M times higher than ordinary linear sensors. Since the signal charge on each line is output from the CCD horizontal shift register, a two-dimensional image can be continuously acquired. TDI mode also dramatically improves sensitivity variations compared to frame mode operation because signals from all pixels are accumulated in each line.
Schematic diagram showing integrated exposure by TDI mode
■High sensitivity over the entire UV to near IR region
The back-thinned (back-illuminated) structure ensures higher sensitivity in the UV through the near IR region, than front-illuminated types (200 to 1100 nm).
【Quantum efficiency vs. wavelength】
■TDI-mode CCD captures a continuous image of fast-moving samples
TDI-CCD is ideal for capturing images of fast moving or rotating objects, and is thus widely used in line scan cameras for industrial robots, etc. The examples below show capturing image examples of fast moving object and fast rotating object (a image of characters printed on a drum that were captured by a CCD operating in frame mode and TDI mode).
■Fast line rate: 50 kHz or more
Using multiple amplifiers (multiple output ports) permits parallel image readout at a fast line rate.
【TDI-CCD sensor structure [Example: S10201-04, 2048(H) x 128(V) pixels, 4 ports on each side x 2 (bidirectional transfer)] 】
