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Frequently Asked Questions about TDI-CCD | ||||||||||||||||||||||||||
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What does TDI stand for and how does it work?
What does TDI stand for and how does it work? TDI stands for 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 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 drastically improves sensitivity variations compared to frame mode operation because signals from all pixels are accumulated in the lines.
What are the benefits of TDI mode? 1. TDI mode provides integrated exposure while synchronizing object movement with the signal charge transfer timing. The signal charge is integrated by a number of times equal to the number of vertical stages, so TDI mode yields enhanced sensitivity compared with ordinary linear image sensors. How does the number of "TDI stages" impact the performance? The number of TDI stages of CCD made by HAMAMATSU is 128 stages. Therefore, 128 times higher signal charge is integrated and it enhances sensitivity about 128 times higher than ordinary linear image sensors. Furthermore, our new TDI-CCD uses a back-thinned structure to achieve even higher quantum efficiency over a wide spectral range from the UV to near the IR region (200 to 1100 nm).
Main applications are sequential imaging of high-speed moving samples, inspection tasks on electronic parts production line, semiconductor inspection and flow cytometery, etc. Does TDI have any disadvantages? The vertical transfer speed must match the moving speed of the object to acquire an image. This means that TDI cannot acquire an image unless the object moves at a constant speed in the same direction. What is the definition of the exposure time? What is the difference between exposure time and integration time? In general, the exposure time is a measure of how long a frame camera is exposed to light. In the TDI operation, the signal charge is integrated while exposed to light, so the integration time defined by the "number of stages" × "1/line rate" is an effective measure. What parameters are important when selecting a TDI sensor / camera? The speed (line rate) and the number of pixels are important criteria for selecting a TDI sensor / camera. Please see our line-up of back-thinned TDI-CCD below.
Do TDI CCD’s operate in MPP (multi-phase pinning) mode? No Does Hamamatsu offer cooled TDI sensors? When is cooling necessary (required)? Basically No. Generally, TDI-CCD is effective in capturing images of a high-speed sample. Since the dark level is sufficiently low in this application, cooling is not required. Our standard product line-up therefore does not include a cooled type. Does Hamamatsu have a timing diagram for TDI operation?
What will happen if the synchronization is less than perfect? Poor synchronization will cause blurred images because the vertical transfer speed must match the object speed. How does binning work in TDI mode? We do not assume vertical binning in TDI mode. What is the difference between standard FFT-CCD (such as S10140 series) and newly developed TDI-CCD? Our TDI-CCD image sensors contain a full array of features ideal for TDI operation. Namely, the vertical registers have good full well specs (Full well: maximum charge the CCD shift register can handle), the transfer speed is sufficiently fast, and these sensors contains high-speed horizontal registers and signal output amps (on-chip amps) to complement the high-speed vertical registers. Compared to the S10140 series, which is a typical back-thinned CCD image sensor and has a vertical register transfer speed of about 100 kHz and readout speed of about 250 kHz, the new S10200 to S10202 series of TDI-CCD image sensors deliver a transfer speed of 1 MHz (Typ.) and readout speed of 30 MHz (Typ.). Another great feature designed for high speed is multiple readout ports for parallel readout. (Structures with multiple readout ports are called "multitaps".) Backed up by this kind of technology, the S10200 to S10202 series are sure to work well at fast line speeds of 50 kHz or 100 kHz.
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