time delay and integration clock

A time delay and integration clock is a timing signal that synchronizes the readout of a linear image sensor as it moves across a scene or as a scene moves past the sensor. The clock pulses match the speed of relative motion between sensor and target, allowing the sensor's pixel array to accumulate charge from the same ground area over multiple exposure cycles. This creates a synthetic long exposure without requiring the sensor to sit idle, effectively multiplying signal strength while rejecting motion blur.

TDI sensors contain multiple rows of photodiodes arranged in series along the direction of motion. As an object or conveyor moves, the TDI clock shifts charge vertically from row to row in lockstep with the object's travel. Each pixel on the sensor reads light from the same physical spot multiple times as that spot passes down the rows. A sensor with 256 rows, for example, integrates 256 times longer than a single-row sensor operating at the same line rate, boosting the signal-to-noise ratio dramatically without increasing exposure time per frame.

Timing and motion coupling

The TDI clock frequency or period must track the actual motion of the object being imaged. If the clock runs too fast, the charge packets shift ahead of the moving scene; if too slow, they lag behind and the integration benefit collapses into blur. Industrial applications like barcode reading on a conveyor belt, automated surface inspection, and document scanning use encoder feedback or calculated velocity to set the clock frequency in real time. Typical speeds range from kilohertz to megahertz depending on object velocity and sensor geometry.

Failure modes include clock skew (when rising and falling edges arrive at different times across the integrated circuit), which causes the charge packets to smear across rows, and phase mismatch with motion, which destroys the synchronization and produces streaked or doubled images. The TDI clock must also maintain low jitter, typically in the nanosecond range, to prevent signal degradation in high-gain applications.

The term "time delay" refers to the incremental shift in charge storage as the clock advances, while "integration" describes the cumulative charge buildup. Together they describe a scanning technique that trades temporal resolution for signal amplitude, making it invaluable in machine vision where speed and sensitivity must both be high but scene contrast or lighting is poor.