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TDI

Time Delay Integration, technology used in line sensors for increasing light sensitivity

When inspecting the surface of continuous materials such as steel or synthetic plastics, and wherever a high level of temporal and geometrical resolution is required, line scan cameras are state-of-the-art. Since the exposure time for one line is determined by the line frequency, the illumination units and lenses used are subject to demanding requirements.

The technology used in a TDI sensor is nevertheless able to deliver usable image information and hence cope with difficult lighting conditions of this type. In order to achieve this, the brightness information of a moving object line is recorded not only by one sensor line, but in sequence by a whole bank of neighbouring lines and the values are simultaneously totalled.

In addition to the possibility of doing without extremely bright illumination components (which therefore act as heat sources) there are a number of further benefits.

  • TDI allows a greater object throughput, since the maximum possible line frequency is not restricted by the lighting conditions.
  • A smaller aperture can be used on the lens, thus providing greater depth of focus.
  • Since the camera angle varies slightly from one TDI line to the next, unwanted surface reflections are averaged out.