Addressing demanding imaging needs with TDI camera technology

Improving both imaging speed and clarity when imaging moving objects with TDI-mode sensor technology

What is TDI?
TDI refers to Time Delay Integration, a specialised detector readout mode used for observing a high-speed moving object under low light conditions normally undetectable by classic CCD imaging.

With an effect similar to that of a linescan camera, TDI is designed to allow continuous movement of the object past the sensor to produce a continuous video image of a moving, two-dimensional object: a definite advantage over stop-then-start capture of traditional technologies.

In the past, certain image-capture deployments traded-off sensitivity in order to capture a fast-moving object. TDI-mode technology preserves sensitivity while not degrading image quality, even given the relative fast movement between the sensor and the objects under image.

How does TDI work?
Based on the concept of accumulating multiple exposures of the same (moving) object, TDI mode effectively increases the integration time available to collect incident light. The technique senses a change in light pattern and shifts them across the CCD in a manner that is synchronised with the movement of the image, in order to integrate more light from the scene. The number of lines on the sensor corresponds to the increase in sensitivity. For example a 256 line TDI sensor provides 256x more sensitivity than a single linescan camera.

Imaging in TDI mode provides dramatically increased responsivity compared to other video scanning methods by permitting much greater scanning speeds in low light, or allows reduced lighting levels (and costs) at conventional speeds and so producing a crisp image.

Points to note if considering TDI technology
TDI requires more care in synchronization and alignment but in general practice these requirements are not difficult to achieve.

A TDI sensor can withstand some misalignment (either translational or rotational) without negative effect on image quality and a total misalignment of one pixel or less across the length of a TDI sensor will not affect image quality.

In most applications, a 96-stage TDI device can comfortably tolerate a 2-4% velocity mismatch between inspection web and imager. This is not difficult to achieve using a web-mounted encoder to supply a sync signal to the camera, even with webs that change speed and this approach is used in thousands of successful applications.

Designed for light-starved applications, TDI typically lacks antiblooming to subdue "glints", and as well, TDI sensors generally use photogates - surface electrodes covering the entire pixel - and while this ensures 100% fill factor, it can attenuate incoming photons in the deep blue region. One camera manufacturer, Teledyne DALSA has produced an anti-blooming device to solve the first problem and has also developed enhanced blue-response TDI sensors to compensate for the latter issue.

Where can TDI technology be used?
TDI CCDs are used in applications that require the ability to operate in extreme lighting conditions, that require both high speed and high sensitivity, for example:
** inline monitoring, inspection and guidance
** sorting
** earth observation satellite (for weather observation, for example)

Adept Electronic Solutions are "The Machine Vision and Imaging Specialists" and distributor of Machine Vision products in Australia and New Zealand. To find out more about any machine vision product please email us at: adept@adept.net.au or call us at Perth (08) 92425411 / Sydney (02) 99792599 / Melbourne (03) 95555621 or use our online contact us page.