demanding imaging needs with TDI camera
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.
Left: Using TDI, the object is
scanned and captured during the entire length of its journey in contrast
to the single line scan process where scan and capture occurs at the
end and so resulting at a single exposure at that point.
Major benefits of TDI
With more effective integration time, the speed of the
target object or inspection web can be increased allowing greater
High-frequency AC or even LED lighting can be used rather than,
high-powered, high-cost, high-temperature halogen lighting with DC
power. This can profoundly lower system build and maintenance costs.
TDI operation effectively averages out fluctuations in light intensity
to represent a DC light source. This factor alonecan justify the increased cost of a TDI sensor versus
the cost of using DC regulated light source and a line scan image
Immune to reflections
TDI capture is not affected by direct sunlight reflections
or reflections from lasers and is able to capture scene details in
both highlight and shadow areas.
** High quantum efficiency
** High resolution
** Wide imaging areas
** Efficient thermal packaging
** Efficient management of high-speed circuitry
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
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
** earth observation satellite (for weather observation, for example)
here to learn more about TDI cameras available through Adept
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: firstname.lastname@example.org
or call us at Perth (08) 92425411 / Sydney (02) 99792599 / Melbourne
(03) 95555621 or use our online contact
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