This means inspections with the fastest line frequencies, sometimes in excess of 100 kHz, are limited to very short exposure times which therefore need more light than is the case with area scan cameras. In this situation the use of standard line scan cameras can be impractical as the lighting intensity required can be excessive. For applications that require higher sensitivities, TDI line scan cameras are used. TDI stands for 'Time Delay and Integration' and describes a sensor design that uses multiple line scan stages. Using this elaborate sensor technology, the line information is copied line by line synchronously with the object movement and exposed with the same image information.
Modern TDI cameras work with up to 256 TDI-stages, providing 256 times the sensitivity compared to standard line scan models.
The advantages of TDI line scan are:
Higher speed: even at high line frequencies the effort for illuminating can be kept small due to the build-up of integration time.
Less light, less expensive: instead of expensive high-powered illumination, fluorescent tubes or LED illumination can be used.
The disadvantages of TDI line scan are:
Sensitivity to misalignment: a TDI sensor will start to introduce cross pixel blurring if there is a total misalignment of more than one pixel along its length.
Sensitivity to speed mismatching: the speed of continuous material and image must be matched to within 2-4 % to prevent interpixel blurring, TDI therefore requires encoder synchronisation.
Intolerant of speed variation when acquiring - for example during run-up and run-down periods.
Does not feature exposure control, so does require constant speed for constant illumination.
However, some of the very latest CMOS based TDI sensors now incorporate exposure control between lines. Overcoming this major limitation of TDI opens up many more applications where controlling the speed of the movement is not possible, such as web inspection.
In this example the pixel charge only accumulates for the exposure time of one line, resulting in a limited amount of time for the illumination to light the target.
The following image represents a single scanned line of a web as it passes under a TDI line scan sensor. As the work passes under a TDI sensor, the image intensity is gradually built-up to form the final output. This allows faster imaging speeds than standard line scan and with less light.
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