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Summary:
The evolution of optical imaging Charge-Coupled-Devices (CCD) has been rapid in recent years. The line detectors have been developed to be used not only in facsimile applications but also for spectrometric purposes. The area sensors have become extremely wide with up to 16 million separate light sensitive elements and they are gradually replacing conventional film materials in certain high-end applications. The use of both sensor types is based on the conversion of light energy into electrons which are accumulated in the photosites during the integration period, i.e. during the exposure of image at the focal plane. The second phase of the operation cycle is the serial readout of the elementary charges which are then amplified for further processing. The feasibility of commercial CCD sensors in print quality monitoring has been studied in the Instrument Laboratory of the Technical Research Centre of Finland. The area sensor is suitable for imaging non-moving objects, such as offset printing plates, and it can also be used to monitor whether a running web in a printing press calls for any manual maintenance. The linear devices offer a good resolution and a wide dynamic range, which might justify their use as sensors in automotive printing presses. The idea of measuring the print density and register using CCD front-end should not be overlooked even though the compensation for the sensor's non-uniform sensitivity, dark drift and readout ringing requires the full capacity of modern 16-bit microprocessors to be benefited.