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receiver with an adjustable critical value. The threshold corresponds to a specific
grey-scale value on the object. If the threshold of the grey-scale value is fallen below
of or exceeded, the printing mark sensor changes its switching status.
Depending on the different types of application (varying reflection with different
colour contrasts), printing mark sensors are often equipped with optical sensors
whose wavelengths can be changed by using different light emitting diodes. Even
bulbs in conjunction with selectable colour filters are used.
Printing mark sensors can be used with fibre optic adaptors. However, in such cases,
luminous radiation is as a rule unfocussed and the sensing width may vary; however
contrast sensitivity quickly diminishes as the distance increases.
Printing mark sensors are also able to detect very small marks. Designs
incorporating an LED light source are for instance able to detect printing marks of a
dimension of 0.5 mm at switching distances of 20 mm, whereas designs using laser
radiation source are able to...
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- Spring '12