Image analysis – Histogram processing – For setting a threshold
Patent
1986-05-09
1987-12-01
Moore, David K.
Image analysis
Histogram processing
For setting a threshold
382 50, 356 71, 209534, 250556, G06K 960
Patent
active
047109637
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to apparatus and a method for sensing documents, such as banknotes or other security notes, having a pattern printed over at least a part of their surface, to assess the degree of soiling.
It is known to sense the condition of a document by detecting light reflected from its surface and then comparing the intensity of the reflected light with a threshold to determine whether the document should be accepted or rejected. Variations in printed intensity and of ink colour present problems with such a method. Filters may be used to match the colour of the incident light to the ink colour, to suppress the colour information, except where black ink is present on the document. However, this does not give very satisfactory results, in our experience. We have also attempted to measure the degree of soiling by measuring the contrast between elemental areas on a note, again without very satisfactory results.
BRIEF DESCRIPTION OF THE INVENTION
According to the present invention, apparatus for determining the fitness of a document by determining its degree of soiling, comprises: a sensor for providng a reflectance signal representing the intensity of light reflectd from a sensed element of the face of the document; and a signal processing circuit responsive to the reflectance signals from the sensor corresponding to sensed elements on the face of the document to provide a "fit" or an "unfit" signal for a document, depending upon the degree of soiling, the signal processing circuit including means for generating a brightness value, derived solely from the reflectance signals whose level exceeds a first threshold, and means for generating a "fit" signal for the document only if the brightness value exceeds a second threshold.
In the preferred embodiment, the document is effectively divided into a large number of areas, each containing a number of elements for sensing, the brightness of each area being individually assessed. Thus, according to another aspect of the invention, apparatus for determining the fitness of a document having some printed regions and some unprinted regions, by determining its degree of soiling, comprises: a sensor for providing a plurality of reflectance signals representing the intensity of light reflected from a plurality of sensed elements of the face of the document; and a signal processing circuit responsive to the reflectance signals including means determining, for each of a number of areas of the document each area comprising a number of elements for sensing, whether the number of sensed elements resulting in a signal level exceeding a first threshold representing the upper limit of brightness for printed areas of the document, reaches a given proportion of the total number of sensed elements for that area; means responsive to the relationship between a second threshold representing the upper limit of brightness for unprinted areas having an excessive degree of soiling, and those signal levels of the elements of the area, which exceed the first threshold, to provide a brightness value for each area; and means responsive to the brightness values of all such areas to generate a "fit" or an "unfit" signal for the document.
However, it is not essential to assess the document area by area; for example, the output of the sensor can be applied to a first comparator, for comparing the sensor output level with a first predetermined threshold, and the sensor output can be integrated over only those periods in which its level exceeds the first predetermined threshold. This provides a total reflectance signal indicative of the total reflectance of the brighter areas of the face of the document; and by comparing this total reflectance signal with the total duration of the periods for which the reflectance signal level exceeds the first threshold, a "fit" or "unfit" decision can be made for the document.
In another form, the apparatus includes means responsive to signals derived from elements of the document whose brightness exceeds a first th
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Chapman Victor B.
Lacey Paul D.
De La Rue Systems Ltd.
Moore David K.
Skinner Anne
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