Apparatus for the optical recognition of documents

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APPARATUS FOR THE OPTICAL RECOGNITION
OF DOCUMENTS

FIELD OF THE INVENTION

The invention relates to an apparatus for the optical recognition of documents.

Such apparatus for the optical recognition of documents are used for example in bank note acceptors for the optical recognition of documents.

BACKGROUND OF THE INVENTION

An apparatus for the optical recognition of documents is known from U.S. Pat. No. 4,319,137, in which a printed sheet can be recognized based upon distinctive features printed thereon. An extended source of white light illuminates a small strip, which runs transversely across the sheet. The light which is either reflected by the sheet or is transmitted through it is simultaneously being detected by three photosensors. Each photosensor only registers the light from a narrow spectral range, for instance, in the red, green or blue color. For each strip the photosensors transfer three signals corresponding to the three colors to an evaluation system.

German patent document DE-PS 37 05 870 describes 25 a device that can be used as a reading head, which can scan a page line by line. The device includes a row of photodiodes to each of which is assigned a pair of lightemitting-diodes (LED's) which are inclined to each other. Each pair of LED's illuminates the sheet in a region located directly in front of its associated photodiode. A collimator is disposed in front of each photodiode and screens all the light that does not directly originate from the region of the sheet directly in front of the photodiode. The reading head produces a monochromatic raster copy of a printed pattern appearing on the sheet.

It is further known from EP-A 338 123, to create the reading head from a group of interchangeable modules arranged in parallel which include a configuration of 40 rows of photodiodes and light sources that optically scan the sheet in a strip like fashion. Each module operates with light of a predetermined color, and produces the signals associated with a monochromatic raster copy of the printed pattern appearing on the sheet.

Finally, from Swiss patent document CH-PS 573 634, a device is known for scanning a sheet with a single photosensor. In such a device, a small circular area on the sheet is sequentially illuminated by single light sources of different spectral color that are disposed at an angle with the plane of the page, the light sources periodically altering the color of illumination. In synchronism with the cyclic illumination of the area, the single photosensor receives light in the particular spectral region that has been scattered into it in a direction perpendicular to the plane of the sheet. Displacing the sheet after each cycle leads to scanning a small strip on the sheet.

In all the foregoing systems, the disposition of the

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tive features that may appear on the surface of a document.

Advantageous embodiments will be presented hereunder.

SUMMARY OF THE INVENTION

The object of the invention is achieved in an apparatus for the optical recognition of documents which extends over the entire width of a transfer plane. Regularly disposed photoelectric elements, whose optical axes create a single sensor plane that is perpendicular to a transfer plane, receive light as altered by the document. The photoelectric elements are regularly disposed in a manner in which their optical axes are contained in a sensor plane perpendicular to the transfer plane. A region of the document, determined by the sensor plane, is illuminated by at least one light line which is inclined with respect to the sensor plane. The light modified by the document is received by the photoelectric elements. The adjacent light sources in each light line are separated by a uniform source distance, which is smaller than the sensor distance between two adjacent photoelectric elements. The light sources emit light within a narrow spectral width in pulses of short duration. Each light source belongs to a color group of a set of color groups, with each source of the same color having the same spectral width. The photoelectric elements convert the modified light into electrical sensor signals. An optical unit determines a first acceptance angle of photoelectric elements. Each of the photoelectric elements has associated with it a second acceptance angle corresponding to a section. Each photoelectric element serves to average the light belonging to each section.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be further clarified by the following figures.

FIG. 1 shows an apparatus for document recognition according to the invention.

FIG. 2 shows an arrangement of light sources and photosensors according to the invention.

FIG. 3 shows a first configuration of light sources.

FIG. 4 shows a second configuration of light sources.

FIG. 5 shows variations of voltage supplies as a function of time.

DETAILED DESCRIPTION

In FIG. 1, item 1 represents a document in the form of a sheet of paper containing monochromatic or polychromatic printed characteristic patterns, which are known to appear on e.g. bank notes. Transfer means 2 drives document 1 along the surface of transfer plane 3 that forms part of the apparatus for the recognition of documents. Above transfer plane 3, photosensitive elements e.g. photosensors 4 are disposed whose optical axes are perpendicular to transfer plane 3 and lie in a sensor plane 5 which is perpendicular to the direction of

of the sheet is such that no directly reflected light from the surface of the sheet ever reaches the photosensors. This is a characteristic feature of these systems.

OBJECT OF THE INVENTION

The object of the invention is to create a cost effective system for the optical recognition of documents, that would enable reliable detection of colored distinc

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Photosensors 4 are at least equidistantly spaced in a row in sensor plane 5, with the row of photosensors 4 being located at a predetermined distance from translation plane 3. Photosensors 4 serve the function of converting light 7 having a broad spectral range into electrical signals. The spectral range encompasses for instance wavelengths of 0.4 fim to 10 ftm, as is e.g. the case for semiconductor silicon photoelements. Light 7