WO1989002133A1

WO1989002133A1 - Optical mark detector

Info

Publication number: WO1989002133A1
Application number: PCT/GB1988/000697
Authority: WO
Inventors: Stephen David Stewart
Original assignee: Drs Data & Research Services Plc
Priority date: 1987-08-29
Filing date: 1988-08-24
Publication date: 1989-03-09
Also published as: GB8720497D0

Abstract

An optical mark detector comprises a light source (2) comprising a transverse array of LEDs (3). The LEDs may be illuminated individually to illuminate a particular portion of a form (4) to be scanned, the width and position of the portion being determined by a mask (5) interposed between the light source (2) and an optical fibre array (6) which transmits light from the mask to the item to be scanned. The presence or absence of a mark within the illuminated portion is determined by a sensor (7) comprising a linear array of optical fibres (9) extending across the full width of the detector, coupled to a single transducer (8). The presence or absence of a mark at a particular position will be determined by the output of the transducer (8) when the particular position is illuminated by the LED known to correspond to that position.

Description

OPTICAL MARK DETECTOR

This invention relates to an optical mark detector, that is to say a device for scanning an item to determine the presence or absence of marks within predetermined zones of the item. Optical mark detectors are used, for example, in the automatic processing of forms on which information has been entered by placing a mark in one or more boxes printed on each form.

Prior art optical mark detectors have comprised means for moving items to be scanned in a longitudinal direction past a transversely arranged array of sensitive elements. The item is illuminated, and each sensitive element provides a signal if a mark having predetermined characteristics passes beneath the sensor. By coordinating the outputs from the sensors with information as to the layout of the form, the presence or absence of marks within each box can be determined. Such devices suffer from the disadvantage that they require a number of sensors equal to the number of parallel bands across the width of the item within which marks are to be expected. The use of a large number of individual sensors and associated electronics is expensive. Further, each detector can only handle items having marked zones corresponding to the position of the sensors. Such detectors are accordingly not versatile.

According to the present invention there is provided an optical mark detector for scanning an item to determine the presence or absence of marks within predetermined zones of the item, the^*sGptical mark detector comprising: a light source operable to illuminate at any moment in time a predetermined portion of the item; and a single sensor having a zone of sensitivity spanning several of said predetermined portions and operable to detect the presence of a mark having predetermined characteristics within any said portion which is within its zone of sensitivity and is for the time being illuminated by the xight source whereby the presence or absence of a mark having said predetermined characteristics within any zone of the item being scanned will be indicated by the output of the sensor when that zone is illuminated by the light source.

With such a device, only a single sensor, or a small number of sensors, need be employed to cover the full width of the item to be scanned. Each portion of the zone of sensitivity of the sensor within which a mark may occur is illuminated in turn, and the presence or absence of a mark within that portion is determined by the output of the sensor at the moment when that portion is illuminated. In a typical application in which a form is being scanned, the parallel bands within which the marks of the particular form in question are to be expected are sequentially illuminated, and the output of the sensor is correlated with the illumination of the parallel bands. In this way, a single sensor is able to cover a large number of potential mark positions.

In a particularly preferred embodiment of the invention the light source comprises a plurality of individual elements, for example light emitting diodes. Such elements are inexpensive and may be energized for a very short period to provide illumination of a particular portion of the item to be scanned.

Preferably, a mask is interposed between the light source and the item to be scanned in order to determine the exact positions and sizes of the portions illuminated by the light source. Merely by changing the ^' masks, the detector may readily be adapted to scan items in which marks are to be expected at any positions across the width of the item. .

Preferably, the light source itself is readily removable from the detector whereby, by changing the light source, the spectral characteristics of the detector may readily be varied.

The invention will be better understood from the following description of a preferred embodiment thereof, given by way of example only, reference being had to the accompanying drawing wherein:

Figure 1 is a perspective view of an embodiment of detector according to the present invention;

Figure 2 is a perspective exploded view of portions of the detector of Figure 1 ; and

Figure 3 is a cross-section on the line Y-Y of Figure 1.

Referring to the drawings, the optical mark detector 1 comprises a light source 2 in the form of an array of light emitting diodes 3. A plurality of diodes 3 are provided which, between them, can illuminate the full width of a form 4 to be scanned by the detector. However, the diodes 3 are individually wired and may be energized individually and separately whereby at any one moment in time only a portion of the form 4 is illuminated by the light source. The exact width and position of the portion of the form 4 which is illuminated by each LED is determined by a mask 5 which is interposed between the light source and the form 4. The mask 5 is readily removable and may be replaced by a different mask whereby the position of the illuminated portions of the form may be varied. In this way, the detector can be customized to a particular form merely by inserting a mask 5 appropriate to the expected positions of marks within the form to be processed.

Light is transmitted from the slots in the mask - to the surface of the form by means of a linear array 6 of optical fibres.

The presence of a mark within the illuminated ^• portion of the form is determined by a sensor 7 which comprises a transducer 8 and an array of optical fibres 9. The ends of the optical fibres 9 adjacent the form 4 are arranged as a linear array extending across the full width of the form. The fibres are then grouped into a circular array to provide an input to the transducer 8. The transducer 8 produces a signal indicative of the presence of a mark having predetermined characteristics within the portion of the form illuminated for the time being by the light source 2. Accordingly, by correlating the output of the transducer 8 with information as to which element of the light ource is for the time being energized, the presence or absence of a mark at any particular position on the form may be determined.

Preferably, the light source is mounted on a circuit board which is readily removable from the detector, whereby the spectral characteristics of the detector may be varied by substituting one light source for another. Preferably, the light source is removeable from the detector without interfering with the mask 5, fibres 6, or sensor 7.

It will be noted from Figure 3 that the orientation of the fibres 6,9 relative to the surface of the form 4 is such as to reduce errors due to mirror like reflections from the surface of the form. Preferably, the fibres 6 and 9 are bonded to appropriate former members 1OA and 10B which are rigidly secured together to provide a robust assembly. By using fibres of a suitably small diameter, and positioning the ends of the fibres sufficiently close to the form to be scanned, the detector of the present invention is able to detect very narrow marks.

Whilst the terms "illuminate" and "light source" have been used herein to described the source of energy used in the detector of the present invention, it will be appreciated that energy may be in the form of other than visible light, depending on the nature of the items to be scanned, and the nature of the marks to be expected. Similarly, although in the preferred embodiment of the invention the sensor 7 is a visible light sensor, it will be appreciated that the sensor could be responsive to other characteristics than visible light, according to the nature of the "illumination" and the nature of the items being scanned.

Claims

1. An optical mark detector for scanning an item to determine the presence or absence of marks within predetermined zones of the item, the optical mark detector comprising: a light source (2) operable to illuminate at any moment in time a predetermined portion of the item (4); and a single sensor (7) having a zone of sensitivity spanning several said predetermined portions and operable to detect the presence of a mark having predetermined characteristics within any said portion which is within its zone of sensitivity and is for the time being illuminated by the light source (2) whereby the presence or absence of a mark having said predetermined characteristics within any zone of the item (4) being scanned will be indicated by the output of the sensor (7) when that zone is illuminated by the light source (2).

2. An optical mark detector according to claim 1 wherein the light source (2) comprises a plurality of individual elements (3) each of which may be selectively actuated to illuminate a predetermined portion of the item.

3. - An optical mark detector according to claim 2 wherein each said individual element (3) is a light emitting solid state device.

4. An optical mark detector according to any preceding claim wherein a mask (5) is interposed between the light source (2) and the item (4) to be scanned to determine the size and exact positions of the portions of the item illuminated by the light source (2).

5. An optical mark detector according to claim 4 wherein the mask (5) is readily removable from the detector and replaceable by a different mask whereby the size and/or position of the portions to be illuminated by the light source may be varied.

6. An optical mark detector according to any preceding claim wherein the light source (2) is readily removable from the detector and replaceable by a different light source whereby the spectral characteristics of the detector may be varied.

7. An optical mark detector according to any preceding claim wherein the sensor (7) comprises a plurality of optical fibres (9) which extend from a linear array adjacent the item to be scanned to a transducer (8) for producing an electrical output in response to light signals received from the fibres.

8. An optical mark detector according to any preceding claim wherein the light source comprises a plurality of optical fibres (6) terminating in a linear array adjacent the item to be scanned.

9. An optical mark detector according to claim 7 and claim 8 wherein said linear arrays of fibres (6,9) are parallel and substantially co-extensive.

10. An optical mark detector according to any of claims 7 to 9 wherein the axes of said fibres (6,9) adjacent the item (4) to be scanned extend obliquely to the plane of the item.