IE50763B1 - A light source for line-by-line scanning of a document - Google Patents

Abstract

1. A light source for scanning a document line-by-line, constituted by a flash tube presenting an inner channel, the ends of which are connected respectively to two gas storage chambers in which are disposed two operating electrodes respectively of the tube, said tube being equipped on the outside with a priming electrode disposed between the two operating electrodes, characterized in that said inner channel is a capillary channel (10) over the useful length of said tube (1) destined to face the document, and that, over the useful length, said tube is a thick-walled tube, the section of which is, for a portion (16) facing the document, substantially formed as a V with a rounded angle in order to form a magnifying glass built into the wall of the tube.

Description

The present invention relates to a light source for scanning a document line by line, in particular by means of a linear bar having the same number of photo-detectors as there are image points along the line as determined by the required definition.

All systems for scanning a line of a document by means of a bar of photodetectors require uniform illumination along the whole length of a scan line. In current practice, a fluorescent tube disposed a short distance from the document is used as a light source and the fluorescent tube is slightly longer than the width of the document. The source can also be formed by two fluorescent tubes which are likewise slightly longer than the width of the document and are disposed symmetrically on either side of the analysis zone at a short distance from the document.

A reflector provided with a narrow slot right across its whole width is associated with the, or each, tube so as to limit transversally the width of the beam which falls on the document and/or to avoid illuminating the bar of photo-detectors directly by means of the tube.

Scanning must often be performed on a continuously moving document, in particular, in high-speed, high-definition systems in which for example, 125 pm high scan lines move past in about 1 ms and are scanned with, a definition of 3x8 image points per square millimetre. This is because the characteristics of present mechanical systems do not make it possible to stop on successive lines of a document for such scanning. However, with the document moving continuously, light sources such as those mentioned hereinabove do not provide sufficient accuracy when scanning successive lines. To obtain sufficently accurate scanning of the lines with continuous high-speed movement of the document, it is necessary to perform the scan in an - 3 appreciably shorter time - at least ten times shorter - than that allotted for movement from one line to the next. Therefore, for the given high definition, this leads to a scan time which is too short and incompatible with the maximum operating fre5 quency of present bars of photodetectors.

Preferred embodiments of the present invention overcome these drawbacks by enabling the scan time to be substantially equal to the time of movement from one line to the next without thereby degrading the scan results. This applies in particular in the case of continuous high-speed movement of the document.

The present invention provides a light source for scanning a document line-by-line, constituted by a flash tube presenting an inner channel, the ends of which are connected respectively to two gas storage chambers in which are disposed two operating electrodes respectively of the tube, said tube being equipped on the outside with a priming electrode disposed between the two operating electrodes, characterized in that said inner channel is a capillary channel over the useful length of said tube destined to face the document, and that, over the useful length, said tube is a thick-walled tube, the section of which is, for a portion facing the document, substantially formed as a V with a rounded angle in order to form a magnifying glass built into the wall of the tube.

Preferably, the light source includes a reflector on the portion of the flash tube opposite to that which forms the magnifying glass.

An embodiment of the invention is described by way of example with reference to the accompanying drawing in which : Figure 1 is a schematic perspective illustration of the light source in accordance with the invention for analysing a document.

Figure 2 is an elevation of part of the end of the light source of Figure 1 on an enlarged scale.

Figure 3 is a cross-section of the light source along line IXI-XIX of Figure 2 on a greatly enlarged scale.

Figure 1 illustrates a source 1 for illuminating a document 2 to be scanned line-by-line while being driven by a drive roller 3 equipped with an end gear 4 which engages with a gear of a drive shaft 5. The light source 1 is powered by a power supply circuit 6 having a control input 7, and is designed to illuminate successive scan lines of the document 4 constituting, for example, part of a facsimile transmitter. As is well-known in the case of line-by-line scanning of a document, a facsimile transmitter includes a linear bar of photo detectors, not shown, which receives the light beam as reflected by the lines successively illuminated by the source.

The source illustrated in Figures 1 to 3 comprises a flash tube 1 which contains a capillary tube 10 for the ionizable gas e.g. xenon. The length, of the capillary tube is substantially equal to the width, of the document 2 and defines the useful part of the tube. Along the length of the capillary tube, the flash tube wall is very thick. The capillary tube 10 is connected to two gas storage chambers 11 and 12 located respectively at each end of the flash tube which is also equipped with two operating electrodes 13 and 14 inside the respective gas storage chambers and with, a - 5 priming electrode constituted by a metal spiral 15 wound round the flash tube in the neighbourhood of one of the electrodes and extended to a point near the other electrode by a metalled track (not visible in the figure) along the outer surface of the tube.

As illustrated in Figure 3, the cross-section of the useful length of the flash tube is substantially triangular. Here, it forms an isosceles triangle whose angles are rounded and whose sides are convex.

Although not illustrated, it will be understood that the cross-section may be differently shaped. However, in any case, it will have a rounded V-shaped portion as referenced 16 in Figure 3 located on the part of the tube which is designed to lie adjacent to the document in order to form a magnifying glass built into the wall of the tube along the useful length thereof. In particular, the portion of the tube opposite to that which forms the magnifying glass may be semi-cylindrical.

The tube is also equipped with a reflector 17 on its portion opposite to that which forms the magnifying glass. The reflector is formed by a layer of white paint on this portion of the outer wall of the tube.

The operating electrodes 13 and 14 and the priming electrode 15 are connected to the power supply circuit 6 which supplies the operating voltage to the operating electrodes and the priming voltage to the priming electrode in response to each control signal at the input 7. The circuit 6 is of known type and is therefore not described here. The rate at which the control signal is applied to the input 7 is defined by the rate at which, the successive scan lines of the document move.

The flash tube thus formed with a capillary tube and a magnifying glass built into it-constitutes a light source whose transversal dimension is small and which can he placed at a short distance from the document to illuminate the successive lines to be scanned. This improves the evenness of illumination along each line. The light source can supply the power necessary to scan each line - 6 by means of the bar of photodetectors in a very short time. Further, for a given useful quantity of electrical energy corresponding to the ionization of a given quantity of gas, and by virtue of the capillary tube, it is possible to adjust the quantity of gas in the light source between the electrodes to match that which corresponds to the given quantity of electrical energy, so that virtually all of the gas contained in the capillary tube is ionized and thus causes a uniform flash to be generated along the tube and hence along the scan line.

The thick wall of the tube confers great meachnical strength on the light source which is subject to vibrations when installed in a scanning assembly or a facsimile transmitter, and further allows the flash tube to perform the function of a magnifying glass for concentrating the light which comes therefrom onto the scan line of the document.

In practice, the diameter of the capillary tube of the light source lies between 0.2 and 1 mm and is preferably 0.25 to 0.5 mm for a vertical definition of 4 to 8 points per millimetre while the flash tube as a whole has a larger transversal dimension which is as little as 5 or 6 mm.

The light source can be used to scan a document whether driven continuously or step by step, at a fixed or a variable speed and in particular at a very high speed: since the corresponding successive triggered flashes supply sufficient illuminating power in very short time periods, virtually all the time taken oy successive lines to move in front of the source can be used to scan the contents of the respective lines. Further, independently of whether the drive is step-by-step or continuous, the rapidity with which each successive line of the document is illuminated makes it possible to form a fixed image of each of these lines on the linear analysis bar which then sees each of the lines as if it were motionless.

Claims (7)

1. A light source for scanning a document line-by-line, constituted by a flash tube presenting an inner channel, the ends of which are connected respectively to two gas storage chambers in which are disposed two operating electrodes respectively of the tube, said tube being equipped on the outside with a priming electrode disposed between the two operating electrodes, characterized in that said inner channel is a capillary channel over the useful length of said tube destined to face the document, and that, over the useful length, said tube is a thick-walled tube, the section of which is, for a portion facing the document, substantially formed as a V with a rounded angle in order to form a magnifying glass built into the wall of the tube.

2. A source according to claim 1, characterized in that it comprises a reflector on a part of the tube which is opposite to that which forms a magnifying glass.

3. A source according to claim 2, characterized in that said reflector is constituted by a deposit of white substance on the outer wall of the tube.

4. A source according to one of the claims 1 to 3, characterized in that said tube has a cross-section which is substantially triangular.

5. A source according to o.ie of the claims 1 to 4, characterized in that said capillary channel has a transversal dimension which lies between 0.1 and 1 mm.

6. A source according to claim 5, characterized in that said capillary channel has preferably a transversel dimension which lies between 0.25 and 0.5 mm.

7. A light source for line-by-line scanning of document, substantially as herein described.