FR2696609A1 - Detector head for facsimile transmission - has aspheric lens converging light rays reflected from printed sheet to array detector - Google Patents

Abstract

The sheets of paper to be transmitted pass through rollers (7), and a band (6) across the page is illuminated by a monochromatic light source (5). Rays are reflected from the page (9) and focussed by an objective (3) with an aspheric lens (4). Rays (2) are focussed from the lens onto a staring array (1), including optical charge coupled detectors. The aspheric lens corrects for the different lengths of dispersion of the rays from the sheet of paper, caused by the varying length of the objective from the line being read. ADVANTAGE - Requires only one lens.

Description

SCALE REDUCTION FAX ANALYSIS HEAD
The present invention relates to a facsimile analysis head, downscaling, comprising means arranged to detect, through an objective comprising an optic, the image of at least one point of a sheet illuminated by means for the transmission of the detected image.

The analysis head, or scanner, of a fax machine is used to transform the image of a sheet, which must be reproduced on a remote fax machine, into electrical signals shaped by the fax machine comprising the analysis head, into view of their transmission over a telephone line.

The sheet to be analyzed, driven by rollers, passes into an area, or strip, lit by a lighting system of the analysis head. The illuminated strip extends transversely to the direction of movement of the sheet and the image of an illuminated line of the sheet arrives, through a focusing objective, at a detector, or, more often a strip of detectors, where it is analyzed. The bar transforms each point of the line analyzed into an analog electrical signal immediately transmitted, through an analog / digital converter, to a buffer memory which thus successively receives the signals of the lines analyzed. The logic of the fax machine transmits on the telephone line, in the desired form, these signals representing the image of the sheet.

The detector array, often made up of charge transfer type sensors, or CCDs (Charge
Coupled Device), allows all points of a line to be detected simultaneously, which eliminates the need for a line scanning device.

In a first type of analysis head, it passes close to the sheet, that is to say that the magnification is unitary. This is called contact image sensors or "Contact Image Sensors" (CIS). The analysis head is enclosed in an opaque casing comprising a window pane, along which the sheet slides. If there is no interposed light guide, then we speak of a full-contact sensor.

In a second type of analysis head, called a reduction in scale, and to which the invention relates, the image of an analyzed line of the sheet is reduced to substantially the length of the detector (CCD), placed at this place, by means of a lens. This makes it possible to analyze the entire line of a sheet when its length clearly exceeds that of the detector.

The objective of the objective is to provide an image of sufficient intensity taking into account the sensitivity of the detectors.

This is why it is widely open. However, a widely open objective provides, all other things being equal, lower quality images, since the peripheral rays are not exactly focused in the image plane, where the detectors are located, so that the focal length of the optics varies depending on whether they are rays close to the optical axis or distant rays.

To remedy this defect, a lens is generally used in facsimile machines consisting of three lenses which, overall, partially correct the aforementioned defect.

Such an objective is expensive, which in a fax machine today has a certain drawback. In addition, its assembly is more sensitive to possible shocks.

The present invention aims to overcome these drawbacks.

To this end, the invention relates to an analysis head of the type mentioned above, characterized in that said optics comprises a single lens.

It was by becoming aware of the fact that the sensitivity characteristics of CCD detectors had improved in recent years that the Applicant realized that one could take advantage of them to degrade the quality requirement of the objective.

Indeed, the objective can be relatively closed, so that the peripheral rays very far from the optical axis of the lens, mainly responsible for the defects linked to the variation of the focal distance, are eliminated.

Advantageously, said lens is aspherical.

Thus is corrected the defect, minor, remaining on the dispersion of the focal distance because it is still transmitted rays which are not exactly confused with the optical axis.

The invention will be better understood with the aid of the following description of the preferred embodiment of the analysis head of the invention, with reference to the single figure in the appendix which is a schematic representation thereof.

The analysis head comprises detectors, here a strip 1 of sensitive CCD optical detectors disposed substantially in the focal plane and on the optical axis 2 of a focusing objective 3 of the analysis head.

The objective 3 comprises a focusing optic made up of a single converging lens 4.

A lighting system 5 of the analysis head, monochromatic in this example, is arranged so as to illuminate a strip 6 (the position of which is limited, in the figure, by two dotted lines) of a sheet of paper crossed by the optical axis 2 and opposite to the strip 1 relative to the objective 3. The strip 6, facing the lens 4, extends parallel to the strip 1 and rollers 7 drive the sheet in a perpendicular direction to the strip 6. The image of line 8 of the sheet located on the optical axis 2 is thus formed on the bar 1.

The lens 4 has, in this example, been obtained by molding a plastic material. The objective here is open to an opening number of "11", and could be even more closed, to a higher opening number, which corresponds to a significant reduction in the opening angle relative to to a known objective, generally open to "5.6". The lens 4 has, here, an aspherical shape, that is to say that its axial section is limited by a curve, corresponding to one of its faces, which has the shape of a fraction of conic to correct the remaining focal distance dispersion defects due to rays distant from the optical axis 2.

The rays 9 distant from the optical axis 2 then converge in the focal plane of the axial rays, which makes it possible to further improve the quality of the image. In this example, the lighting system 5 being monochromatic, the chromatic dispersion of focal distance is not felt.

Claims (3)

1. Facsimile analysis head, downscaling,  comprising means (1) arranged to detect, at  through an objective (3) comprising an optic (4),  the image of at least one point on a leaf (6)  illuminated by lighting means (5) for the  transmission of the detected image,  characterized by the fact that said optic (4)  has a single lens (4). 2. Optics according to claim 1, in which  said lens (4) is aspherical. 3. Optics according to one of claims 1 and 2 in  which said lens (4) is molded of material  plastic.