GB1587533A - Optical sensor apparatus - Google Patents

Description

(54) OPTICAL SENSOR APPARATUS (71) We, XEROX CORPORATION of Xerox Square, Rochester, New York, United States of America, a corporation organised under the laws of the State of New York, Unites States of America, do hereby declare the inven tion, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an optical sensor apparatus of the kind in which a line-by-line op tical image is produced of an object, and is de tected by a photosensitive array comprising a line of discrete photosensitive devices.

A typical arrangement of this kind includes scanning optics for continuously scanning an object in a line-by-line fashion, and producing a line image on a linear array of charge coupled devices (CCDs). The CCDs are arranged to pro vide a succession of instantaneous electrical outputs, the outputs of individual devices depending on the amount of light falling on them.

In this way, each optical image line is represented by a set of electrical signals which correspond with the light intensity at a series of sampling points substantially at the centres of the CCDs.

One commercially available for of CCD array is a linear array of 1728 elements, which is use ful in a number of document reading applications, since it corresponds to 210 sampled points per inch for A4 size sheets or 173 per inch for B4. If greater resolution is required, it is of course possible to use a CCD array with a greater number of elements. However, this approach suffers from the disadvantage that such arrays are not so easily or cheaply available as the 1728 element arrays. Alternatively, it is possible to use two or more arrays, and arrange for the electrical outputs to be manipulated in such a way as to take into account the gaps between the arrays. This technique, however, suffers from the disadvantage that it requires rather complicated electronics.

It is an object of the present invention to provide an optical sensor apparatus in which these disadvantages are at least partially overcome.

According to the present invention there is provided an optical sensor apparatus comprising scanning means for producing on a linear photosensitive array a succession of op tical line images of successive line portions of an object, the photosensitive array comprising a plurality of discrete, regularly spaced, photo sensitive devices each of which is adapted to produce an electrical output dependent upon the intensity of light incident thereon, means for displacing the image relative to the array in a direction parallel with the length of the array in a sequence of steps each of which is an in tegral submultiple of the spacing of the photo sensitive devices, and for repeating the sequence of steps for each line image, and means for receiving the electrical outputs of said devices said outputs being in the form of successive sets of signals, one set for each of said steps, the sets of signals for each sequence of steps representing interlaced samplings of one of said line portions of the object.

An optical sensor apparatus in accordance with the invention will now be described, by way of example, with reference to the accom panying drawings, in which: Figure 1 is a diagrammatic, perspective view of one embodiment of the apparatus of the in vention; Figure 2 is a graph for use in explaining the operation of the invention; and Figure 3 is a diagrammatic representation of a stylus array for utilising the electrical out put provided by the invention.

Referring to Figure 1, a lens 1 is used to focus light from a scanning apparatus, not shown, onto a photosensitive array 2. The scanning apparatus may be, for example, a full- and half-rate reflector scanning apparatus, which produces a line-by-line image of a docu ment. Thus, in the arrangement shown in Figure 1, the line image falling on the array 2 is a continuously changing image correspond ing with the continuous line-by-line scanning of the document. The photosensitive array 2 conveniently consists of a CCD array, such as a Fairchild 1728 element array, and such an array is arranged to provide a succession of electrical outputs, each output representing a line of the object. The CCD elements effec tively sample the light intensity falling on them at 1728 regularly spaced positions along the image line.

In the apparatus of the invention, the sam pling rate is increased by displacing the image, during the time in which each line portion of the object is being scanned, relative to the array by an integral submultiple of the spacing between the CCD elements, and reading the CCD outputs in each position. Although this submultiple could be any number, a simple and convenient number is 2, so that the image is displaced along the direction of the length of the array by one half of the element spacing between successive readouts of the CCDs. The CCD array is thus read twice for each line scan, and the signals are stored and fed to an output device in the correct order. This effectively doubles the sampling rate of the CCD array.

The necessary displacement of the image relative to the array is typically only a few microns, e.g. on the Fairchild 1728 element array, it is 7 microns. To achieve this small displacement in a controlled manner, a thin transparent plate such as a glass plate 3 may be used. In the arrangement shown in Figure 1, a thin glass plate 3 is situated between the lens 1 and the photosensitive array 2. The plate is angularly displaced about an axis substantially parallel with the short edges of the array 2, so as to displace the image falling on the array 2 by the appropriate distance. The plate 2 can be vibrated by a device to give the two discrete positions at which the CCD array reads the image, or the plate can be continuously oscillated. The latter method may be more convenient, since a typical vibration rate for the glass plate is 1,500 Hz.

The required oscillation of the plate can be achieved by using a quartz crystal attached to one side of the plate. In a typical arrangement, a glass plate of about 3mm thickness would be angularly displaced by only one-third of a degree to achieve the required 7 micron displacement. The oscillation would tend to blur the image; however, the modulation transfer function (MTF) associated with a 7 micron blur is considerably better than the MTF of the CCD elements themselves.

Referring now to Figure 2, there is shown a graph of one line of an image of an imaginary object. The curve 4 represents light intensity in the image as a function of the distance along the photosensitive array. In the first position of the glass plate 3, the image is sampled at all the points marked A on the graph, and at the second position of the plate 3 the image is sampled at all the points marked B. There is, of course, a short delay between the two readouts for a given line scan, but this can be taken into account in the output electronics.

It is possible in this way to combine the two sets of electrical signals into a single set, or alternatively it is possible to adapt the device which utilises the signals, for example a printout device, to use the two sets of signals.

One arrangement for doing this is shown in Figure 3, which represents a set of styli which are energised in response to the electrical signals to provide a printout corresponding with the original object. The styli and a copy sheet are arranged for relative movement, and the two sets of styli, which are arranged in staggered rows as shown, are energised in succession by the two sets of signals. In this way, it can be arranged that the printouts corresponding to the two sets of signals are superimposed along the same line on the copy sheet.

The apparatus of the invention provides the advantage that it allows inexpensive standard CCD devices to be used instead of more expen sive non-standard devices. A compact optical system can be used even with high image quality requirements. One particularly suitable form of compact scanning system is that described in our co-pending patent application No. 22870/77 (Serial No. 1586112). The output signals can readily be utilised without the need for complex electronics.

WHAT WE CLAIM IS: 1. An optical sensor apparatus comprising scanning means for producing on a linear photo sensitive array a succession of optical line images of successive line portions of an object, the photosensitive array comprising a plurality of discrete, regularly spaced, photosensitive devices each of which is adapted to produce an electrical output dependent upon the intensity of light incident thereon, means for displacing the image relative to the array in a direction parallel with the length of the array in a sequence of steps each of which is an integral submultiple of the spacing of the photosensitive devices, and for repeating the sequence of steps for each line image, and means for re ceiving the electrical outputs of said devices, said outputs being in the form of successive sets of signals, one set for each of said steps, the sets of signals for each sequence of steps representing interlaced samplings of one of said line portions of the object.

2. The apparatus of Claim 1 wherein said array of photosensitive devices comprises a linear array of charge coupled devices.

3. The apparatus of Claim 1 or Claim 2 wherein said means for displacing the image is such as to displace it by one half of the spacing of the photosensitive devices.

4. The apparatus of Claim 3 wherein said means for displacing the image comprises a transparent plate mounted for angular vibration in the optical path between the object and the photosensitive array.

5. The apparatus of Claim 4 including a quartz crystal attached to the transparent plate for vibrating it.

6. Copying apparatus including the optical sensor apparatus of any one of Claims 3 to 5, and a printout arrangement including a set of styli arranged in two staggered rows, each row of styli being arranged to make marks on a copying sheet in dependence upon a respective set of signals from the photosensitive elements.

7. An optical sensor apparatus substantially as hereinbefore described with reference

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. the object is being scanned, relative to the array by an integral submultiple of the spacing between the CCD elements, and reading the CCD outputs in each position. Although this submultiple could be any number, a simple and convenient number is 2, so that the image is displaced along the direction of the length of the array by one half of the element spacing between successive readouts of the CCDs. The CCD array is thus read twice for each line scan, and the signals are stored and fed to an output device in the correct order. This effectively doubles the sampling rate of the CCD array. The necessary displacement of the image relative to the array is typically only a few microns, e.g. on the Fairchild 1728 element array, it is 7 microns. To achieve this small displacement in a controlled manner, a thin transparent plate such as a glass plate 3 may be used. In the arrangement shown in Figure 1, a thin glass plate 3 is situated between the lens

1 and the photosensitive array 2. The plate is angularly displaced about an axis substantially parallel with the short edges of the array 2, so as to displace the image falling on the array 2 by the appropriate distance. The plate 2 can be vibrated by a device to give the two discrete positions at which the CCD array reads the image, or the plate can be continuously oscillated. The latter method may be more convenient, since a typical vibration rate for the glass plate is 1,500 Hz.

The required oscillation of the plate can be achieved by using a quartz crystal attached to one side of the plate. In a typical arrangement, a glass plate of about 3mm thickness would be angularly displaced by only one-third of a degree to achieve the required 7 micron displacement. The oscillation would tend to blur the image; however, the modulation transfer function (MTF) associated with a 7 micron blur is considerably better than the MTF of the CCD elements themselves.

Referring now to Figure 2, there is shown a graph of one line of an image of an imaginary object. The curve 4 represents light intensity in the image as a function of the distance along the photosensitive array. In the first position of the glass plate 3, the image is sampled at all the points marked A on the graph, and at the second position of the plate 3 the image is sampled at all the points marked B. There is, of course, a short delay between the two readouts for a given line scan, but this can be taken into account in the output electronics.

It is possible in this way to combine the two sets of electrical signals into a single set, or alternatively it is possible to adapt the device which utilises the signals, for example a printout device, to use the two sets of signals.

One arrangement for doing this is shown in Figure 3, which represents a set of styli which are energised in response to the electrical signals to provide a printout corresponding with the original object. The styli and a copy sheet are arranged for relative movement, and the two sets of styli, which are arranged in staggered rows as shown, are energised in succession by the two sets of signals. In this way, it can be arranged that the printouts corresponding to the two sets of signals are superimposed along the same line on the copy sheet.

The apparatus of the invention provides the advantage that it allows inexpensive standard CCD devices to be used instead of more expen sive non-standard devices. A compact optical system can be used even with high image quality requirements. One particularly suitable form of compact scanning system is that described in our co-pending patent application No. 22870/77 (Serial No. 1586112). The output signals can readily be utilised without the need for complex electronics.

WHAT WE CLAIM IS: 1. An optical sensor apparatus comprising scanning means for producing on a linear photo sensitive array a succession of optical line images of successive line portions of an object, the photosensitive array comprising a plurality of discrete, regularly spaced, photosensitive devices each of which is adapted to produce an electrical output dependent upon the intensity of light incident thereon, means for displacing the image relative to the array in a direction parallel with the length of the array in a sequence of steps each of which is an integral submultiple of the spacing of the photosensitive devices, and for repeating the sequence of steps for each line image, and means for re ceiving the electrical outputs of said devices, said outputs being in the form of successive sets of signals, one set for each of said steps, the sets of signals for each sequence of steps representing interlaced samplings of one of said line portions of the object.

2. The apparatus of Claim 1 wherein said array of photosensitive devices comprises a linear array of charge coupled devices.

3. The apparatus of Claim 1 or Claim 2 wherein said means for displacing the image is such as to displace it by one half of the spacing of the photosensitive devices.

4. The apparatus of Claim 3 wherein said means for displacing the image comprises a transparent plate mounted for angular vibration in the optical path between the object and the photosensitive array.

5. The apparatus of Claim 4 including a quartz crystal attached to the transparent plate for vibrating it.

6. Copying apparatus including the optical sensor apparatus of any one of Claims 3 to 5, and a printout arrangement including a set of styli arranged in two staggered rows, each row of styli being arranged to make marks on a copying sheet in dependence upon a respective set of signals from the photosensitive elements.

7. An optical sensor apparatus substantially as hereinbefore described with reference

to Figures 1 and 2 of the accompanying drawings.