A method of and apparatus for correcting the output signals of a plurality of photodetecting elements

Abstract: When an array of photodetecting elements has to be calibrated, normally a reference surface is scanned. However, since the reference surface is not an ideal surface but may include scratches or dust particles the signals read out from the array have to be corrected. The present invention provides a method of correction in which the statistical distribution of the signals read out over the surface is evaluated for each photodetector. By this evaluation signals due to dust and scratches can be disregarded before calculating a mean value representative of the sensitivity of each individual element.

Description: The invention relates to a method of correcting the output signals from a plurality of photodetecting elements, in which a reference surface is scanned to provide a plurality of output signals corresponding to the light intensity detected by each photodetector. A correction value is calculated for each photodetecting element and subsequent output signals from the photodetecting elements are then corrected utilising the correction values. The invention also relates to an apparatus for correcting output signals in accordance with the method. The method and apparatus are preferably utilized in a scanner, more preferably in a copying apparatus. Description of prior art Scanning of documents is conventionally performed utilizing a plurality of photodetecting elements, such as a photoelectric cell array, eg of the CCD type. A problem with such arrays is that they do not provide uniform output signals. This non-uniformity is caused by the differences in sensitivity between the cells, fluctuations in the dark current, variations in illumination as well as transmission characteristics of the optical system. In order to compensate for this non-uniformity, the signals from the photoelectric cells have to be corrected. Such correction is performed by scanning a uniform, preferably white, reference surface prior to the scan of the document and calculating correction values from the result of this scan. However, a further problem occurs since the reference surface is not completely uniform but contains defects such as scratches and dust. In the prior art various calculations have been proposed to overcome this problem. It has eg been proposed to substitute values falling below a threshold with the mean value of the whole row of pixels or to simply average over a plurality of measurements to reduce the influence of dust particles. A further approach is disclosed in document US-A-5 062 144 (corresponds to EP-A-0 336 410). According to this document the reference surface is divided into regions within which regions an average of a plurality of measurements is calculated for each photoelectric cell. The regions are assumed to be wider than a dust particle on the surface. From these averaged values for each region the maximal intensity value for each pixel is determined and used for the correction. However, it has been found that not only may dust particles give rise to false low values but also that scratches may provide too high intensity values. A method which, in part, considers this problem is disclosed in US-A-5 347 370. The method therein disclosed does not take the highest value but eg the second or third highest value in accordance with a calculating algorithm. In order to provide a simple apparatus with need for only a small memory, such a value (called the "median" value) is determined firstly for every block and then from these block values a global "median" value is calculated. This method has the disadvantage that it takes all the values into account for determining the "median" value which means that a wide dust particle or scratch may effect the measurement too much and that no averaging is performed to reduce influence from noise. Present invention It is an object of the present invention to provide a correction method/apparatus which overcomes the disadvantages of the prior art methods and which provides accurate results even when larger dust particles or scratches are present on the reference surface. It is a further object to provide an apparatus for correcting signals in accordance with this method. This object is achieved as set out in claims 1 and 10. This method/apparatus does not require that the reference surface is divided into blocks, since dust particles and scratches are disregarded by the statistical evaluation. Further, because false values due to dust or scratches or noise are disregarded when calculating the correction value a higher accuracy is achieved. The invention will now be explained by reference to the accompanying drawings wherein: Fig. 1 is a schematic view of a scanning apparatus according to a preferred embodiment of the invention; Fig. 2 shows the position of the reference strip and the construction of the cover in detail; Fig. 3 is an example of the signals read out from the detector over one scan line; Fig. 4 is a schematic diagram showing sets of signals as outputted from the photodetecting elements; and Fig. 5 is a diagram showing a possible statistical distribution of detected signals for one photodetecting element. In Fig. 1 a scanning apparatus which forms part of a copying machine is schematically shown, which comprises a transparent glass plate 1 over which a document to be scanned is positioned by a transporting means (not shown). The transporting means sucks the document on the back side and positions it in the correct scanning position at a small distance over the transparent glass plate 1. At one end of the plate a reference surface in a cover 4 is provided. During a reference scan the reference surface is scanned in a main scan direction S by a scanning system, schematically shown at 5, comprising light source means and mirrors. The scanned image line in a sub scan direction, preferably perpendicular to the main scan direction, is then focussed by imaging means 6 onto the photoelectric detecting means 7, which may be in the form of a CCD array. The signals are read out and converted into digital signals in a conventional manner by the read-out and converting means 8. The digital signals are then processed by signal processing means 9. The signal processing means 9 contains means for calculating a correction value for each pixel and performing correction during scanning action. Such means may include means for averaging the signals output from the CCD array, means for determining the statistical distribution, means for calculating a correction value based upon evaluation of the statistical distribution and storage means for storing data needed for the evaluation and for storing the calculated correction values. The reference scan is preferably performed each time the apparatus is switched on. With reference to Fig. 2 the reference surface will now be nearer described. The reference surface is preferably in the form of a strip 2 positioned in a cover 4. In order to provide approximately the same conditions for the reference surface as for the document to be scanned, the reference strip 2 is not fastened to the transparent plate 1 but is loosely placed onto the plate. As can be seen in Fig. 2 the support plate 4, eg a metal plate, is machined into an end part 3 of the transparent plate 1. When a document to be scanned is transported over the transparent plate it is important that no sharp edges are present which could damage the document. Therefore the support plate 4 is machined into the transparent plate and so shaped that no sharp edges are provided. Further, the angle of the bend of the plate is important, a too sharp bend could damage the original document transported over the glass plate. The reference strip 2 is eg formed of a sheet of white material suitable as a white standard; such materials are well known in the art. On both sides of the sheet 2 there may provided glue for gluing the support plate 4 to the transparent plate 1. The sheet 2 is then positioned loosely under the support plate 4 between the gluing portions. Of course, the sheet 2 may also be glued to the support plate, or may be in the form of a layer coated onto the support plate. To prevent stray light from being reflected on the support plate and reentering the optical system, the plate may be coated black. Fig. 3 gives an example of signals read out from the detecting array during a single scan. As can be seen in the figure, the signal values for the different photodetecting elements are subjected to important variations besides the small variations due to differences in sensitivity between the individual photodetecting elements as well as noise. Fig. 4 shows an example of sets of output signals from the array detector. The three different curves schematically indicates how three scanned lines may differ in output signals. If eg a dust particle is present on the reference surface a dip will occur for one or more of the detecting elements depending on the width of the dust particle. If the dust particle is large compared to the size of the detecting elements, then the dip in output signal will also occur for a plurality of scanned lines. On the other hand, if a scratch is present on the reference surface an output signal is provided which is higher than the normal signal for a white surface. Further, also the illuminating means, such as halogen lamps, are subjected to changes in light intensity which will have to be corrected. Moreover, the detected signals are also influenced by the optical system, stray light and reflections in the transparent plate as well as electronic noise and dark currents. In order to eliminate the influence of the dark currents, preferably a part of the array is covered by an opaque plate and the signals from this part is taken as a measure of the dark current. Since the present invention utilizes the statistical distribution for evaluation, it is not necessary to divide the surface into blocks as in the prior art. The surface can be scanned either continuously or step wise. In the latter case, the same part of the surface is measured a plurality of times and thereafter the scanning system is moved to another part which is then also measured a plurality of times. This procedure is repeated for the whole reference surface. Fig. 5 gives an example of the statistical distribution of a plurality of output signals for a single detecting element. In the diagram of Fig. 5 only the signal values between 70 and 100 are shown. The actual measurement range is preferably from 0 to 255 with 0 representing ideal white and 255 representing ideal black. A large dust particle will eg result in a plurality of occurrences with a very low intensity, as can be seen to the right in the diagram of Fig. 5. In this exams at least three different dust particles can be seen to have influenced the measurement: the first gave rise to values around 90-91, the second around 95 and the third around 97. On the other hand, a scratch will give rise to a number of occurrences having a high intensity, as can be seen to the left in the diagram, around the values 77-78. By evaluating the plurality of output signals for each detecting element in this way, disturbing signals due to eg dust and scratches can effectively be eliminated. This is done by neglecting signals which are too high or too low when calculating the correction value for the respective detecting element. Further, by calculating a mean value of all relevant signals, errors due to eg noise are effectively reduced. In a preferred embodiment such evaluation is performed in the following manner: (a) determine the statistical distribution of the output signals for each detecting element; (b) for a specific element determine the signal value (=max value) which has the highest number of occurrences (in the example of Fig. 5 this would be the value M); (c) determine - in accordance with specified data for a specific application - an area (A) around the max value in which the signal values should be taken into account (in Fig. 5 the area (A) corresponds to the values A1...An); (d) calculate from the thus selected values a mean value; (e) designate the calculated mean value as the correction value for the specific detecting element to be used when correcting the output signals from this element; (f) repeat steps (b) to (e) for each detecting element; and (g) store correction values for use during scanning action. The above cited method steps are preferably implemented to be performed by the signal processing means 9. In another embodiment of the present invention the output signals over a plurality of scanned lines may be averaged to provide a plurality of averaged signals for each detecting element. The scanned lines averaged may either be taken from the same image line - which is then scanned a plurality of times - or from a plurality of adjacent image lines. The evaluation of the statistical distribution is then performed on the basis of the averaged signals. This has the advantage of reducing noise before the statistical evaluation is performed and thus enhancing the possibility of neglecting signals due to dust and scratches. The area selected around the max value is so determined that as many of the relevant signal values as possible are taken into account. Preferably 50%, more preferably 70-80%, of the total values should be within the designated area. The size of the area may be determined by conventional statistic analysis, eg based upon the standard deviation. Preferably, the size of the area is based upon a predetermined typical standard deviation calculated at manufacturing. In practice, an area defined to between 2 and 5 times the standard deviation, more preferably to 4 times the standard deviation, gives good results. Although in Fig. 5 the area A is shown as being symmetrical around the max value M, the area could also be determined unsymmetrically depending on the circumstances. Another way of determining the area is to start at the max value (M) and then increase the number of values considered relevant on both sides of the max value until a satisfactory number of values have been included. This increase may be performed symmetrically around the max value (M) or, if preferred, unsymmetrically so as to eg take a specific number of signal values into account on each side of the max value.

Claims: A method of correcting the output signals from a plurality of photodetecting elements (7), comprising: (a) scanning a reference surface (2) to provide a plurality of output signals corresponding to the detected light intensity for each photodetecting element at a plurality of scanning positions; (b) processing step (9) for calculating a correction value for each photodetecting element and for correcting the output data from the photodetecting elements (7) on the basis of said correction values; characterised in that (c) said correction value is determined by evaluating the statistical distribution of said plurality of output signals for the respective element. A method according to claim 1, characterised in that the determination of the correction value includes determining a signal value (M) having the maximum number of occurrences. A method according to claim 2, characterised in that an area (A) around the value (M) of relevant signal values is determined. A method according to claim 3, characterised in that a mean value is calculated from the signal values lying within said determined area (A). A method according to claim 3 or 4, characterised in that said area (A) corresponds to an area around the value (M) of between 2 and 5 times, preferably 4 times, the standard deviation for a predetermined statistical distribution. A method according to claims 3 or 4, characterised in that said area (A) is determined by increasing an area around the value (M) until a desired number of values is included in said area. A method according to any preceeding claim, characterised in that a plurality of said output signals for each detecting element are averaged to provide a plurality of averaged signals for each element, and in that said evaluation of the statistical distribution is made on said averaged signals. A method according to any of claims 1 to 7, wherein the reference surface is continuously scanned. A method according to any of claims 1 to 7, wherein the reference surface is scanned stepwise. Apparatus for correcting output signals from a plurality of photodetecting elements (7), comprising: a reference surface (2); a plurality of photodetecting elements (7); means for scanning (5) the reference surface (2) so as to provide a plurality of output signals corresponding to the light intensity detected by each photodetecting element (7) at a plurality of scanning positions; processing means (9) for calculating a correction value for each photodetecting element; and means for correcting the output data from the photodetecting elements on the basis of said correction values; characterised in that the processing means (9) further comprises means for determining said correction value by evaluating the statistical distribution of said plurality of signals for the respective element. Apparatus according to claim 10, characterised in that the means for determining (9) the correction value includes means for determining a value (M) having the maximum number of occurrences. Apparatus according to claim 11, further characterised by means for determining (9) an area (A) around the value (M) of relevant values. Apparatus according to claim 12, further characterised by means for calculating (9) a mean value from the signal values lying within said determined area (A). A method according to claim 12 or 13, characterised in that said area (A) is determined to an area around the value (M) of between 2 and 5 times, preferably 4 times, the standard deviation for a predetermined statistical distribution. A method according to claim 12 or 13, characterised in that said area (A) is determined by increasing an area around the value (M) until a desired number of values is included in said area. Apparatus according to any preceeding claim, further characterised by means for averaging (9) a plurality of said output signals to provide a plurality of averaged signals, wherein said evaluation of the statistical distribution is made on said averaged signals. Apparatus according to any of claims 10 to 16, wherein the scanning means (5) scans the reference surface continuously. Apparatus according to any of claims 10 to 16, wherein the scanning means (5) scans the reference surface stepwise.

Inventors: AERTS WILHELMUS J, AERTS, WILHELMUS J.
Applicants: OCE TECH BV, OCE-TECHNOLOGIES B.V.
Application year: 1995
Main classification: H04N1
IPC codes: H04N1, G06T7, G06T1