Method of obtaining white reference data used for correcting non-uniformity in photoelectric cell array

Abstract: In an image scan reader, a white reference plate (3) is attached to a transparent plate on which an original is to be placed. Respective optical densities on a plurality of scanning lines (L A1 -L An , L B1 -L Bm ) are detected, and are averaged within respective regions (R A , R B ). The maximum values of the respective averaged densities are selected for each pixel to be used for correcting non-uniformity of CCD cells which is employed in reading the white reference plate and the image of the original.

Description: The present invention relates to a method of obtaining a white reference data which is used for correcting non-uniformity in photoelectric cell array, which is provided in an image scan reader such as process scanner. Description of Background Arts As well known in the art, a linear photoelectric cell array is provided in an image scan reader in order to read an original image for each scanning line, where the linear photoelectric cell array has a number of photocells. The respective photoelectric output signals of the photocells are not uniform even if they read an original image having uniform optical density because of non-uniformity in sensitivity of the photocells and illumination of an original as well as fluctuation of dark currents in the photocells. In order to compensate the non-uniformity, the photoelectic signals are so corrected that uniform signals can be obtained when an original in a uniform density is read with the linear photoelectric cell array. The correction is known in the art as "shading correction". see e.g. DE-A-3719553 providing such a shading correction. In the shading correction, a reference white plate having an uniform density is prepared, and white reference signals are generated by reading the white plane with photoelectric cell array. A shading correction table is then produced with reference to respective levels of the white reference signals. However, a white reference plate having complete uniformity in density is hardly obtained in practice, since various defects such as dusts and scratches often exist on white reference plates. Consequently, a manufacturer and a user of the image scan reader are obliged to conduct the shading correction with a defective white reference plate. In one of improvements in the shading correction, the white reference plane is read with the linear photoelectric cell array through an optical system being out of focus, whereby the defects on the white reference plate are smoothed in the projection onto the linear photoelectric cell array. However, this improvement has a disadvantage that the smoothing is not sufficient if the original image is projected onto the linear photoelectric cell array through a reduction optical system having large depth of field. When the white reference plate is displaced for from the original so that sufficient smoothing can be obtained, the condition of illumination on the white reference plate comes different from that of the original, whereby the shading correction losts its accuracy. In another improvement of the shading correction, the white plate is read for a plurarity of scanning lines, to thereby obtain a plural sets of white reference signals. Then, the plural sets of the white reference signals are compared with each other, and one of them having maximum levels is employed for the shading correction. Although the improvement has an advantage in that it can avoid influence from dark defects on the white reference plate, electric noises or temporary peaks which may cause in the linear photoelectric cell array and a signal processor are fetched as "maximum levels", whereby the meaningless peaks are undesirably used for the shading correction. The invention is as set out in the claims. The present invention is intended for a method of obtaining a white reference data which is so used for correcting non-uniformity in output signals of a linear photoelectric cell array that the output signals of the linear photoelectric cell array are selectively enhanced or suppressed according to the white reference data when the output signals are obtained by reading an image of an original with the linear photoelectric cell array. The present intention is also intended for an image scan reader having a function of correcting the non-uniformity. Accordingly, an object of the present invention is to obtain a white reference data without undesirable influence from noises and defects on a white reference plane. Another object is to obtain a white reference data with as optical system being in focus. Another object is to generate a white reference data regardless of the depth of field of an optical system. These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a schematic diagram showing an image scan reader according to a preferred embodiment of the present invention, Fig. 2 is a schematic diagram showing a white reference plane 3a which is conceptionally divided into two areas R A and R B , and Fig. 3 is a flow chart showing process steps conducted in the image scan reader. Fig. 1 shows an image scan reader 1 of a flat bed type to which a preferred embodiment of the present invention is applied. The image scan reader 1 comprises a transparent plate member 2 on which an original (not shown) is placed with its face laid down. A white reference plate 3 is attached to an end portion of the transparent plate 2. The white reference plate 3 has the shape of a strip, and its longitudinal direction is parallel to a main scanning direction Y. The white reference plate 3 may be a conventional one, and its bottom surface is a white reference plate 3a. A projection lens 4 being in focus is provided under the transparent plate 2, through which the density distribution on the white reference plane 3a or the image of the original on the transparent plate 2 is projected onto the sensing surface of a CCD linear photosensor 5. The CCD linear photosensor 5 has a linear array of CCD cells (not shown). A main scanning along the direction Y is attained by serially accessing the CCD cells along the alignment thereof so as to generate a series of photoelectric signals S I . The image reading with the CCD linear photosensor 5 is conducted while moving the transparent plate 2 along the direction (-X), whereby a subscanning along the direction X is attained for the white reference plane 3a or the original being placed on the transparent plate 2. The photoelectric signals or linear image signals S I thus obtained from the CCD linear photosensor 5 are amplified in an analog amplifier 6, and then converted into a digital data S D by an A/D converter 7. When the white reference plane 3a is read with the CCD linear photosensor 5 in order to obtain a white reference data for shading correction, the digital data S D is a "white density data" expressing two-dimensional distribution of optical densities on the white reference plane 3a as density distribution on a pixel array. On the other hand, when the original is read after the reference data has been obtained, the digital data S D is an "image data " expressing the image of the original for each pixel. Since the characteristic construction of the present invention exists in a process of obtaining the white reference data for the shading correction, the following description will be mainly directed to the former stage in which the white reference plane 3a is read and the digital data S D is the white density data. The white density data S D is delivered to a circuit 10 for generating the white reference data. The circuit 10 generates the white reference data S for one scanning line through a process which will be described later, and the white reference data S is delivered to an image processor 8. After the delivery of the white reference data S, the image of the original is read with the CCD linear photosensor 5 and the image data thus obtained is transmitted to the image processor 8 through a transmission path (not shown) while bypassing the circuit 10. Shading correction for the image data is attained in the image processor 10 on the basis of the white reference data S. Gradation correction and other controls for reproducing the image on a photosensitive material through scanning are also attained in the image processor 8. The image processor 8 may include a divider for dividing the white reference data S by an integer, which will be described later. The controller 9 is operable to control the circuit 10. B. Detail of Preferred Embodiment In the process of generating the white reference data S, the white reference plane 3a is concepitonally divided into a plurality of regions R A and R B (Fig. 2) aligned along the subscanning direction X. Since the white reference plane 3a has the shape of a strip extending along the main scanning direction Y, the regins R A and R B are partial strips extending along the longitudinal direction of the white reference plane 3a and being parallel with each other. It is to be noted that the division of the white reference plane 3a is only conceptional one which is introduced for clear understanding of the present invention, and the division is not an actual division of the white reference plane 3a. However, if desired, the white reference plate 3 may be actually divided into two strips of sub-plates to separate the regions R A and R B from each other. That is, the regions R A and R B may be defined on a single plate, or alternatively, they may be defined on different sub-plates, In the preferred embodiment, the regions R A and R B exist on a single reference plane 3, as shown in Fig. 2, in order to avoid increase of members in number. On the regoins R A and R B , parallel scanning lines L A1 -L An (n ≧ 2) and L B1 -L Bm (m ≧ 2) are supposed, respectively. The integers n and m may be identical to each other, or alternatively, they may be different from each other. These scanning lines are conceptionally defined in scanning of the regions R A and R B , and they are not real lines drawn on the white reference plane 3a. Since the division of the white reference plane 3a and the establishment of scanning lines L A1 -L An and L B1 -L Bm are conceptionally attained through data processings accompanied with the scanning of the white reference plane 3a, the white reference plate 3 itself may be a simple white plate. In response to a start command, the optical densities on the scanning lines L A1 -L An belonging to the first region R A are read or detected for each pixel by the CCD linear photosensor 5 (the process step 101 in Fig. 3). The detection is conducted while moving the transparent piate 2 along the direction (-X) with the white reference plate 3 placed thereon, whereby the respective optical densities on the scanning lines L A1 -L An are serially read or detected in that order. A one-dimensional white density data S D for the first scanning line L A1 which is thus obtained has K components, where K is the number of the potocells provided in the linear CCD linear photosensor 5. The number K also expresses the number of pixels on one scanning line being discriminated from each other with respective Y-coordinate values. The data S D is stored in a line memory 12a (Fig. 1) for each pixel through a selector 11a. Then, the white density data S D for the first scanning line L A1 is read out from the line memory 11a for each pixel as a data S a synchronously with the detection of white density data S D for the second scanning line L A2 , and is supplied to an adder 16 through a selector 15. The other input of the adder 16 is the one-dimensional white density data S D for the second density data. Those two inputs of the adder 16 are added to each other therein for each pixel, to become a data S I . Prior to the addtion in the adder 16, the selector 11a is switched to connect the line memory 12a to a selector 13, and the selector 13 connects the adder 16 to the line memory 12a, so that a loop cyclically connecting the line memory 12a, the selector 15, the adder 16 and the selectors 13 and 11a is established. Consequently, the data S I is transmitted to the line memory 12a, and is stored for each pixel at corresponding addresses in the line memory 12a, while serially deleting the old data having been stored therein. That is, the old data in the line memory 12a is replaced with the new data S I through a read-modify-write operation. Another line memory 11b and a comparator 14 are being disenabled during the aforementioned process step, and do not contribute the operation of the loop. Each of the line memories 11a and 11b has a storage capacity corresponding to one scanning line. The controller 9 is operable to generate address signals ADRS and control signals CONT, by which read/write control of the line memories 11a and 11b is attained. The other elements in the circuit 10a are also controlled in their operation timings by the control signals CONT. The read-modify-write operation is repeated for all scanning lines L A1 -L An belonging to the first region R A , so that respective white density data for the scanning lines L A1 -L An are accumulated or summed up along the subscanning direction X for each Y-coordinate value (the process step 102). A data S ACC (not shown) expressing the accumulated values or sums is being stored in the line memory 11a. Since the accumulation is conducted only along the subscanning direction X, the data S ACC includes a number of components corresponding to respective Y-coordinate values or pixels. For example, the component D₁ of the data S ACC expressing the sum at the position Y=Y₁ (Fig. 2) is; D₁ = I A1 + I A2 + ... + I An (1) where I A1 -I An are the values of the white density data S D at respecive pixels having the coordinate Y=Y₁ on the scanning lines L A1 -L An . Similarily, the other components of the data S ACC express the respective sums at the other Y-coordinate values, respectively, and therefore, the data S D consists of K-components. The data S ACC is substantially equivalent to an average S AV of the respective white density data for the scanning lines L A1 -L An since the data S ACC expresses the sums and the number or factor n is a constant in the definition of the average S AV : S AV = S ACC /n (2) The value of the average S AV can be obtained by dividing the value of the data S ACC by the number n . If the number n is equal to m , the average S AV can be directly obtained by providing a 1/n-divider at the front stage of the circuit 10. However, since the average S AV can be easily estimated from the data S ACC , such a 1/n-divider may be omitted as shown in Fig. 1. In the present invention, the term "average" is used in its broad sense, and it may be any quantity expressing collective character of white density data. The data S ACC is directly proportional to the simple average S AV , and therefore, the data is included in the concept of the "average" in the broad sense. After the "averaged" data for the first region R A is obtained in the line memory 12a in the form of the data S ACC , similar operation is conducted for the second region R B (the process steps 103 and 104). In the process for the second region R B , the line memory 12b is used and the selector 15 selects an output S b of the line memory 12b. The selector 11b connects the line memory 12b to the A/D converter 7 in order to fetch the white density data S D for the scanning line L B1 into the line memory 12b, and then, is switched to the other side for transmitting the output of the adder 16 to the line memory 12b. An accumulation or summation through read-modify-write operation is carried out in another loop consisting of elements 12b, 15, 16 and 11b. Therefore, when the detection of respective density levels is completed for the scanning lines L B1 -L Bm belonging to the second region R B , sums or "averages" of respective white density data for the scanning lines L B1 -L Bm are obtained in the line memory 12b in the form of an "averaged" data including a plurality of components which express the averaged white density level for each Y-coordinate value or pixel position. For example, the component D₂ corresponding to the position Y = Y₁ is the sum or average of respective white density data L B1 -L Bm at the position Y = Y₁ on the scanning lines L B1 -L Bm , i.e., D₂ = I B1 + I B2 + ... + I Bm (3) The first and second averaged data being stored in the line memories 12a and 12b respectively are read out from the same for each pixel or Y-coordinate value, to be delivered to the comparator 14. The comparator 14 compares the respective averaged data with each other to specify maximum one MAX for each Y-coordinate value (the process step 105). The maximum values MAX for respective Y-coordinate values are serially output from the comparator 16 as a maximum white data S M . The selectors 13 and 11a are switched toward the comparator 16, and the maximum white data S M is stored in the line memory 12a for each pixel or Y-coordinate value. As the storing operation progresses, the averaged data which has been stored in the line memory 12a is serially deleted and replaced with the data S M . After the maximum white data S M for all of the Y-coordinate values is stored in the line memory 12a, the data S M is read out from the line memory 12a in time series, to be delivered to the image processor 8 as the white reference data S. The white reference data S includes K components corresponding to the number of the pixels or the coordinate values along the direction Y. At each Y-coordinate value, the corresponding component expresses the maximum one of the first and second averaged data. If D₁ > D2 at Y = Y₁ , the component of the white reference data S for Y = Y₁ is the larger one D₁ of them. From the expressions (1) and (3), the following relation holds: where n = m . A similar relation holds for other values of Y. In the next process step 106, the 1/n divider which is provided in the image processor 8 divides the white reference data S by the number n (= m ) , to thereby generate a data S C (not shown). On the other hand, when n ≠ m , a 1/n divider is inserted between the comparator 14 and the line memory 12a, while a 1/m divider is inserted between the comparator 14 and the line memory 12b. In this case, the 1/n divider in the image processor 8 is omitted since the data S C being just a rigorous average of the white density data is obtained in the circuit 10 in the form of: The other components of the data S c corresponding to Y-coordinate values other than Y = Y₁ are also obtained as values similar to the expression (5) or (6). After the data S c is obtained, the image of the original is read with the CCD linear photosensor 5 for each pixel on each scanning line through the combination of the main scanning and the subscanning (the process step 107), and the image data is transmitted to the image processor 8. The image processor 8 corrects the image data on the basis of the data S c which expresses the non-uniformity in the CCD cells (the process step 108). That is, the image data for pixels having a Y-coordinate value at which the data S c indicates relatively small value (i.e., "dark" value) is relatively enhanced, and/or those having a Y-coordinate value at which the data S c indicates a relatively large value (i.e., "light" value) is relatively suppressed. The corrected image data is when subjected the gradation correction and other processings (the process step 109), to be delivered to an image recorder (not shown) for image recording or reproducing of the original image for each pixel (the process step 110). C. Advantage of Preferred Embodiment and Modifications According to the preferred embodiment, the respective white density data for the plurality of the scanning lines L A1 -L An (L B1 -L Bm ) are averaged in each of the regions R A and R B , and therefore, smoothed is fluctuation in optical density which caused by small defects such a small dust 21 and a scratch 22 (Fig. 2). Furthermore, if a relatively large dust 23 exists on the white reference plane 3a, the influence of the dust 23 on the white reference data S can be eliminated by using the maximum data within respective averaged white density data for the regions R A and R B , since the dust 23 is not so large as to extend from one region to the other region in most cases. Consequently, the white reference data S is a desirable one in that the influence of the defects is substantially eliminated and the data S is comparable to that obtained from an ideal or complete white reference plane having no defects. If the image scan reader 1 is provided with a white reference plate and a black reference piate in order to obtain a white reference data and a black reference data, respectively, the circuit 10 may be used for obtaining the black reference data as well as the white reference data. Since the circuit 10 has a relatively simple structure, the same can be producted without increase in cost. The white reference plane 3a may be conceptionally or actually divided into three or more regions. In general, when the number of regions is N (N ≧ 2), the process steps of detecting two-dimensional distribution of optical densities in a J-th region (1 ≦ J ≦ N) and averaging the same in the subscanning direction X are repeated for all of the integers J within 1 ≦ J ≦ N. The average of the white density data may be a square root of a square sum, a geometical mean, or the like. The maximum white data S M expresses the whitest density value for each pixel within the respective averaged data, and therefore, it is the maximum value when the white density data S D expresses lightness, while it is the minimum value when the white density date S D expresses darkness. The later case is corresponding to the modification where the white density data S D is obtained through a circuit for taking a complement of the photoelectric signals S I . That is, the term "maximum" is used for indicating the lightest optical density in the present invention so that local darkness which is caused by the defects on the white reference plane is eliminated from the white reference date. The white reference plane may be a part of the transparent plate 2 which is painted white.

Claims: A method of obtaining a white reference data (S) which is used for correcting nonuniformity in output signals (S I ) of a linear photoelectric cell array (5) to selectively enhance or suppress the output signals (S I ) of said linear photoelectric cell array (5) according to said white reference data (S) when the output signals (S I ) are obtained by reading an image of an original with said linear photoelectric cell array (5), said method comprising the steps of: (A) reading a white reference plate (2), (B) obtaining white reference data (S) on a plurality of scanning lines (L A1 , L A2 , ..., L An ), and (C) averaging said data (S), characterized in that (1) preparing a white reference plane (3a) which includes first to N-th regions (R) being arrayed in a first direction X, where N is an integer larger than one, and where said first to N-th regions (R) are strip regions (R) whose respective longitudinal directions are parallel to said second direction Y, (2) generating first to N-th averaged data (S D ) through the steps of: (2a) selecting a J-th region (R J ) within said first to N-th regions, where J is an integer satisfying the condition 1 ≦ J ≦ N, (2b) by means of said linear photoelectric cell array (5), detecting distribution of optical densities in said J-th region (R J ) to generate J-th white density data expressing said distribution, (2c) averaging said J-th white density data in said first direction X to generate J-th averaged data which includes a plurality of K components being assigned to respective positions being discriminated from each other with Y-coordinate values, where Y is a second direction being perpendicular to said first direction X, and (2d) repeating the steps (2a) through (2c) while serially updating the integer J under the condition 1 ≦ J ≦ N, to thereby generate said first to N-th averaged data (S D ) , (3) comparing said first to N-th averaged data (S D ) with each other to select maximum one (MAX) of said first to N-th averaged data (S D ) for each position (Y i ) indicated with a Y-coordinate value, whereby a plurality of selected maxima (D max ) corresponding to the respective Y-coordinate values (Y i ) are identified, and (4) generating white reference data (S C ) from said plurality of said selected maxima (D max ). Method according to claim 1, characterized in that the step (2b) includes the step of: (2b-1) by means of said linear cell array (5), detecting one dimensional distribution of optical densities in each of several scanning lines (L A1 , L A2 , ..., L An (n ≧ 2); L B1 , L B2 , ..., L Bm (m ≧ 2)) which are extending in said second direction Y and are arrayed in said first direction X, to thereby generate a plurality of one-dimensional density data (I An ; I Bm ) as said J-th white density data, and the step (2c) includes the steps of: (2c-1) averaging said plurality of said one-dimensional density data (I An ; I Bm ) in said first direction X to generate said J-th averaged data (S AV ). Method according to claim 2, characterized in that said maximum one (D max ) expresses whitest optical density within respective components K of said first to N-th averaged data (S AV ) being assigned to a same Y-coordinate value. Method according to claim 3, characterized in that a same number (n) of scanning lines (L A1 , L A2 , ..., L An (n ≧ 2)) is defined on each of said first to N-th regions (R), each of the plurality of said one-dimensional density data (S D ) has K components, where K is an integer expressing a number of pixels being discriminated from each other with respective Y-coordinate values on one scanning line, and the step (2c-1) includes the step of: (2c-1) summing respective components of the plurality of said line density data (S D ) for each Y-coordinate value to thereby generate said J-th averaged data (S ACC ). Method according to claim 4, characterized in that said linear photoelectric cell array is provided in image scan reader (1), said image scan reader (1) is operable to read said image of said original through a combination of a main scanning and a subscanning, and said first (X) and second (Y) directions are a subscanning direction and a main scanning direction in said image scan reader, respectively. Image scan reader (1), with : (1) supporting means for supporting an original, (2) a white reference plate (3) attached to said supporting means, wherein a surface of said reference plate (3) is in a plane defined by first direction (X) and second (Y) directions perpendicular to each other, (3) linear photosensor array means (5) extending in said direction to face said supporting means, (4) means for relatively moving said supporting means and said linear photosensor array means in said first direction, (5) means for enabling said means of 3) and 4) to detect distribution of optical densities on said white reference plate (3) for M scanning lines each of which extends in said second direction (Y), where M is an integer larger than one, (6) means for averaging said distribution for n scanning lines within said M scanning lines for each respective position along said second direction to generate a first averaged data, where n is an integer larger than one, and (7) reading means reading an image of said original for each scanning line with said means of 3) and 4) , to thereby generate an uncorrected original image data, and (8) correcting means for said original image data employing white reference data (S). characterized by (9) means for averaging said distribution for m scanning lines within said M scanning lines for a position along said second direction to generate second averaged data, where M is an integer satisfying the condition M = n + m and each of n and m is an integer larger than one, (10) comparing means for comparing said first and second averaged data with each other to select a maximum value (MAX) of said first and second averaged data for each position along said second direction to obtain a set of maximum values, and (11) storing and memory means for preparing white reference data (S M ) from said set of said maximum values (MAX). Image scan reader according to claim 6, characterized in that said n scanning lines are defined on a first region (R A ) of a surface of said white reference plate, said m scanning lines are defined on a second region (R B ) of said surface, and said first and second regions (R A ,R B ) are strip regions being arranged in parallel to each other.

Inventors: MURAKAMI SHIGEO DAINIPPON SCRE, MURAKAMI, SHIGEO DAINIPPON SCREEN MFG.CO., LTD.
Applicants: DAINIPPON SCREEN MFG, DAINIPPON SCREEN MFG. CO., LTD.
Application year: 1989
Main classification: H04N1
IPC codes: H04N1, G06T1