Optical adjustment arrangement for a scanning system

Abstract: An optical adjustment arrangement for a scanning system comprises a main part (21;22) having defined thereon a triangle with a center point (29; 32) and first and second base points (27, 28; 30, 31). An optical part (23; 24) is positioned onto the main part (21; 22) so that the center point (29; 32) is aligned with the optical axis (50) of the optical part. The right and left sides of the optical part (23; 24) are adjusted separately and independently by first and second adjustment means (51, 52; 53, 54). The position of the adjustment means (51, 52; 53, 54) coincides with the base points of the triangle (27, 28; 30, 31). When one of the first and second adjustment means (51, 52; 53, 54) is moved in a direction perpendicular to the plane defining the triangle, the main part (21; 22) with the optical part (23; 24) is rotated around the line connecting the other of the first and second base points (27, 28; 30, 31) with the center point (29; 32). Stabilising parts (25; 26) acting as flexible hinges are preferably provided around the main part (21; 22).

Description: The invention relates to an optical adjustment arrangement for a scanning system, comprising an optical part to be adjusted, a mounting onto which the optical part is mounted and a frame onto which the mounting is fixed. Preferably the optical part is an array of light detecting elements or an imaging device. The invention also relates to a multiple optical adjustment arrangement comprising at least first and second optical adjustment arrangements of the above mentioned kind. Description of prior art Scanning systems for scanning a document are well known in the art. In such systems a document to be scanned is detected, line by line, by a camera consisting of photodetecting elements and an imaging device such as a lens. The photodetecting elements have to be adjusted both with respect to the focus and with respect to the position of the scan line. Depending upon the maximum width of documents to be scanned and the space available for the optical path either a single photodetecting array or a plurality of adjacently positioned arrays may be utilised. In particular when scanning large documents, ie up to 1000mm width, the prior art approaches have been to provide a plurality of photodetecting arrays aligned with each other. This is because the space available for the optical path is often very restricted which would require a very large array for scanning at a reasonable resolution. Such large arrays are not normally available on the market or are extremely expensive. Multiple array arrangements are eg known from the documents EP-B-0 235 201 and US-A-4 571 637. However, in such systems the problem occurs that the different arrays have to be aligned with each other very precisely in order to provide a high quality image. Also there is a problem of overlapping regions or space between the arrays. Thus, as a result the adjustment arrangements for such systems become very complex. In the above cited EP document a complex structure with four adjustment means causing a plastic deformation is utilised to align each array with the others. Further, for adjustment of focus a special measurement stand (figure 7) is needed. Another construction is known from the above cited US document, wherein for each array five separate and independently operating adjustment means are needed for each camera. Moreover, it has been found that a further problem occurs in that the array is not flat but actually has a skew which also has to be compensated for. Present invention It is an object of the present invention to provide an optical adjustment arrangement which is simple to handle and to manufacture and which may be easily recalibrated if necessary. The adjustment arrangement is equally well suited for the adjustment of the detecting array and the imaging device. It is capable of compensating for the problem of skew and to allow a very accurate calibration with respect to both focus and position of the scan line. This object is solved by providing a mounting comprising a main part onto which the optical part is mounted, having a center point and first and second base points defining a triangle; the center point being fixed with respect to the frame and in alignment with the optical axis of the optical part; and first and second adjustment means for separately adjusting the first and second base points in a direction transverse to the plane containing the triangle; wherein upon adjustment of one of the base points the main part rotates around a line defined by the center point and the other of the two base points. Preferably a multiple arrangement is provided comprising at least first and second such optical adjustment arangements. The invention will now be described in detail with reference to the accompanying drawings, wherein: Fig. 1 shows a scanning apparatus in which the optical arrangement according to the present invention may be used; Fig. 1a shows a plate for pressing the document to be scanned against a transparent plate; Fig. 2 is a schematic drawing for explaning the basic principles of the adjustment arrangement; Fig. 3 shows a preferred multiple arrangement; Fig. 3a schematically shows the relation between the arrangement and the scan line; and Fig. 4 shows a calibration sheet used for scan line adjustment. In Fig. 1 a scanner 1 for large documents of up to 1000mm width is shown. Such scanners are eg utilised in the engineering field when large drawings are to be scanned. In operation a document to be scanned is moved over a transparent plate defining a scan line in a sub scan direction perpendicular to the direction of movement. In the engineering field handling large documents such as drawings, these documents are often stored as folded documents. Before scanning the document is then unfolded. This results in that there are wrinkles on the document. Thus, when such a document is scanned the scanned image may be of unsatisfactory quality if the document is not pressed flat onto the transparent plate. The scanner 1 is therefore equipped with a pressing plate 9 for pressing the document against the plate. Since in a large width scanner also the transparent plate has to be of large width, a further problem occurs in that the plate is not quite flat over the whole width. Therefore, the pressing plate has to be of flexible material so as to be able to press the document tightly against the transparent plate over the whole width. Referring now to Fig. 1a an example of such a pressing plate is shown. The plate is preferably made of aluminium with a white coating thereon. The white coating serves the purpose of acting as a calibration surface which is scanned each time the scanner is switched on. It has been found that such a white coating adheres better to aluminium than to the plastic materials utilised in the prior art. This is of importance to assure a high quality surface for calibration purposes over a long time. As seen in the figure, the pressing plate has a plurality of cut out portions formed in the plate. These cut out portions have a double function of both providing space for the feed rollers and assuring the necessary flexibility of the plate. Referring back to Fig. 1 again, when a document is scanned each scan line is imaged onto a single photodetecting array 5. To be able to image such large documents onto a single photodetecting array, the optical path 2 has been made very long. This is achieved by utilizing the length of the stand 6 of the scanner for the optical path 2. A folding mirror 3 is positioned in the stand 6 near the bottom to further increase the length avalilable for the optical path. Imaging means 4 such as a lens focusses the image of the scan line onto the photodetecting array 5. By providing such a long optical path 2 it is possible to image a scan line of a very large width onto a single CCD array and, thus, the problems of the prior art systems having a plurality of arrays which have to be carefully aligned are overcome. The photodetecting array 5 is preferably a CCD array with about 7500 detecting elements wherein each detecting element has a dimension of about 7x7 um. The optical path 2 thus requires a length of about 1000 mm in order to provide for the necessary magnification ratio. The stand 6 of the scanner is constructed as self-bearing without any frames. The stand consists of two side plates 7 and, mounted thereto, back and front plates 8. The optical parts, ie the mirror 3, the lens 4 and the array 5 are all mounted onto the back plate as shown in the figure. In order to provide a high quality image of the document the optical elements, ie the lens and the photodetecting array have to be carefully adjusted manually. A problem with such an array of photodetecting elements is that the array itself is not perfectly planar but has a skew. This skew is in the range of up to 60 um which is too much to assure a scanned image of high quality. Therefore, first the array 5 itself has to be adjusted so as to provide a sharp image over the whole length of the array 5. Then the imaging means, ie the lens 4, has to be adjusted so as to assure that the image focussed onto the array 5 is exactely aligned with the scan line. With reference to Fig. 2 the basic principles of an optical adjustment arrangement in accordance with the present invention will now be described. An important criteria when constructing such an adjustment arrangement is that the adjustments of the right and left side have to be totally independent of each other, ie the adjustment of one side is not allowed to influence a previous adjustment of the other side. Further, the adjustment should be easy to perform with a minimum of adjustment means being necessary. The basic principle of the present invention is to use a triangle construction. In Fig. 2 an optical part 10 is shown as being positioned within a triangle 11. The optical part 10 is fixed onto a mounting 17. The optical part is to be adjusted in a direction transverse to the plane defining the triangle. It should be noted that the mounting part itself does not have to be in the form of a triangle as long as there are provided points defining the triangle. The triangle is defined by a center point 14 and first and second base points 12 and 13. The center point 14 is fixed with respect to a frame (not shown). The base points 12, 13 are separately and independently adjustable in a direction transverse to the plane defining the triangle. Upon adjustment of the first base point 12 the mounting is rotated around a line 15 defined as the line between the center point 14 and the second base point 13. During this first adjustment the second base point 13 acts as a fixed point. When the mounting is rotated around line 15 the optical part 10 is also moved in the same way. Thus, by adjusting the base point 12 back and forth it is possible to find the optimum position of the left side of the optical part 10. In practice, this may be done by observing the intensity of light passing through or impinging upon the left side of the optical part 10. In the same way the right side of the optical part is adjusted by adjusting the second base point 13. The main part is then rotated around the line 16 between the first base point 12 and the center point 14. During this second adjustment the first base point 12 acts as a fixed point and thus the first adjustment already made is not influenced by the second adjustment. In this way the right and left sides of the optical part can be separately and independently adjusted. A preferred embodiment of the present invention will now be described with reference to Fig. 3. In this figure there is shown a multiple arrangement comprising two optical adjustment arrangements for adjusting both the detecting array and the lens of the scanning apparatus of Fig. 1. Of course it is not necessary to utilise the arrangement of the present invention in such a multiple arrangement but each part could be used also in a single arrangement. In Fig. 3 a multiple arrangement 20 is comprised of a first arrangement for adjusting a photodetecting array 23 so that a sharp image is formed over the whole width of the array, and a second arrangement for adjusting the position of the scan line with respect to the array. The scan line is positioned by adjusting a lens 24 mounted onto the second arrangement together with the first arrangement onto which the array is mounted. Thus, the first and second arrangements are coupled to each other. The array 23 is mounted onto a first main part 21. The first main part 21 has a center point 29 which is in alignment with the optical axis 50 of the multiple optical arrangement. Further, first and second base points 27, 28 are provided. The center point 29 and the base points 27, 28 define a triangle. The base points 27, 28 are separately and independently adjustable by first and second adjustment means 51, 52 in a direction transverse to the plane defining the triangle, as shown in the figure by arrows A. The array of photodetecting elements 23 is mounted onto the first main part 21 so that a specific element on the left and right side respectively lies on the respective line from one of the base points 27, 28 to the center point 29. The specific elements are selected depending upon the number of elements needed for the image and the amount of skew of the array. In Fig. 3a the multiple arrangement 20 is shown in relation to the scan line indicated with numeral 70. The width of the scan line to be imaged is determined by the number of detecting elements on the sensor. The specific elements may be the respective outermost element of the array or may be selected to a specific number from the outermost element. In the example of Fig. 3 the array comprises 7500 elements of which 7200 are used for the scan line image. In order to achieve a maximum accuracy over the whole width of the array, the specific elements are chosen so that 600 elements lie outside the triangle on each side, ie 6300 elements lie inside the triangle. This is because otherwise, due to the skew of the array, the inaccuracy for the elements in the middle of the array would be too high. On the left and right side of the main part 21 a respective stabilizing part 25 is provided. The stabilising part 25 assures that the base points 27, 28 of the main part 21 are both adjustable and acting as fixed points during adjustment of the other side. To perform this double function the stabilising parts are formed as flexible hinges with respect to the base points. Each stabilising part 25 comprises a center point 43, 44 and first and second base points 39/40, 41/42. The center point and the base points define a triangle which is opposed to the main part triangle. The base points 39/40, 41/42 are fixed onto the main part 22 of the second arrangement and the center points 43, 44 are connected to the base points 27, 28 of the first main part 21. In the case of a single arrangement the base points of the stabilising parts would be fixed to a frame onto which the main part is also fixed. In the embodiment of Fig. 3 the base points 39/40, 41/42 of the stabilising parts lie on a common line with the center point 29 of the first main part 21. As can be seen in Fig. 3, preferably two stabilising parts are positioned around the main part, one on each side. The stabilising parts are only flexible in the direction indicated by arrows B and are stiff in all other directions. Next the second arrangement for adjusting the scan line position will be described. This arrangement is mainly identical to the first arrangement. A lens 24 is mounted onto the second main part 22. The second main part 22 comprises a center point 32 and first and second base points 30, 31 defining a triangle. The center point 32 is aligned with the optical axis 50 of the multiple optical arrangement 20 and is fixed with respect to the frame 60. The first and second base points 30, 31 are separately and independently adjustable by first and second adjustment means 53, 54 in a direction transverse to the plane defining the triangle, as shown in the figure by arrows A1. This direction is also transverse to the direction of adjustment of the first main part 21. In the second arrangement the main part 22 does not coincide with the triangle. This clearly shows that the form of the main part is not restricted to a triangle as long as there are provided base and center points defining a triangle in the above stated manner. The positions of the base points 30, 31 are given from the fact that a line from the respective base point 30, 31 through the center point 32 will define the outer limits of the image to be imaged by the lens onto the 6300 elements lying within the triangle. With reference to Fig. 3a this means that the lines drawn from base points 30 and 31 respectively through the center point 32 will - when continued through the optical system - coincide with the outer limits of the part of the scan line 70 imaged onto these 6300 elements. To the left and to the right of the second main part 22 there are provided stabilising parts 26. The stabilising parts 26 are constructed in the same way as the stabilising parts 25 in the first arrangement, ie to function as flexible hinges with respect to the base points 30, 31 of the second main part 22. The stabilising parts 26 each have first and second base points 35/36, 37/38 and a respective center point 33, 34. The base points are fixed with respect to the frame 60 and lie on a common line with the center point 32 of the second main part 22. The center points 33, 34 of the stabilising parts are connected to respective base points 30, 31 of the main part. As can be seen in Fig. 3, in the second arrangement the center points 33, 34 of the stabilising parts 26 are not directly connected to the base points 31 and 32 of the main part 22 as in the first arrangement. This is due to the fact that the triangle of the main part 22 does not coincide with the form of the main part 22. However, this is not a problem as long as the base points 31, 32 are connected to the center points 33, 34 of the stabilising parts. The second main part further acts as a frame with respect to the first main part 21; ie the center point 29 and the base points 39-41 of the stabilising parts 25 are fixed to the second main part 22. The base points 35 to 38 and the center point 32 of the second arrangement are fixed onto a frame 60 which is schematically indicated in Fig. 3. The frame 60 may eg be fixed in the stand of the scanner shown in Fig. 1. Also in the second arrangment the stabilising parts 26 are only flexible in the direction indicated by arrows B1 and stiff in all other directions. Next the adjustment procedure will be described in more detail. When the arrangement of Fig. 3 is utilised in a scanner of the kind shown in Fig. 1 the following adjustment procedure will be executed. The position of the imaging means with respect to the array is pre-adjusted to provide an optimum focus in the middle of the array (23) by moving the lens (24) back and forth and detecting the signal of a predetermined number of detecting elements in the middle of the array (23). Next the skew of the detecting array will be compensated for by adjusting the left and right sides of the array separately in order to achieve a sharp image over the whole of the array. To assist in this procedure a calibration sheet is positioned onto the transparent platen. Such a calibration sheet preferably consists of a line pattern which enables detection of maximum contrast. The sheet is positioned so that the image of the line pattern will be imaged onto a predetermined number, eg 100, of detecting elements of the array on each side. First the left side of the array is adjusted by movement of the adjustment means 51 and the signals from the left side elements are read out. In an example the adjustment means consists of a screw of the M3 size. This means that one turn of the screw corresponds to a movement of 0.5 mm in the adjustment direction. An adjustment of 50 um would thus correspond to 36 degrees; ie a high accuracy can be easily achieved. When the adjustment screw is turned a signal indicating the contrast of the detected intensities is read out and the screw is adjusted until a predetermined value has been reached. Thereafter, the right side of the detecting array is adjusted in the same way by movement of the adjustment means 52. When the detecting array has been thus optimally adjusted it has to be aligned with the position of the scan line. This is because the mechanical tolerances are too high to assure an optimal alignment without manual adjustment. Upon adjustment of the scan line position another calibration sheet is placed on the transparent plate in a specific position. The calibration sheet is preferably in the form shown in Fig. 4. Such a sheet has a black surface to provide a high contrast. The sheet is positioned so that the smallest width is exactely in the middle of the scan line 70 as indicated in the figure. By detecting the intensity of light impinging upon a predetermined number of photodetecting elements, the left and right side of the scan line position can be optimally adjusted by turning the adjustment screws 53 and 54 respectively. When the position of the scan line is adjusted a signal corresponding to the number of elements being covered by the black sheet is read out. If the number of elements is too high then the position of the scan line is adjusted in one direction by movement of the adjustment means 53 and 54 respectively. If the number of elements then increases it is noted that this was the wrong direction and the adjustment means 53 and 54 respectively is moved in the other direction until an optimal signal has been achieved. This procedure is done for both the left and the right side of the scan line by utilising the first and second adjustment means 53 and 54. The above description is only one example of an embodiment of the invention as contained in the claims. In particular the invention is not limited to a double arrangement but a single arrangement or an arrangement with three or more individual arrangements come within the scope of the claims. Further, the invention is not limited to a large format document scanner of the type shown in Fig. 1 but could equally well be utilised in a scanning system for small documents. The optical adjustment arrangement may be implemented in a separate scanning apparatus or form part of a scanning unit in a copying machine.

Claims: Optical adjustment arrangement for a scanning system, comprising: an optical part (23; 24) to be adjusted; a mounting (21; 22) onto which the optical part is mounted; and a frame (22; 60) onto which the mounting is fixed; characterised in that the mounting comprises: a main part (21; 22) onto which the optical part (23; 24) is mounted, having a center point (29; 32) and first and second base points (27/28; 30/31) defining a triangle; the center point (29; 32) being fixed with respect to the frame (22; 60) and in alignment with the optical axis (50) of the optical part (23; 24); and first and second adjustment means (51/52; 53/54) for separately adjusting the first and second base points (27/28; 30/31) in a direction transverse to the plane containing the triangle; wherein upon adjustment of one of the first and second base points (27/28; 30/31) the main part (21; 22) rotates around a line defined by the center point (29; 32) and the other of the first and second base points (27/28; 30/31). Arrangement according to claim 1, characterised in that the mounting further comprises at least one stabilising part (25; 26) arranged around the main part (21; 22). Arrangement according to claim 2, characterised in that the at least one stabilising part (25; 26) is formed so as to function as a flexible hinge with respect to the adjustable base points (27/28; 30/31) of the main part (21; 22). Arrangement according to any of claims 2 or 3, characterised in that the at least one stabilising part (25; 26) has a center point (43, 44; 33, 34) and first and second base points (39/40, 41/42; 35/36, 37/38) defining a stabilising triangle; the center point (43, 44; 33, 34) of each stabilising part being connected to one of the base points (27/28; 30/31) of the main part (21; 22). Arrangement according to claim 4, characterised in that the triangle of the at least one stabilising part (25; 26) is defined as being opposed to the triangle of the main part (21, 22). Arrangement according to any of claims 4 or 5, characterised in that the base points (39/40, 41/42; 35/36, 37/38) of the at least one stabilising part (25; 26) are fixed with respect to the frame (22; 60). Arrangement according to any of claims 2 to 6, characterised in that two stabilising parts (25; 26) are provided, one on each side of the main part (21; 22). Arrangement according to any preceding claim, characterised in that the optical part to be adjusted is a light detecting device, preferably an array of light detecting elements (23). Arrangement according to claim 8, characterised in that the array of light detecting elements (23) is mounted onto the main part (21) so that each side of the triangle in the sense of a line connecting the center point (29) with one of the base points (27, 28) coincides with a respective predetermined first and second detecting element of the array (23). Arrangement according to claim 9, characterised in that upon adjustment of the detecting array (23) the respective predetermined first and second elements are read out. Arrangement according to claim 10, characterised in that a respective predetermined number of detecting elements adjacent the first and second elements are read out upon adjustment. Arrangement according to claim 11, characterised in that the array (23) is adjusted by firstly adjusting the first base point (27) with the first adjustment means (51) until a desired value is read out from the first element and the corresponding predetermined number of adjacent elements, and secondly the second base point (28) is adjusted with the second adjustment means (52) until a desired value is read out from the second detecting element and the corresponding predetermined number of adjacent elements. Arrangement according to any of claims 1 to 7, characterised in that the optical part comprises an imaging device (24), preferably a lens. Arrangement according to claim 13, characterised in that triangle of the main part (22) is so defined that the sides of the triangle defining a line from the center point (32) to the respective base point (30, 31) define the outer limits of the object to be imaged by the imaging device (24). Arrangement according to claim 14, characterised in that a light detecting device, preferably an array of light detecting elements, is positioned in alignment with the imaging device (24) and in that upon adjustment of the imaging device (24) the light intensity impinging upon the light detecting device is read out. Arrangement according to claim 15, characterised in that the imaging device (24) is adjusted by fistly adjusting the first base point (30) of the main part (22) with the first adjusting means (53) until a desired value is read out from the detecting device, and secondly adjusting the second base point (31) of the main part (22) with the second adjustment means (54) until a desired value is read out from the detecting device. Arrangement according to claims 1 to 16, characterized in that it comprises at least first and second optical adjustment arrangements. Arrangement according to claim 17, wherein the main part (21) of the first optical adjustment arrangement is fixed to the main part (22) of the second optical adjustment arrangement. Arrangement according to claim 17 or 18, wherein the first optical adjustment arrangement comprises an array of detecting elements (23) as the optical part and the second optical adjustment arrangement comprises an imaging device (24) as the optical part. Arrangement according to claim 19, wherein the first optical adjustment arrangement is used to adjust the focus of the detecting array (23) and the second optical adjustment arrangement is used to adjust the position of an image imaged onto the detecting array (23) by the imaging device (24). Arrangement according to claims 1 to 20, characterized in that said arrangement is part of an optical scanner.

Inventors: BOERSTAL HENDRICUS G J, PUIK ERIK C N, BOERSTAL, HENDRICUS G.J., PUIK, ERIK C.N.
Applicants: OCE TECH BV, OCE-TECHNOLOGIES B.V.
Application year: 1995
Main classification: H04N1
IPC codes: H04N1, G02B7