Focus controlling method and system for an image capturing system

[0003] The present invention relates to a focus controlling method and system, and, more specifically, to a focus controlling system for an image capturing system. The magnification ratio of the system can be controlled through a closed loop control method and system according to the variation in the position of the object being scanned.

[0004] An image capturing system like a scanning system is an image processing tool which is popularly employed for acquiring the image of documents, photographs, or even objects. The image is acquired and then transferred to an accessible signal or data for a processor or a computer. Without limiting the scope and application of the present invention, the operation of a scanner is illustrated as follows. In general, the document is illuminated by a light source when being scanned. The image is then transferred onto image or photo sensing devices through one or a set of lenses. Generally speaking, the photo sensing devices like CCD (charge-coupled device), CIS (contact image sensor), or photo-sensitive MOS (metal oxide semiconductor) devices, can be employed. The photo sensing devices can transform the image to accessible signals. In general, the scanning systems can generally be classified to at least three categories by its operation characteristics, including a handhold scanning system, a sheet feeding scanning system, and a flatbed scanning system.

[0005] For achieving the high quality of the image being captured, a flatbed scanning system is used frequently. FIG. 1 illustrates a schematic side view of a portion of a flatbed scanning system. In the application of the flatbed scanning system, a document or an object being scanned 10 is placed on a transparent plate 12. An image capturing device 14 is moved under the transparent plate 12 to capture the image of the document 10 line-by-line. The quality of the image acquired is greatly dependent on the characteristics of the image capturing device 14. The parameters like resolution and depth of focus (DOF) have a great influence on the quality of the image transferred.

[0006] Under the raised requirement of image quality and the increasing processing ability of related processors or computers, the resolution of the scanner is significantly raised to capture high quality images. In general, the depth of focus of the image capturing device 14 is quite limited, especially for high resolution scanning systems. However, in most of the image capturing applications, some documents 10 are covered with a document wrapper or a transparent document wrapper 10a, as shown in FIG. 2. The insertion of the document wrapper 10abetween the document 10 and the image capturing device 14 increases the distance between the two. The optical path between the object being scanned and the lens of the optical system is then changed. If the focus point, or namely the length of the optical path, of the image capturing device 14 is remained unchanged, the magnification ratio would be varied under the increased object distance. The change of magnification ratio on the scanned width might cause the captured image to be distorted in size or the length-to-width ratio. Besides, the object being scanned can be left outside the range of designed depth of focus, the resolution and clearness of the image is greatly influenced. The image quality is then damaged under the shifted focus point.

[0007] For most image scanning devices, the focus point of the object being scanned is fixed. For high resolution scanning systems, the depth of focus is generally very short and the tolerance to the variation in the document height is extremely low. However, lots of document are scanned accompanied by a document wrapper with a considerable thickness. The object distance of the document 10 from the image capturing device 14 is increased. In order to adapt for the change of document height, an improved scanning system must be provided with an adaptive focus controlling system. Therefore, the magnification ratio and the resolution of the scanning process can be adjusted and the quality of the image captured can be maintained.

[0008] The present invention discloses a focus controlling method and system for an image capturing system. The length of the optical path between the scanned object and the lens of the image capturing device can be controlled. Under the variation in the position of the scanned object, the magnification ratio of the system can be controlled or maintained within a tolerable range through a closed loop control method and system.

[0009] The focus controlling system of the present invention can include a calibration paper, an image capturing device, an input interface, and a processing and controlling device. The image capturing device is employed for scanning the calibration paper. The image capturing device has a focus point adjusting mechanism. The input interface is utilized for receiving a location data of a scanning point. The processing and controlling device is responsive to the input interface and the image capturing device to control the focus point adjusting mechanism. As an example, the image capturing system can be a flatbed scanning system.

[0010] The focus controlling method of the present invention includes the following steps. First, a scanning point is set or detected and a compensated magnification ratio for the scanning point is then calculated. A focus point of a image capturing device is adjusted and a calibration paper is scanned. The adjusting step and the scanning step are then repeated until a magnification ratio of the calibration paper is approximately equal to the compensated magnification ratio. The focus controlling process is then finished with a justified magnification ratio for the raised height of the object being scanned.

[0011] The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by referencing the following detailed description, when taken in conjunction with the accompanying drawings as follows.

[0012] FIG. 1 illustrates a schematic side view of a portion of a prior art flatbed scanning system.

[0013] FIG. 2 illustrates a schematic side view of a document with a document wrapper placed in a prior art flatbed scanning system.

[0014] FIG. 3 illustrates a schematic side view of a focus controlling system in accordance with the present invention.

[0015] FIG. 4 illustrates a schematic diagram of the optical path of a scanning system and a calibration paper having width identification marks in accordance with the present invention.

[0016] FIG. 5aillustrates a pulley-screw type moving mechanism for the image capturing device in accordance with the present invention.

[0017] FIG. 5billustrates a slide-screw type moving mechanism for the image capturing device in accordance with the present invention.

[0018] FIG. 5cillustrates a wire-worm type moving mechanism for the image capturing device in accordance with the present invention.

[0019] FIG. 5dillustrates a screw-driving mechanism for the image capturing device in accordance with the present invention.

[0020] FIG. 6 illustrates a schematic side view of documents with and without a document wrapper placed on a flatbed scanning system in accordance with the present invention.

[0021] FIG. 7 illustrates a schematic flowing diagram of the focus controlling method in accordance with the present invention.

[0022] The present invention discloses a focus controlling method and system for an image capturing system. The length of the optical path between the scanned object and the lens of the image capturing device can be adjusted according to the change in the position of the object being scanned. With the adaptive control on the focus point of a image capturing device, the magnification ratio and the resolution of the system can be controlled through a closed loop control method and system in the present invention.

[0023] Referring to FIG. 3, a schematic side view of the focus controlling system in the present invention is illustrated with related functional blocks. The focus controlling system for an image capturing system can include a calibration paper 20, an image capturing device 22, an input interface 24, and a processing and controlling device 26. Without limiting the scope and application of the present invention, a flatbed scanning system is employed solely as an illustrative example of the present invention. Upon acknowledging the spirit of the present invention, skill artisans can apply the focus controlling method and system in other kinds of image capturing system without further indication of embodiments.

[0024] In a flatbed scanning system, a transparent plate or glass 28 is used for placement with the object being scanned, like documents, photographs, or even objects. In this case, the calibration paper 20 can be attached to a surface of the document plate 28 of the scanning system. The calibration paper 20 can be embedded into a housing 30 located on the peripheral area of the document plate 28. Alternatively, the calibration paper 20 can also be placed directly onto the glass 28 when it is needed. For achieving the focus point correction purpose of the present invention, the calibration paper 20 has at least one set of width identification marks. An example of the width identification marks is illustrated in FIG. 4 as marks 20aand 20b. An example, the marks 20aand 20bcan be black or dark blocks on a calibration paper 20 with a white background.

[0025] Turning to FIG. 3, the image capturing device 22 is utilized in the scanning system for capturing an image as well as scanning the calibration paper 20. For controlling the focus point, the image capturing device 22 has a focus point adjusting mechanism. In the preferred embodiments, the image capturing device 22 can have one or more movable reflecting mirrors. The focus point can then be varied through the movements of the reflecting mirror or mirrors. In the case, the image capturing system 22 includes two movable reflecting mirrors 22aand 22b. The reflecting mirrors 22aand 22bcan be attached to a mirror box 22c. The box 22ccan be moved in relative to the image capturing portion 22dof the image capturing device 22, by the driving of a moving mechanism 22e. In the case, a driving motor like a stepping motor can be included in the moving mechanism 22eto move the box 22c. Therefore, the object distance, namely the length of the optical path between the object being scanned and the lens of the image capturing device 22, can be varied to adjust the focus point and the magnification ratio of the system.

[0026] Various kinds of moving mechanisms can be employed to drive the mirror box 22cto move in relative to the image capturing portion 22d. Referring to FIG. 5a, a pulley-screw type moving mechanism can be used. The mirror box 22ccan be moved in relative to the image capturing portion 22dthrough the rotation of two parallel screw rods 22fand 22g. Flexible devices like springs 22scan be inserted between the mirror box 22cand the image capturing portion 22d. The screw rods 22fand 22gcan be driven by the motor 22mby a belt and pulley mechanism.

[0027] Another example of moving mechanism is illustrated in FIG. 5bas a slide-screw type. A screw rod 22hlocated aside the mirror box 22ccan be driven directly by the motor 22m. Two sliding blocks 22h1 and 22h2 can be moved by the rotating screw rod 22h. The position of the mirror box 22cin relative to the image capturing portion 22dcan be controlled by relating the position of the mirror box 22cto the profile variation on the sliding blocks 22h1 and 22h2, by contacting through bearing like wheels. In the same way, the flexible devices like springs 22scan be inserted between the mirror box 22cand the image capturing portion 22d.

[0028] In FIG. 5c, a wire-worm type moving mechanism is shown. As discussed above, the flexible devices like springs 22scan be inserted between the mirror box 22cand the image capturing portion 22d. The relative position between the mirror box 22cand the image capturing portion 22dcan be constrained and controlled by the length of a wire 22w. The length of the wire 22wis adjusted by a winding wheel 22j, which can be driven by the motor 22mthrough a worm rod 22k.

[0029] A fourth type of the moving mechanism is illustrated in FIG. 5d. In the screw-driving type moving mechanism, the mirror box 22ccan be moved in relative to the image capturing portion 22dthrough the rotation of the screw rod 22p, under the driving of the motor 22m. One or more guiding rails like the rails 22rshown in the figure can be employed to guide the motion of the mirror box 22c. Therefore, with the operation of the screw rod 22pand the motor, the focus point can be controlled. In addition, various kind of image sensing elements, like CCD (charge-coupled device), CIS (contact image sensor), or photo-sensitive MOS (metal oxide semiconductor) transistors, can be employed in the image capturing device 22. In the case, charge-coupled devices can be utilized in the image capturing device 22 for capturing the image being scanned.

[0030] Turning to FIG. 3, the input interface 24 is an interface device for receiving a location data of a scanning point. In general, the input interface 24 can be a processing system which is connected with the scanning system. As an example, a computer for controlling the operation of the scanner and processing the image signal can be the input interface 24. Under the variation in the raised object height, the variation of the location of the scanning point can be inputted from the input interface 24. In general, the value of the raised object height can be determined from the type or thickness of document wrappers used on the document being scanned. A built-in menu with several frequently-employed height can also be set within the interface 24. The user can directly select the listed items of heights without the effort in measuring and re-entering the numbers. Therefore, the focus controlling system can adjust the focus point or the scanning point according to the specified variation in object location. In the case, the scanning point can be set as a raised height of an object being scanned, in compared with the standard height of putting the object directly on the document plate 28. As a consequence, the raised height can be set as about the thickness of the document wrapper covering over the scanned document, by which the object height is increased.

[0031] In the focus controlling system, the processing and controlling device 26 is responsive to the input interface 24 and the image capturing system 22 to control the adjustment in the focus point. In this case, the processing and controlling device 26 can include a processor 26aand a motor controller 26b. The processor 26acan have accompanying memories for processing and calculating data. Besides, the processor 26acan be implemented with the processing resource of the computer connecting to the scanning system. Therefore, the hardware design and cost of the scanning system can be reduced.

[0032] Referring to FIG. 4, a schematic diagram of the optical path in an image capturing device of the present invention is illustrated. For adjusting the focus point, the calibration paper 20 is scanned. The scanned image is transferred to an image 32 through a lens set 34. An object distance p is the distance between the object being scanned, or the calibration paper 20 in the case, and the lens 34. A image distance is the distance between the lens 34 and the projected image 32. For the scanning system as shown in the figure, the magnification ratio m of the system is

[0033] m=p/q

[0034] If a document 36 is placed directly on the document plate 28 as shown in FIG. 6, the magnification ratio m can be kept with the prescribed value. The image can be acquired correctly under the designed depth of focus and resolution range. However, if a document 36ais placed onto the document plate 28 with a document wrapper 38 in-between, the object distance is increased with the raised height x, namely the thickness of the document wrapper 38. In the prior art scanning system, the focus point is fixed and the increased object distance of p+x would result in a raised magnification ratio. The raised magnification ratio m′=(p+x)/q. In addition, the document 36amight located out of the focus point and the depth of focus. The image quality is then damaged under a distorted magnification ratio and a reduced resolution.

[0035] In order to compensate for the increased object height x, the object distance p of the lens 34 to the calibration paper 20 should be reduced with the same amount. If the object distance of the lens 34 to the calibration paper 20 is reduced to p−x, a compensated magnification ratio m1 of the calibration paper 20 can be calculated as:

[0036] the compensated magnification ratio m1=(p−x)/q=(a standard object distance−a raised height of an object being scanned)/a standard image distance.

[0037] Therefore, the focus point of the scanning system is adjusted to get a modified magnification ratio m1′ of about the value of the compensated magnification ratio m1. Therefore, the variation in the object height can be compensated and the correct magnification ratio can be acquired. By the operation of the processing and controlling device 26, the focus point is adjusted to maintain an nearly constant magnification ratio of the scanned image.

[0038] Referring to FIG. 7, a schematic flowing diagram of the focus controlling method in the present invention is shown. At first, a step 40 of detecting or setting a scanning point is performed. A location of the scanning point can be set as a raised height x of an object being scanned. Alternatively, the scanning point can also be acquired through detecting the height of the document surface by the system. The raised height can be set through the input interface 24 in FIG. 3. In the step 42 of FIG. 7, a compensated magnification ratio for the set location is calculated. As what described above, the compensated magnification ratio for the set location can be calculated as:

[0039] the compensated magnification ratio m1=(p−x)/q=(a standard object distance—a raised height of an object being scanned)/a standard image distance.

[0040] With the system disclosed in the present invention, the compensated magnification ratio m1 can be calculated with the processor 26ain FIG. 3. Next, the focus point of the image capturing device is adjusted in the step 44. As an example, the focus point can be adjusted using one or more movable reflecting mirrors, which can be driven by the moving mechanism 22edescribed above. After the focus point is adjusted, the calibration paper is scanned in the step 46. The calibration paper 20 is designed to have width identification marks of a predetermined width. Therefore, the magnification ratio can be calculated by the ratio of the marked width to the actual image width on the image sensor. In the case, the magnification ratio can be calculated by the processor 26a.

[0041] Finally, the adjusting step 44 and the scanning step 46 are repeated until the image capturing device has a magnification ratio m1′ of about the compensated magnification ratio m1. The repetition can be performed with the judgement criteria on the magnification ratio in the step 48. If the difference between the magnification ratio m1′ and the compensated magnification ratio m1 is smaller than a tolerable criteria E, the process of adjusting the focus is finished and stopped. If the difference between the magnification ratio m1′ and the compensated magnification ratio m1 is not small enough, the adjusting step 44 and the scanning step 46 are repeated until the magnification ratio m1′ is approximately equal to the compensated magnification ratio m1.

[0042] Therefore, after the focus point is adjusted, the raised object height or increased object distance can be compensated with the shifted focus. The actual magnification ratio of scanning the object with changed position can be justified to the designed value. The focus point is refocused onto the actual position of the object being scanned. The document can then be scanned with appropriated depth of focus and nearly optimized resolution. The quality of the scanned image can be raised greatly with the closed loop control method and system of the present invention.

[0043] As is understood by a person skilled in the art, the foregoing description of the preferred embodiment of the present invention is illustrative of the present invention rather than a limitation thereon. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims. The scope of the claims should be accorded with the broadest interpretation so as to encompass all such modifications on the similar structure. While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.