Abstract:
An image scanner comprising an image pick-up device and a planar light source. Backlight is generated by the planar light source and transmitted through a object. The image of the object is focused on the image pick-up device. In scanning operations, the planar light source and the image pick-up device are synchronously moved to scan the object.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to an image scanner of low manufacturing cost and high scanning quality. 
     2. Description of the Related Art 
     Generally speaking, flat bed type image scanners are classified into two different kinds in accordance with their different operation ways: reflection-type scanners and transmission-type scanners. The reflection-type scanners are used for scanning non-transparent media such as paper, objects and books. On the other hand, the transmission-type scanners are used for scanning transparent media such as films and slides. 
     The present invention is related to the transmission-type scanners. Conventional transmission-type scanners include two kinds of light sources: movable and fixed. FIG. 1 is a schematic diagram of a conventional transmission-type scanner provided with a movable light source, wherein a tubular lamp  11  and an optical module  14  are simultaneously moved to scan a transparent object  12  disposed on a piece of glass  13 . The image of the transparent object  12  is focused on a charge-coupled device (CCD)  15  contained in the optical module  14 . Such a scanner can be put into practice in various ways. For example, what is disclosed in Taiwanese Patent No. 311791. 
     In the above example, the distribution of intensity of illumination of the tubular lamp  11  is very concentrated, as shown in FIG.  2 . If at any moment the movement of the optical module  14  and that of the tubular lamp  11  are not simultaneous, then the light received by the charge-coupled device  15  will be greatly reduced. Thus, the resultant image signal obtained from the charge-coupled device  15  is poor. Therefore, the movement of the optical module  14  and that of the tubular lamp  11  should be always kept simultaneous during the scanning process. However, the tubular lamp  11  and the optical module  14  generally are driven by different mechanisms so that keeping the tubular lamp  11  and the optical module  14  simultaneous is pretty complicated and difficult. 
     FIG. 3 depicts a conventional scanner provided with a fixed light source, where the light source is a planar light source  31  disclosed in, for example, Taiwanese Patent No. 292032. The planar light source  31  includes two tubular lamps  311 , a reflection sheet  315 , a lighting guide  317  and a diffuser  316 . The reflection sheet  315  is provided on the top of the planar light source  31  to reflect the light of the tubular lamps  311 . The lighting guide  317  is used to uniformly distribute the light reflected by the reflection sheet  315  via a plurality of light-guiding dots  318  provided thereon. The diffuser  316  is also utilized to uniformly distribute the light reflected by the reflection sheet  315 . Both the lighting guide  317  and the diffuser  316  are of great size. In operations, the planar light source  31  is stationary while an optical module  34  is moved to scan a transparent object  32  disposed on a piece of glass  33 . The image of the object  32  is focused on a charge-coupled device (CCD)  35  contained in the optical module  34 . 
     Due to the use of the reflection sheet  315 , diffuser  316  and lighting guide  317 , the distribution of intensity in this example is much more uniform than that of the previous example. Nevertheless, the intensity difference between the middle and sides of the planar light source is 5%-10% that does not satisfy the requirement for a scanner of high revolution. In addition, forming light-guiding dots  318  on the lighting guide  317  is labor consuming. The greater size of the lighting guide  317  is, the more light-guiding dots  318  are required; thus, the manufacture of lighting guides is difficult and expensive. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a scanner that solves the above-mentioned problems. 
     In accordance with the object of the present invention, an image scanner comprising an image pick-up device and a planar light source is provided. Backlight is generated by the planar light source to transmit through a transparent object. The image pick-up device is used to retrieve the image of the transparent object, i.e., the image of the transparent object is focused on the image pick-up device. In scanning operations, the planar light source and the image pick-up device are simultaneously moved to scan the transparent object. 
     As described above, the planar light source of the present invention is moved to project backlight onto the simultaneously moving image pick-up device. Accordingly, the size of the planar light source can be small. A lighting guide in the planar light source is also of small size and has fewer light-guiding dots provided thereon; thus, manufacture of the lighting guide of the present invention is easier than that of the prior art. In addition, the distribution of light intensity in the present invention employing the small lighting guide is more uniform than that in the prior art employing the large lighting guide. The difference between the illumination on the middle of the transparent object and the illumination on the sides of the transparent object is less than 1% in the present invention, that is superior to 5%-10% in the prior art. Therefore, the scanning quality of the present invention is superior to that of the prior art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
     FIG. 1 depicts a conventional image scanner provided with a movable light source; 
     FIG. 2 shows the distribution of intensity of illumination of a tubular lamp; 
     FIG. 3 depicts a conventional image scanner provided with a fixed light source; and 
     FIG. 4 depicts an image scanner provided with a movable planar light source in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 4 depicts an image scanner provided with a movable planar light source  41  in accordance with the present invention, wherein the planar light source  41  includes a tubular lamp  411 , a reflection sheet  415 , a lighting guide  417  and a diffuser  416 . The dimension of the planar light source  41  along the longitudinal axis of the image scanner is D (in FIG. 4, the direction of the longitudinal axis is indicated by arrows A 1 , A 2 ). The reflection sheet  415  is utilized to reflect light of the tubular lamp  411 . The lighting guide  417  is utilized to uniformly distribute the light from the reflection sheet  415  via a plurality of lighting-guide dots  418  provided thereon. The diffuser  416  is also used to uniformly distribute the light. In the present invention, the lighting guide  417  and the diffuser  416  are of small dimension D. During the scanning operations, both the planar light source  41  and an image pick-up device  44  should simultaneously move in same speed toward same direction to scan the transparent object  42  disposed on the glass  43 . The image of the object  42  is focused on a charge-coupled device (CCD)  45  disposed in the image pick-up device  44 . 
     During the scanning operation toward the transparent object, the image pick-up device  44  and the planar light source  41  should simultaneously move in the same direction with same speed. The planar light source  41  projects light transmitting through the object  42  and finally received by the moving image pick-up device  44 . Accordingly, the size of the planar light source  41  is unnecessary to cover the whole object  42  and the planar light source  41  can be in the small dimension D. However, due to the moving speed mismatch between the image pick-up device  44  and the planar light source  41 ; there is a maximum relative position difference between the image pick-up device  44  and the planar light source  41  during the scanning operation. In order to keep the light always uniformly distributed toward the transparent object, we must make the dimension D larger then the relative position difference. To sum up, the minimum dimension D must be larger than the possible maximum relative position difference between the image pick-up device  44  and the planar light source  41 . The lighting guide  417  in the planar light source  41  is also of small dimension D and has some light-guiding dots  418  provided thereon; thus, the manufacturing of the lighting guide of the present invention is easier than that of the prior art. In addition, the distribution of light intensity in the present invention employing the small lighting guide is more uniform than that in the prior art employing the large lighting guide. The difference between the illumination on the middle of the object  42  and the illumination on the sides of the object  42  is less than 1% in the present invention, that is superior to the 5%-10% in the prior art. Therefore, the scanning quality of the present invention is superior to that of the prior art. 
     Furthermore, the planar light source  41  of the present invention needs only one tubular lamp  411  as the light source due to the use of the small lighting guide  417 . That is more economical than the use of two tubular lamps in the prior art. 
     The above embodiment introduces how the arrangement of the present invention is applied to a transmission-type scanner. However, it should be understood that the present invention is also applicable to the image scanner disclosed in U.S. Pat. No. 5,467,172, which can be selectively used in a transmission-type operation or a reflection-type operation. That is, the arrangement of the present invention is applicable to the scanner disclosed in U.S. Pat. No. 5,467,172 to scan transparent media during the transmission-type operation. 
     While the invention has been described by way of example and in terms of the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.