Abstract:
A method for increasing signal to noise ratio is disclosed. The method can automatically detect saturation output voltage of the photosensors via adjusting exposure time or illumination intensity so as to obtain optimum output voltage of the photosensors as well as high signal to noise ratio that can generate high quality images.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for increasing signal to noise ratio, and more particularly to a method for increasing signal to noise ratio that can automatically detect saturation output voltage of photosensors via adjusting exposure time or illumination intensity so as to obtain optimum output voltage of the photosensors. 
     2. Description of the Related Art 
     Optical scanners are used to capture and digitize images. For example, an optical scanner can be used to capture the image of printed matter on a sheet of paper. The digitized image can then be electronically stored and/or processed with character recognition software to produce ASCII text. The typical optical scanner includes a light source, a linear array of photoelectric sensing elements (generally a CCD sensor or a CMOS sensor, or a CIS sensor), an analog amplifier, an analog to digital converter (ADC), a controller and a random access memory (RAM). 
     The CCD sensor includes a large number (e.g., 2000) of photoelectric sensing elements arranged in a linear array. Each photoelectric sensing element will capture light representing a single pixel of the image. The array will capture a line of pixels. By moving the CCD sensor across a document, the entire document can be scanned one line at a time. 
     The conversion into digital signals of light reflected from or transmitted through the document takes place in essentially three steps. First, each photoelectric sensing element will convert the light which it receives into an electric charge. The magnitude of the charge will depend on the intensity of the light and the exposure time. Second, the charges from each of the photoelectric sensing elements are converted into analog voltages via the analog amplifier. Finally, the analog voltages are digitized by the analog to digital converter for digital image processing and storage in the RAM. 
     In conventional optical scanners, the CCD sensor is slowly scanned across a document. The photoelectric sensing elements are continuously exposed. After a fixed exposure time, a line of charges (representing a line of pixels of the image) are dumped from the photoelectric sensing elements to one or more analog shift registers. Once the charges are dumped, the photoelectric sensing elements resume generating charges in response to the light to which they are exposed. However, before the next line of charges can be dumped, the analog shift registers must be cleared and the charges stored therein must be processed. 
     The processing time for the data in the CCD sensor includes the time required to serially shift a line of charges from the analog shift registers, to convert the charges to analog voltages, to digitize the voltages, to perform any desired image processing and to store the digital representation of each pixel in the RAM. Once all pixels or charges of a line have been processed, the charges of the next line can be dumped from the photoelectric sensing elements. Thus, the time required to process all pixels or charges of a line would be equal to the exposure time of the CCD sensor. Conventional optical scanners fix the exposure time equal to this processing time. For example, if it takes one microsecond to process each charge or pixel, then a 2000 pixel line would require two milliseconds for full processing. Thus, the exposure time for the CCD sensor would be equal to a fixed two milliseconds. 
     The CCD sensor will generally have a fixed noise level. Thus, to optimize the signal to noise ratio of the scanner, it is desirable to maximize the optical signal received at the CCD sensing element. By maximizing the optical signal, with a fixed noise level, the signal to noise ratio can be maximized. However, image quality provided by conventional optical scanners is always limited or hardly upgraded since their expose time and illumination intensity are fixed and a minimum saturation voltage of photoelectric sensing elements, instead of an actual saturation voltage, is set as standard system value. It is desirable to provide a method for increasing the signal to noise ratio of the sensor to overcome the limitations of the prior art. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a method for increasing signal to noise ratio and image quality. 
     It is another object of this invention to completely utilize the saturation output voltage of the photosensors in an image scanner. 
     It is a further object of this invention to provide a method for automatically detecting saturation output voltage of the photosensors via adjusting exposure time or illumination intensity so as to obtain optimum output voltage of the photosensors. 
     To achieve these objects, and in accordance with the purpose of the invention, the invention provide a method for increasing signal to noise ratio. The out voltage of photosensors such as charge coupled devices (CCD) or complemental metal oxide semiconductor sensors is usually derated or lower than the input voltage of the analog to digital converter (ADC). The method for increasing signal to noise ratio of the invention is used to find out the actual saturation voltage of the photosensors and then increases the output voltage of the photosensors by increasing exposure time tint or lamp illumination to near the actual saturation voltage of the photosensors so that the signal to noise ratio can be increased and image quality can be upgraded. The method comprises the following steps. First of all exposure time T n  or illumination L Xn  is set. Next a white target chart for said exposure time T n  (illumination L Xn ) is scanned. Then output digital data D n  via an N bit analog to digital converter are generated. Next output digital data D n  is compared with 2 N −1. Then exposure time T n  is increased to exposure time T n+1  if output digital data is smaller than 2 N −1. Next white target chart is scanned for exposure time T n+1 . Then output digital data D n+1  are generated via N bit analog to digital converter. Next output digital data D n+1  are compared with said output digital data D n . Exposure time T n  is saved when output digital data D n+1  is not larger than output digital data D n  Exposure time T n+1  is further increased when output digital data D n+1  is larger than output digital data D n . However, if output digital data is not smaller than 2 N −1, then exposure time T n  decreased to exposure time T n+1 . White target chart is scanned for said exposure time T n+1 . Output digital data D n+1  are generated via said N bit analog to digital converter. Then output digital data D n+1  are compared with output digital data D n  Exposure time T n+1  is saved when output digital data D n+1  is not larger than output digital data D n . Exposure time T n+1  is further decreased when output digital data D n+1  is larger than output digital data D n . 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  shows a flow chart of the method for increasing signal noise ratio; 
         FIG. 2  shows a flow chart of how the method of this invention is utilized in a scanning procedure; and 
         FIG. 3  shows a diagram of saturation voltage V sat  versus input illumination energy E. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     It is to be understood and appreciated that the method for increasing signal noise ratio described below do not cover a complete system and method. The present invention can be practiced in conjunction with various software and hardware that are used in the art, and only so much of the commonly practiced components and steps are included herein as are necessary to provide an understanding of the present invention. 
     The present invention will be described in detail with reference to the accompanying drawings. It should be noted that the drawings are in greatly simplified form. 
     Referring to  FIG. 1 , a flow chart of the method for increasing signal to noise ratio in accordance with one embodiment of the invention is shown. Under common circumstance, the out voltage of photosensors such as charge coupled devices (CCD) or complemental metal oxide semiconductor sensors is lower than the input voltage of the analog to digital converter (ADC). The method for increasing signal to noise ratio of the invention is used to find out the actual saturation voltage of the photosensors and then increases the output voltage of the photosensors by increasing exposure time tint or lamp illumination to near the actual saturation voltage of the photosensors so that the signal to noise ratio can be increased and image quality can be upgraded. First of all, after step  102 , original exposure time tint T 0  or original lamp illumination L X0  is set in step  104 . Since illumination energy E equals the product of exposure time tint T and lamp illumination L, increasing either exposure time tint T or illumination L will increase illumination energy E. Next in step  106 , a white target chart is scanned and the photosensors convert the light which it receives into an electric charge. The magnitude of the charge depend on illumination L X0  (the intensity of the light) and exposure time tint T 0  set forth. Then the charges from the photosensors are converted into analog voltages via the analog amplifier. Finally, the analog voltages are digitized to form a maximum data D 0  by the analog to digital converter for digital image processing. If a 8 bit analog to digital converter is utilized, data D 0  is from 0 to 255. In step  108 , maximum data D 0  is compared to 255 if a 8 bit analog to digital converter is utilized. If maximum data D 0  is smaller than 255, then exposure time tint T n  or lamp illumination L Xn  should be increased and the white target chart is scanned again in step  110 . The magnitude of the charge now depends on illumination L Xn+1  and exposure time tint T n+1 . Then the charges from the photosensors are converted into analog voltages via the analog amplifier. Finally, the analog voltages are digitized to form a maximum data D n+1  by the analog to digital converter for digital image processing. The maximum data D n+1  is then compared to the previous maximum data D n  (starting from D 0 ) in step  112 . If the maximum data D n+1  is not larger than the previous maximum data D n , that means that the output voltage of the photosensors has been saturated and exposure time tint T n  or illumination L Xn  generating the maximum data D n  is saved in step  114 . On the contrary, if the maximum data D n+1  is larger than the previous maximum data D n , that means that the output voltage of the photosensors has not been saturated and the process goes back step  110  until the output voltage of the photosensors is saturated. 
     If maximum data D 0  is not smaller than 255, then exposure time tint T or lamp illumination L Xn  should be decreased and the white target chart is scanned again in step  116 . The magnitude of the charge depend on illumination L Xn+1  and exposure time tint T n+1 . Then the charges from the photosensors are converted into analog voltages via the analog amplifier. Finally, the analog voltages are digitized to form a maximum data D n+1  by the analog to digital converter for digital image processing. The maximum data D n+1  is then compared to the previous maximum data D n  (starting from D 0 ) in step  118 . If the maximum data D n+1  is smaller than the previous maximum data D n , that means that the output voltage of the photosensors has been saturated and exposure time tint T n+1  or illumination L Xn+1  generating the maximum data D n+1  is saved in step  120 . On the contrary, if the maximum data D n+1  is not smaller than the previous maximum data D n , that means that the output voltage of the photosensors has not been saturated and the process goes back step  116  until the output voltage of the photosensors is saturated. 
     Referring to  FIG. 2 , a flow chart of how the method of this invention is utilized in a scanning procedure is shown. In step  202 , an user interface operated in personal computer is launched. Then default scan parameters are set up in step  204 . Next optimum exposure time or optimum illumination is found in step  206  or  208  via the method described above and shown in  FIG. 1 . New scan parameters obtained in step  206  or  208  are then set up before starting scan in step  212 . 
       FIG. 3  shows a diagram of output voltage V 0  of photosensors versus input illumination energy E(L x ×T). As shown in the figure, output voltage V 0  increases linearly with input illumination energy E until saturation exposure SE is reached, wherein ADS means average dark signal. Output voltage V 0  equals saturation voltage V sat  at saturation exposure SE. 
     Other embodiments of the invention will appear to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples to be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.