Abstract:
A light exposure control device and method that allows three excitation signals to be generated by a counter and sent to a CCD module after the CCD module has been exposed to an image. Inside the CCD module, a red photodetector, a green photodetector and a blue photodetector can be separately triggered to capture the necessary image signals. The excitation signals sent to each CCD photodetector are independent from each other. Furthermore, each CCD photodetector reacts only to a specific excitation signal. Therefore, constraints caused by unrelated excitation signals are avoided, and so exposure time can increase considerably.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application Ser. No. 87103177, filed Mar. 5, 1998, the full disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a light exposure device and its operation. More particularly, the present invention relates to a light exposure control device suitable for scanners that provides independent excitation signals for triggering each photodetector such as, for example, a charge coupled device (CCD). 
     2. Description of Related Art 
     A charge coupled device (CCD) is a kind of semiconductor device. When a CCD is illuminated by a light source, the intensity of the photons is transformed into a quantity of accumulated electric charges. In general, the stronger the intensity of light beam shining on a CCD, the greater the amount of electric charges generated by the CCD will be. Therefore, the amount of electric charges stored inside a CCD will vary according to the intensity of external light. Utilizing the light intensity/electric charge relationship, a vast number of these CCDs can be arranged systematically into an array forming a CCD module. When photons coming from a light source strike a picture, a corresponding light image is formed. If a photosensitivity CCD module is positioned to receive light signals from the image, image data can be captured. After some transformations of the image data, the transformed image data can be used in a number of applications. For example, a scanner can rely on photosensitive CCD modules to extract a color image from a picture. The method of image extraction includes utilizing three CCD photodetector strips, which are sensitive to red, green and blue light respectively, in the CCD module. During light scanning operation, the red, green and blue lights coming from the image are captured by the corresponding photodetectors in the CCD module. The three primary colors of an image captured by the CCD module are then output to a converter for transforming the image into digital data. Next, the digitized data of the three primary colors are re-grouped, re-creating the original image received from the CCD module. The output of image signals from the CCD module is controlled by an excitation signal. Whenever the CCD module receives an excitation signal, one of the three primary colors from the image is captured. 
     FIG. 1A illustrates the method of excitation for a conventional CCD module. As shown in FIG. 1A, the CCD module  10  has three CCD photodetector strips. In the fabrication of CCD photodetectors, three different photodetectors each having sensitivity for a particular part of the color spectrum can be separately manufactured so that the three primary colors can be registered. Subsequently, when the three primary colors of the image captured by the photodetectors are recombined, the originally exposed color can be reproduced. The extraction of three primary colors red, green and blue are carried out by a red photodetector  100 , a green photodetector  110  and a blue photodetector  120  respectively. In operation, when the picture  4  is illuminated by light source  2 , exposure signals  6  will be generated and in turn are sent to the CCD module. Since the exposure signals  6  already contain a mixture of the three primary colors, an excitation signal  15  can be simultaneously generated and sent to the red  100 , green  110  and blue  120  photodetectors for capturing the corresponding red, green and blue parts of the exposure signals  6 . Hence, in a conventional design, signaling lines  105 ,  115  and  125  can be connected together so that excitation signal  15  can be simultaneously applied to the respective red  100 , green  110  and blue  120  photodetectors. 
     FIG. 1B is a timing diagram showing the relationship between the excitation signal  6  and the CCD photodetector  15  of FIG.  1 A. Since the red  100 , green  110  and blue  120  photodetectors are connected together, they will be simultaneously triggered by the same excitation signal  15 . For example, as shown in FIG. 1B, when pulses  132 ,  134 ,  136 ,  138 ,  140 ,  142  and  144  are produced, three photodetectors including red, green and blue will all be triggered. Because each excitation signal has a cycle time T 1 , the extraction of a particular color from the image cannot be conducted at a time interval greater than T 1 . In other words, the light exposure, trigger and data extraction cycle for each CCD photodetector must be finished before the end of a cycle T 1 . 
     As an illustration, assume that before the pulse  132  is generated, all three photodetectors are in the light-gathering state. Furthermore, assume that the time from the generation of pulse  132  to the beginning of the next pulse  134  represents a full cycle T 1 . Within the cycle time T 1 , all three photodetectors have already completed their respective light gathering operations. Therefore, when pulse  132  arrives, all three CCD photodetectors  100 ,  110  and  120  are simultaneously triggered. Thereafter, if red light needs to be extracted from the image, the red light from the image can be captured by the red photodetector  100 . Moreover, the next round of light exposure is carried out within the cycle time T 1  marked by pulse  132  and pulse  134 . Within the cycle time T 1  between pulse  132  and the next pulse  134 , again all three photodetectors have completed a photodetection operation. Hence, when the next pulse  134  arrives, all three CCD photodetectors  100 ,  110  and  120  are simultaneously triggered. Thereafter, if green light needs to be extracted from the image, the green light from the image can be captured by the green photodetector  110 . Moreover, the next round of light exposure is carried out within the cycle time T 1  marked by pulse  134  and pulse  136 . Using similar operational steps, when the next pulse  136  arrives, the blue light from the image can be captured by the blue photodetector  110 . By repeating the above steps, three primary colors of an image can be continuously captured and converted to primary color data. Subsequently, when the stream of three primary color data are properly recombined back together, the original color picture is reproduced. 
     In the conventional method, the same excitation signal is used to trigger red, green and blue CCD photodetectors. Therefore, the period from light exposure to data extraction for each photodetector must not exceed one excitation cycle. From an alternate viewpoint, since all three CCD photodetectors are triggered by the same excitation signal, each one of the photodetectors is constrained to work together with the other photodetectors. In other words, light exposure is limited to a period within one excitation signal cycle. To achieve the image capture within constraints, exposure time of the CCD photodetectors must match the excitation signal. In a world where the quality and speed of scanners are both critical for market success, production cost of a CCD module is bound to increase and thus may lower its power to compete in the market. 
     Furthermore, in order to complete the exposure of CCD photodetectors within a short interval, the necessary light intensity of the light source must be increased. A light source having a high intensity is not only expensive, but also consumes more power and generates a higher surrounding temperature due to heating. Consequently, the working life of electronic devices will be shortened. 
     In addition, in order to shorten the exposure time and to increase scanning signal precision so that the quality and speed of operation of a scanner can be maintained, optical elements, CCD module and other related devices must be designed to have a high signal-to-noise (S/N) ratio. Consequently, the production cost of the scanner is raised. 
     In summary, a conventional light exposure control device has the following defects: 
     1. A high-intensity light source is required, leading to large power consumption and high operating temperature, thereby shortening the working life of electronic devices. 
     2. The signal-to-noise (S/N) ratio of optical elements and CCD modules has to be increased, thereby increasing production cost. 
     3. To capture a clear image from the CCD within a short interval requires highly sensitive photodetectors, thereby leading to an increase in production cost and a decrease in its ability to compete in the market. 
     In light of the foregoing, there is a need to improve light exposure control device. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a light exposure control device so that the length of exposure for each CCD photodetector can be extended without affecting scanning speed. In addition, the signal-to-noise (S/N) ratio can be increased without increasing the production cost of a CCD module. Therefore, competitiveness in the market will soar. 
     In a second aspect, the present invention provides a light exposure control device that permits the use of a lower-intensity light source so that power consumption and operating temperature can be reduced. Hence, working life of electronic components can be extended. 
     In a third aspect, the present invention provides a light exposure control device capable of using devices having a lower signal-to-noise (S/N) ratio so that production cost is lower and commercial value is higher. 
     To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a light exposure control device. The light exposure control device is capable of delivering three independent excitation signals from a counter so that a red photodetector, a green photodetector and a blue photodetector can be triggered separately. Since each excitation signal delivered to each CCD photodetector is independent from each other, and each CCD photodetector furthermore reacts to a specific excitation signal, the CCD photodetector is able to avert the effect of other unrelated excitation signals. Therefore, the length of exposure for each CCD photodetector is greatly increased. In addition, the excitation signals can be generated by an oscillator or a frequency generator. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings, 
     FIG. 1A illustrates the method of excitation for a conventional CCD module; 
     FIG. 1B is a timing diagram showing the relationship between the excitation signal and the CCD photodetector of FIG. 1A; 
     FIG. 2A shows a light exposure control device according to one preferred embodiment of this invention; and 
     FIG. 2B is a timing diagram showing the relationship between the excitation signal and the three primary color excitation signals for the device as shown in FIG.  2 A. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
     FIG. 2A shows a light exposure control device according to one preferred embodiment of this invention. As shown in FIG. 2A, when a picture  4  is illuminated by a light source  2 , exposure signals  6  are generated. Unlike conventional methods, this invention utilizes a counter  200  to separately control the triggering of each CCD photodetector. In operation, the counter  200  receives excitation signal  15  and then uses signal lines  205 ,  215  and  225  to respectively trigger a red photodetector  100 , a green photodetector  110  and a blue photodetector  120 . Therefore, the red part of the exposure signals  6  can be independently captured and output from the red photodetector  100 , the green part of the exposure signals  6  can be independently captured and output from the green photodetector  110 , and the blue part of the exposure signals  6  can be independently captured and output from the blue photodetector  120 . The excitation signals mentioned above can be generated by an oscillator or a frequency generator. 
     For example, when only red image signals are required, an excitation signal is delivered to the red photodetector  100  so that the red image signals can be captured. Because green image signals and blue image signals are unwanted, there is no need to trigger the green or the blue photodetector. Similarly, the method by which the CCD photodetectors are triggered to extract green or blue image signals is the same. The main advantages of this excitation method is that whether a CCD photodetector is triggered or not depends on the need for any one of the three primary color signals, and each can be independently triggered. With independent triggering, each CCD photodetector can work separately thereby saving unnecessary excitation and light exposure activities in other CCD photodetectors. 
     FIG. 2B is a timing diagram showing the relationship between the excitation signal  15  and the three primary color excitation signals for the device as shown in FIG.  2 A. As shown in FIG. 2B, the excitation signal profile is the same as in FIG.  1 B. However, in this invention, an additional counter  200  capable of receiving an excitation signal  15  and generating three different excitation signals is installed. Through the actions of three excitation signals, three CCD photodetectors can be triggered independently. The relationship between the excitation signals are: a first excitation signal passes through a first signal line  205  to trigger the red photodetector  100 ; the second excitation signal passes through a second signal line  215  to trigger the green photodetector  110 ; and a third excitation signal passes through a third signal line  225  to trigger the blue photodetector  120 . As shown in FIG. 2B, through the action of counter  200 , only one of the three excitation signals is triggered within one cycle time T 1  of excitation signal  15 . In other words, only one of the three excitation signals that includes the first excitation signal, the second excitation signal or the third excitation signal is selected for triggering within one cycle time T 1 . The method of triggering using a counter  200  is one major aspect of this invention. 
     As an illustration, assume that before the pulse  132  is generated, all three photodetectors are in the light-gathering state. When red image signals need to be extracted, the first excitation signal is delivered to the red photodetector  100 . As soon as pulse  232  arrives, the red photodetector  100  is triggered, permitting the output of stored red image signals to be registered before the arrival of pulse  232 . Within the cycle from pulse  232  to  238 , there will be sufficient time for the red photodetector  100  to charge up again through light exposure and get ready for the next round of red image signals extraction. As soon as the next pulse  238  arrives, stored red image signals from a previous cycle can be extracted, and then the next round of light exposure is initiated. Within the cycle from pulse  238  to  244 , there will be sufficient time for the red photodetector  100  to charge up again through light exposure and get ready for the next round of red image signals extraction. As soon as the next pulse  244  arrives, stored red image signals from a previous cycle can be extracted, and then the next round of light exposure is initiated again. 
     Similarly, when green image signals need to be extracted, the second excitation signal is delivered to the green photodetector  110 . As soon as pulse  234  arrives, the green photodetector  110  is triggered permitting the output of stored green image signals to be registered before the arrival of pulse  234 . Within the cycle from pulse  234  to  240 , there will be sufficient time for the green photodetector  110  to charge up again through light exposure and get ready for the next round of green image signals extraction. As soon as the next pulse  240  arrives, stored green image signals from a previous cycle can be extracted, and then the next round of light exposure is initiated. Similarly, when pulse  236  arrives, the blue photodetector  120  is triggered permitting the output of stored blue image signals registered before the arrival of pulse  236 . Within the cycle from pulse  236  to  242 , there will be sufficient time for the blue photodetector  120  to charge up again through light exposure and get ready for the next round of blue image signals extraction. As soon as the next pulse  242  arrives, stored blue image signals from a previous cycle can be extracted, and then the next round of light exposure is initiated. 
     The sequence of excitations shown in FIG. 2B can be regarded as a reference only. In practice, since all three CCD photodetectors can be independently triggered, image data for any one color can be extracted by triggering the corresponding CCD photodetector any time as required. Hence, the actual triggering sequence may not need to follow any order. 
     Since all three CCD photodetectors are separately triggered, whenever the same image signals are captured, this invention is capable of extending the exposure of a CCD photodetector up to 3 times the conventional method. Hence, the light exposure control device of this invention is able to provide sufficient exposure time for the CCD photodetector to gather light signals  6  for obtaining a clearer and more complete picture. 
     In summary, the advantages of this invention include: 
     1. Exposure time for the CCD photodetector is extended so that the extracted image data can be clearer and more complete. With a longer exposure time, the CCD photodetector can have a lower sensitivity, thereby lowering the cost of production for the CCD module. Hence, the product is more competitive. 
     2. With extended exposure time, light intensity of light source can be lower. Therefore, power can be saved and the operating temperature can be reduced. Thus, operational life of electronic devices is longer. 
     3. With longer exposure time, optical elements, CCD module and other related devices can have a smaller signal-to-noise (S/N) ratio. Hence, both the quality and speed of operation for a scanner can be maintained while the production cost can be lowered. Consequently, the product can better compete in the market. 
     Although a CCD photodetector is used to explain the operation of the light exposure control device in the above, other photosensitive devices having similar functions (for example, CIS photodetector) can be used in the above invention as well. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.