Abstract:
A portable dual-mode scanner device capable of being powered through a universal serial bus connection where the individual components of the portable scanner are selectively and controllably powered so as to function within the power limitations of the usb port (i.e., 5 Volts, 2.2 watts) without appreciable degradation of a captured image whether that image resides on a transparent media (i.e., a negative) or an opaque media (i.e., paper) is described.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation-in-part of co-pending US application Ser. No. 09/154,395, entitled “lightweight mobile scanners”, filed Sep. 16, 1998, commonly assigned, by Darwin Hu, et al, two of which are the inventors thereof. This application is also related to U.S. Pat. No. 6,054,707, entitled “Portable scanners capable of scanning both opaque and transparent materials”, commonly assigned, by Alpha Hou, one of the co-inventors hereof. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a scanning system and more particularly relates to a mobile scanner capable of scanning both transparent and opaque materials that itself does not have a separate processor and is not energized by a separate power supply, wherein the power required to support the operation of the mobile scanner is derived from a universal serial bus (USB) connection with a computing device. 
     2. Description of the Related Art 
     There are many applications that need optical scanners to convert paper-based objects, such as texts and graphics, to an electronic format that can be subsequently analyzed, distributed and archived. One of the most popular optical scanners is flatbed scanners that convert scanning objects, including pictures and papers, to images that can be used, for example, for building World Wide Web pages and optical character recognition. Another popular optical scanner is what is called sheet-fed scanners that are small and unobtrusive enough to sit between a keyboard and a computer monitor or integrated into a keyboard to provide a handy scanning means. Most optical scanners are referred to as image scanners as the output thereof is generally in digital image format. 
     With the increasing popularity of notebook computers, there is an emerging need for portable scanners suitable for use as notebook computer accessories. Battery powered handheld scanners have been used to fill this role. To be compact and energy efficient, many of these handheld scanners are manually operated, namely a user would have to hold a handheld scanner over a document to be scanned and move it from one end to another end so as to scan in the entire scanning document. However, the quality of images is not always satisfactory, typically suffering from jittered motions of the manual operations. 
     With the emergence of sheet-fed scanners, such as those Paper Port series from Visioneer Inc., the sheet-fed scanners gradually replace handheld scanners. Because a sheet-fed scanner operates automatically, namely a scanning document is well controlled to pass through an image sensor in the sheet-fed scanner; the resultant images are generally satisfactory. 
     The current sheet-fed scanners in the market are standalone devices whereas they generally operate with a computer. In addition to an image sensor, an illumination source and a motion mechanism, a typical sheet-fed scanner has a micro-processor, memory, post-processing circuitry and interface circuitry. Further to energize all the parts in the scanner, there is an external power adapter converting a high AC voltage to a low DC voltage. With all the parts together, a sheet-fed scanner is indeed a self-contained system having an interface to a computing system. 
     Generally there is a micro-processor in the sheet-fed scanners that controls and operates all the parts to work in synchronization. To reduce the cost of a scanner, the micro-processor in use is typically an 8-bit microprocessor, for example, 8088 from Intel, which is considered nowadays a primitive processor in light of available 32-bit or even 64-bit microprocessors. The overall system performance is thus severely limited by the performance of an 8-bit microprocessor. 
     There exists a need for a scanner that is compact, energy efficient and lightweight enough to be truly compatible with laptop computers. Additionally, there is a need for mobile scanners that are capable of scanning not only regular papers but also transparent materials (i.e., negative or positive films, transparencies for an overhead projectors or X-ray films). The demand for mobile scanners with the dual-scanning capability is ever increasing in both consumer and business markets. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is therefore an object of the present invention to provide a portable dual-mode scanner device capable of being powered through a universal serial bus connection where the individual components of the portable scanner are selectively and controllably powered so as to function within the power limitations of a universal serial bus connection port (i.e., 5 Volts, 2.2 watts) without appreciable degradation of a captured image whether that image resides on a transparent media (i.e., a negative) or an opaque media (i.e., paper). 
     According to one aspect of the present invention, the scanner itself comprises an image sensing module, a motion mechanism and an interface engine. As an important feature in the present invention, the scanner is not powered by an external power adapter rather it is powered through a universal serial connection to a host computer. The image sensing module, including a one-dimensional image sensor, an optical system, a first illumination source (for opaque scanning) and optionally a second illumination source (for transparent scanning), is responsible for imaging content containing media such as a document. The motion mechanism is responsible for moving the document through the image sensing module at a steady speed. Both of the image sensing module and the motion mechanism are coupled to the interface engine that comprises a control circuit. In operation, the circuit receives system control signals from the computing device through the universal serial connection. It then generates logical control signals for the image sensing module and the motion mechanism to operate in synchronization. Further the interface engine draws a power supply from the computing device through a universal serial bus to selectively and controllably energize the image sensing module and the motion mechanism to operate. 
     According to another aspect of the present invention, the disclosed scanner comprises two demountable cases. The main case houses the image sensing module and the motion mechanism and the base case houses the second illumination source. The first illumination source in the image sensing module provides front illumination to an opaque scanning object while the second illumination source provides back illumination to a transparent scanning object, as such the disclosed scanner is capable of scanning both opaque and transparent materials, a dual scanning feature that has been long sought in the scanning market. 
     Accordingly, one of the objects in the present invention is to provide a mobile scanner that is of high performance and low cost and so lightweight that it can be used in any conditions. 
     Other objects, together with the foregoing are attained in the exercise of the invention in the following description and resulting in the embodiment illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
     FIG. 1 shows a schematic configuration in which the present invention may be practiced; 
     FIG. 2 shows a systemic diagram of a prior art scanner device that converts a paper-based scanning object to a corresponding digital image; 
     FIG. 3 illustrates a block diagram of a mobile scanner according to one embodiment of the present invention; 
     FIG. 4A shows a cross-section view of a mobile scanner that may correspond to the one shown in FIG. 1; 
     FIGS.  4 B and FIG. 4C depict respectively a pictorial view of front illumination and back illumination with respect to an image sensor; 
     FIGS. 4D to  4 F show respectively three different views of the internal structures of a mobile scanner according to one embodiment; 
     FIG. 5 shows an internal block diagram of an interface engine implemented in a USB connection that may be used in the present invention to couple the scanner to a computing device; 
     FIG. 6 illustrates an internal functional diagram of an image sensing module in one embodiment of the present invention; and 
     FIGS. 7A to  7 C illustrate a flowchart of a process that is executed in a computing device to control the interface engine in FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, in which like numerals refer to like parts throughout the several views. FIG. 1 shows a schematic configuration in which the present invention may be practiced. Mobile scanner  100  is connected, through a universal serial bus cable  113  to a compatible universal serial bus port (not shown) on a computing device  102 . Computing device  102  which may be an IBM PC or PC-compatible notebook computer includes an USB port coupled to the PC bus and controlled by a microprocessor in the computing device. The microprocessor is typically a 32-bit microprocessor such as a Pentium III processor from Intel Corporation. 
     When the connector  115  on the universal serial bus cable  113  is received at the universal serial bus port (not shown) on computing device  102 , not only does an interface engine in scanner  100  receive a power supply from the computing system, typically 5V, but it also becomes integrated into the computing system, receiving system control signals from the microprocessor resident in computing device  102 . Through universal serial bus cable  113  and the associated interface engine, mobile scanner  100  can communicate with computing device  102 . Further computing device  102  operates an application program under an operating system, for example, Microsoft Windows 98 or Linux. The application program, as will be described in more detail below, is a compiled and linked version of a process that controls the operations of mobile scanner  100  via the universal serial bus connection. 
     Scanner  100  scans, line by line, a scanning object  110 , such as a piece of paper or film with text and/or graphics on it. The scanning result, which is typically a digital representation of scanning object  110 , is transferred to computer  102  through universal serial bus cable  113 . The digital representation may be converted by the application program to a standard image format such as TIFF (Tag Image File Format) or BMP (Bitmap File Format), that may be manipulated for desired visual effects by another application program, such as PhotoShop 5.0 from Adobe Systems. The digital representation or manipulated digital image can be displayed on display monitor  104 . 
     Computing device  102  is further provided with one or more removable storage devices which may be read or written to, a fixed disk drive (not shown) for storing image files and application program files, a keyboard  106  for permitting input of text data, such as titles and names for scanned image files, and a pointing device  108  such as a mouse or the like which is also provided to permit execution of commands, for example, to display the scanned object and manipulated images thereof on display monitor  104 . 
     FIG. 2 shows a systemic diagram of a prior art scanner  200  that converts a paper-based scanning object  220  to a corresponding digital image  222 . Scanner  200  comprises an image sensing module  204 , an analog-to-digital circuitry  206 , a motion mechanism  208 , a microprocessor  210  and memory  212 . Motion mechanism  208  is controlled by microprocessor  210  which works in synchronization with image sensing module  204  such that image sensing module  204  images scanning object  220  while passing through image sensing module  204  at a steady speed. 
     Image signals from image sensing module  204  are then digitized by analog-to-digital circuitry  206  to produce digital signals (raw data) representing scanning object  220 . To have a common interface with most of the computing devices, scanner  200  further comprises a post-signal processing circuitry  214  and an interface  216 . Post-signal processing circuitry  214  generally performs digital signals enhancement and often presents the digital signals in a standard image format, such as TIFF or BMP. Interface  216  prepares the images for real-time transmissions through a standard interface, such as IEEE RS-232, via a serial port to a computing device. In addition, power supply  218 , typically heavy and bulky, is provided to scanner  200  to energize all the parts in scanner  200  to operate. 
     Referring now to FIG. 3, there is shown a block diagram of a mobile scanner  300  according to one embodiment of the present invention. Mobile scanner  300  comprises fewer components than found in scanner  200  of FIG.  2 . The principle components of scanner  300  are an image sensing module  304  and a motion mechanism  312 . It should be noted that there is no separate power supply to energize image sensing module  304  and motion mechanism  312  to work. The elimination of a separate power supply will considerably reduce the overall weight, size and portability of a scanner. As will be further described below, the power for image sensing module  304  and motion mechanism  312  is derived from computing device  302  with which scanner  300  operates. 
     Image sensing module  304  comprises an image sensor  306 , an optical lens  308  and an illumination system  310 . Preferably, image sensor  306  is an array of Complementary Metal-Oxide Semiconductor (CMOS) photodetectors, each producing a charge signal when being exposed to incident light. Generally, the number of photodetectors in the array depends on the maximum size of a scanning document the scanner is designed to accommodate and the resultant image resolution. For example, a regular paper has a size of 8.5×11 inches. For 300 dpi (dot per inch) resolution, the number of photodetectors is 9×300=2,700 wherein 0.5 inch is added to account for a margin of the 8.5-inch width. 
     Optical lens  308  collects incident light from a scanned object illuminated by illumination source  310  onto image sensor  306 , thereby an image of the scanning object is produced. Motion mechanism  312  is responsible for moving the scanning object through image sensor  306  so that the entire scanning object can be scanned. According to one embodiment of the present invention, both image sensing module  302  and motion mechanism  304  are housed in a compact case that can be made of a kind of light but rigid plastic material. 
     An interface engine such as USB interface  316  houses all the electronic components such as A/D converter, a processor, memory and etc. to control image sensing module  302  and motion mechanism  304  to operate synchronously in addition to support data communication with a host computing device. Depending on an exact implementation, the interface engine may be housed in the same compact case that houses image sensing module  302  and motion mechanism  304 . Alternatively, the interface engine may be housed in a separate case, shown in the figure, connected to the serial connection cable. Other possible embodiments include an integration of the interface engine with the serial connection cable. 
     FIG. 4A shows a cross-section view of a mobile scanner  400  according to the present invention and mobile scanner  400  may correspond to mobile scanner  100  of FIG.  1 . As shown in the figure, scanner  400 , appearing rounded triangle shaped, comprises two demountable portions, a main case  402  and a base case  404 , both preferably made of an identical material. Main case  402  houses an image sensing module  406  and motion mechanism of which only a rubber surfaced rod  408  is shown. A scanning object  420  is received from receiving opening  422 , then moved by moving rod  408  to pass an optical path  426  and exited from exiting opening  424 . When scanning object  420  passes optical path  426 , scanning object  420  is illuminated by front illumination source  428 , reflected light from scanning object  420  is collected by lens  412  and focused upon image sensor  410 . It is noticed that illumination source  428  is referred to as front illumination source because it provides front illumination to a scanning object that is opaque. In other words, when scanning object  420  is transparent, no reflected light can be collected from the scanning object illuminated by illumination source  428 . 
     Base case  404  comprises a second illumination source  432  which provides back illumination and can be removably mounted to main case  402  when there is a need to scan a scanning object that is transparent. The transparent sheet-like scanning object may include negative or positive films, transparencies for an overhead projector or X-ray films. 
     FIG. 4B and 4C demonstrate respectively the front illumination for an opaque scanning object and the back illumination for a transparent scanning object from the perspective of optics. In FIG. 4B, scanning object  420  is an opaque scanning material, such as a piece of paper, having a front surface facing to image sensor  410 . Front illumination source  410  illuminates the front surface of scanning object  420 , light reflected from the surface is collected by optical lens  412  and focused upon image sensor  410  so that an image of the surface is obtained. In FIG. 4B, scanning object  420  is a transparent scanning material, such as a piece of film. Back illumination source  432  provides illumination from the back of transparent scanning object  420  such that light transmitted through transparent scanning object  420 , namely transmitted light, is collected by optical lens  412  and focused upon image sensor  410  so that an image of the transparent scanning object is obtained. 
     It should be pointed out that base module  404  is one of the distinctive features of the present invention. With base module  404  mounted to main module  402 , mobile scanner  400  is capable of scanning both transparent and opaque scanning document. Typically, a user uses main module  402  for scanning paper-sheet materials in which case illumination source  428  provides front illumination to the materials passing through optical path  426 . When the user decides to scan transparent materials, base module  404  can be mounted onto main module  402 , in which case illumination source  432  is on to provide back illumination to the transparent materials. Those skilled in the art understand that a switch can be placed on the bottom of main module  402 , the switch is so designed that it can be automatically turned on to switch the power supplied to illumination source  428  to illumination source  432  when base module  404  is mounted to main module  402 . 
     Accordingly to another embodiment of the present invention, a light source (emitter)  434  and a photodetector  436  are used to detect if scanning object  420  is opaque or transparent. Preferably, light source  434  and photodetector  436  are aligned so that photodetector  436  is always activated by light source  434 . When an opaque sheet is received from receiving opening  422 , photodetector  436  becomes inactivated because of the blocked light source  434  by the opaque sheet. A control circuit, not shown in the figure and understood to those skilled in the art, can automatically turn on front illumination source  428 . Similarly, when a transparent sheet is inserted into the scanning gap, photodetector  436  remains activated because light source  434  can go through the transparent sheet. Therefore the same control circuit can automatically turn on back illumination source  432 . 
     For completeness, FIGS. 4D to  4 F illustrate a top view, a side view and a front view of the internal structure of main module  402 . Elongated part  452  corresponds to image sensing module  406  of FIG.  4 A and includes an image sensor, a (front) illumination source and a rod lens array. Shaft  454  including one or more rubber-surfaced tube  456  is rotated by a motor  458  through a gearbox  460 . When a scanning object, not shown, is inserted into the scanner, motor  458  causes shaft  454  to rotate at a speed adjusted by gear box  460  and the scanning object is then carried along by rubber surfaced tube  456  to move against elongated part  452  so that the image sensor therein can image the scanning object completely. It should be pointed out that, fundamentally different from the scanners in the market, there is no microprocessor and other electronic components in main module  402  to control the operation of the image sensor and the illumination source. The overall system performance of the scanner is not fixed rather depending on a host computer with which the scanner is coupled to operate. 
     Referring now to FIG. 5, there is shown an internal block diagram of interface engine  500  implemented with universal serial bus connector  502  and universal serial bus interface  504 . Universal serial bus connector  504  preferably meets the well-known Universal Serial Bus (USB) standards as defined in the Universal Serial Bus Specification Revision 1.1 which is incorporated by reference. Universal serial bus connector  502  includes differential data input/output (I/O) lines, a ground reference GND, as well as the integral power node all of which are preferably coupled through universal serial bus interface to the device circuits shown. 
     Read Only Memory (ROM)  508  provides information to the computing device on what functions or procedures universal serial bus interface  504  performs and what system resources (I/O, IRQ, etc) it requires. Clock  506 , typically an oscillator circuit, provides a central clocking signal to all the parts so that connected components work in synchronization. 
     According to one embodiment, image sensing module  514  outputs analog signals generated from the charge signals in the photodetectors of the image sensor when image sensor is impinged with incident light (reflected or transmitted light). The analog signals are received by an analog-to-digital (A/D) converter  512  through an analog signal line and the resultant digitized (digital) signals are uploaded to the computing device via the universal serial bus connection. The illumination source in the image sensing module  514  is controlled and synchronized by control signals from control signals circuitry  506  that operates under system control signals from an associated computer device (i.e., computing device  102  of FIG. 1) via USB interface  504 . Typically, the illumination source comprises three primary colored lights, such as red, green and blue. To reproduce a color image, three primary color intensity images must be obtained. In other words, A/D converter  512  receives three analog signals respectively for each of the colored lights and produces respectively three digital signals. 
     The control signals from control signals circuitry  506  ensures that only one of the three lights is “ON” at one time for a specific period and each of the lights is successively turned on. Similarly, to ensure that drive mechanism  520  to operate in synchronization with image sensing module  514 , motor drive  518  receives control signals from control signals circuitry  506 . It is understood to those skilled in the art that control signals circuitry  506  is controlled by system control signals generated from the computing device via the universal serial bus connection and comprises many logic circuits so as to generate control signals with different timings. As will be explained below, the system control signals are generated in the computing device that operates an application program, the application program implementing a process that uses the computing device to ultimately controls the operation of the scanner. Upon receiving the system control signals, USB interface  504 , which typically an ASIC circuit, produces interface control signals to synchronize all parts in the interface engine. With respect to the interface control signals, control signals circuitry  506  generates a set of scanner control signals. 
     Each of the scanner control signals is used to synchronize the operation of a particular part. For example, three illumination control signals are generated to control the operation of the illumination source comprising the three colored lights. The timing in the illumination control signals is so designed that only one of the lights is turned on for a specific period at one time and the three lights are successively turned on. Similarly sensor control signals and motor control signals are generated with respect to the illumination control signals so that the image sensing module and the motion mechanism can work harmoniously to produce images of scanning objects. Additionally, sequential power regulator  526  sequentially powers the system components in accordance with instructions received from control signals circuitry  506  to minimize overall power requirements. 
     Optionally, there is one protection control signal from control signals circuitry  504  to motor driver  518 . The protection control signal is used to protect the motor drive mechanism  520 . For example, in the middle of scanning a document, in the event of a malfunction, the protection control signal can immediately cause motor drive  518  to stop the operation of the motor in drive mechanism  520 . 
     To further understand the principles of the present invention, FIG. 6 shows an internal functional diagram of sensing module  600  according to one embodiment of the present invention. As illustrated, there are two illumination sources, first one  602  is for back illumination and second one  605  is for front illumination. The front illumination source comprises a red LED  604 , a green LED  606  and a blue LED  608 , each controlled respectively by a control signal, GLED, RLED or BLED at respective connectors. 
     For simplicity, back illumination source  602  is shown as one light and one connector  603 . Those skilled in the art understand that back illumination source  602  may be implemented with a single LED or a fluorescent light controlled by an “ON” signal at connector  603 , and alternatively with three colored lights similar to red LED  604 , green LED  606  and blue LED  608 , in which case connector  603  should be implemented with three separate connectors, and the corresponding control signals may be provided from USB interface  504  of FIG.  5 . As described above, either illumination source  602  is turned “ON” for a transparent scanning object or the LEDs are successively turned “ON” for an opaque scanning object. 
     The rod lens array  610  collects either the reflected light from the opaque scanning object illuminated by one of red LED  604 , green LED  606  and blue LED  608  or transmitted light from the transparent scanning object illuminated by back illumination source  602  and focuses the light onto image sensor  612 . Image sensor  612  comprises, for example, N photodetectors. Each of the photodetectors collects light cast thereon during each integration process and generates a pixel signal. Upon the completion of the integration process, the pixel signals, each respectively generated by one of the photodetectors, are sequentially readout to the video bus  616  as a scanning signal via readout switch array  614  controlled by control signals from USB interface  504  of FIG.  5 . It should be noted that image sensor  612  may be a CMOS type sensor and those skilled in the art will understand that the description works the same for CCD type sensors. 
     Switch array  614  comprises the same number of the readout switches as the number of the photodetectors in the image array  612 . It is understood to those skilled in the art that each of the readout switches may be implemented by a diode that becomes “On” or “passing through” when a proper voltage from USB interface  504  of FIG. 5 is applied across. As shown in the figure, the scanning signal is coupled to a gain &amp; offset control circuit  618 . The scanning signal is processed, including amplified and offset, in gain &amp; offset control circuit  618  with respect to a desired adjustment and subsequently output as a signal V out  to USB interface  504  of FIG.  5 . 
     FIGS. 7A to  7 C show flowcharts of the process that can be used to control the operation of the interface control card and subsequently of the image sensing module and the motion mechanism and should be understood in conjunction with FIG.  5 . According to one embodiment of the present invention, a compiled and linked version of the process is executed in a computing device with which the scanner operates. 
     In FIG. 7A, the process starts with a calibration procedure to get various calibration data that will be used to refine image raw data during actual image acquisition. Because of various practical reasons, the image sensor in the image sensing module may not produce intensity signals or valid signals from all of the photodetectors. To avoid undesirable (bad) signals, at  704 , one or more lines of scanning signals are acquired to detect if there are any bad signals in the acquired scanning signals. As described above, this is done through the interface engine that produces the digital signals of the scanning signals. If there are any digital signals that have values beyond ±50% of the average of the entire line of digital signals, these pixels are marked “bad”. 
     At  706 , the process is moved to get correct exposure times for the three respective colored lights. Typically, the three colored lights are red, green and blue light tubes stimulated by respective one or more red, green and blue LEDs. It is known that the sensitivity of the image sensor to the colors and the light intensities from the three colored lights are different. To avoid any biased illumination, a set of calibrated timing parameters must be derived from an actual test. There can be many ways to derive this set of timing parameters. According to one implementation, the set of timing parameters is derived with respect to the maximum values in the three respective lines of signals obtained under independent and respective illumination of the three colored lights. 
     At  708 , the process is moved to obtain the most appropriate gain and offset from the actual signals. Preferably a few hundreds of lines of signals are acquired from the same scanning lines under the same lighting conditions (reduced to 5% of the adjusted full capacity). These signals are then averaged to derive an offset, referred to as BlackData, of each of the three colored lights. At  710 , the process is moved to obtain the respective maximum values, referred to as WhiteData, from a few hundreds of lines of signals that are acquired under the same lighting conditions (adjusted full capacity). Hence a set of WhiteData and BlackData is obtained for each colored illumination. 
     At  712 , the process is moved to calculate the gain value, referred to as GainData, with respect to the respective obtained set of WhiteData and BlackData. 
     FIG. 7B shows the process (image acquisition) with the control of the illumination source and motion mechanism. It should be noted herein that the process does not have to identify which illumination source is in operation. Typically, before the acquisition process starts, a cleanup procedure is executed at  720 . The purpose of cleanup procedure is to ensure that no signal residues are in the image sensor. At  722 , for every scanning line, the colored lights are respectively and successively turned on for a specific time by the respective exposure timing parameters obtained in the calibration process. To be specific with one implementation, the green light is kept “ON” for a duration of GLEDOntime, the red light is kept “ON” for a duration of RLEDOntime and the blue light is kept “ON” for a duration of BLEDOntime. Under each of the lighting condition, three lines of digital signals are respectively and subsequently obtained before the motion mechanism advances the scanning object for the next scanning at  724 . 
     FIG. 7C shows the process for image acquisition. At  732 , lines of image data are captured under the respective exposure timing parameters obtained in the calibration procedure. At  734 , those marked “bad” signals are replaced using an average of neighbor signals. 
     Generally, the scanner is designed to scan at a fixed resolution, for example, 300 dpi or 600 dpi. If a user decides to have an image resolution other than what the scanner can provide, the scanned digital signals are extrapolated or interpolated at  736  to produce the desired resolution. Before the digital signals are finally output, for example, for display, the digital signals are corrected with the gain and offset obtained empirically before. At  740 , the corrected digital signals (image pixels) are regrouped to comply with the traditional RGB data format. 
     The present invention has been described in sufficient detail with a certain degree of particularity. It is understood to those skilled in the art that the present disclosure of embodiments has been made by way of examples only and that numerous changes in the arrangement and combination of parts may be resorted without departing from the spirit and scope of the invention as claimed. Accordingly, the scope of the present invention is defined by the appended claims rather than the forgoing description of embodiments.