GB2328577A - Optical image scanning system with improved image resolution - Google Patents

Abstract

A method for improving the quality of an output image in an optical image scanning system. A step motor and a CCD sensor array are used to scan and acquire images. The timing of running the step motor and exposing the CCD sensor are carefully controlled so that the sensor is exposed to the reflected light of a scanned object when the motor is not moved. While the motor steps, the CCD sensor is turned off. The blurring effect of scanning is reduced and the modulation transfer function is improved.

Description

METHOD FOR IMPROVING THE IMAGE RESOLUTION IN AN OPTICAL IMAGE SCANNING SYSTEM FIELD OF THE INVENTION The present invention relates to a method for improving the resolution of an output image in an optical image scanning system, and more particularly to a method for improving the quality of an output image of a flat-bed scanner.

BACKGROUND OF THE INVENTION In the early days, a single image pixel (i.e. a picture element) scanned by an optical imaging system of a scanner was digitized into only one bit value. The bit value represents either "on" or "off' for an image pixel. In recent years, the charge-coupled device (CCD) of a desk-top type scanner can be used to recognize 256 gray levels of brightness. In the scanning process, most optical image scanning systems scan across the imaged patterns or documents to acquire image data information.

In traditional methods of scanning, each pixel area is imaged to a CCD sensor element.

The CCD sensor moves across one pixel distance during the sampling period of each image pixel. The area actually sensed by the CCD sensor covers one pixel height and two-pixel width of the scanned pattem when the image pixel is acquired due to the scanning motion and the covered area of each sensor cell. Therefore, in the scanning direction two adjacent pattern pixels are sensed by each sensor cell in the CCD array. FIG. 1 shows an ideal output image for one pixel height range after scanning a black-and-white bar pattem. The scanning direction is along the right direction of the horizontal axis. As shown in the figure, the image is only one pixel height because there is no motion in the vertical direction and two adjacent vertical pixels do not interfere with each other.

FIG. 2 shows the intensity of CCD sensor integration for scanning the black-and-white bar pattern during the acquisition of an image pixel. As described above, the scanning direction is along the right direction of the horizontal axis. During the sampling of an image pixel, the CCD sensor oversees two pattern pixels. In other words, in the conventional scanning system, the CCD moves from one pattern pixel to the next pattern pixel during the exposure time of each image pixel. Therefore, there is a blurring effect from the two adjacent pattern pixels in the horizontal direction due to the scanning motion. The resolution in the horizontal direction is not as good as that of the vertical direction.

As an example, the output image of a conventional scanning system for a black-andwhite bar pattern does not have a sharp contrast as shown in FIG. 1. The output image for a "black" pixel of the original picture pattern becomes non-black and the output image for a "white" pixel becomes non-white. FIG. 3 illustrates the ratio of black and white of a scanned signal in one pixel range after scanning a black-and-white bar pattern picture with the conventional methods. Due to the motion in the horizontal direction of the image scanning system, the cells in the CCD array sense the adjacent pixels in both forward and backward directions for about 1/8 of a pixel width without considering other noise. The signal sampled for an image pixel by the CCD sensor integration can only achieve 3/4 of the actual signal from a pattern pixel as indicated in FIG. 2. This causes "non-black" and "non-white" pixels to be seen when a black-and-white bar pattern is scanned. Assuming that the gray level for a black pixel is 0 and the gray level for a white pixel is 100, the gray level for such a non-black pixel is 100*(1/8 + 1/8) = 25, and the gray level for a non-white pixel is 100*3/4 = 75. FIG. 4 shows the actual voltage level after integrating the signal of FIG. 3. The brightness of the output image is higher and its gray level is also higher if the voltage value of a CCD cell is higher. Thus, for an ideal case of a conventional scanning method, the gray level for a nonwhite pixel is equal to three times of the gray level for a non-black pixel. This smoothing effect causes some blurring in the scanning direction, degrades the resolution and the contrast of an image acquired in an optical scanning system.

SUMMARY OF THE INVENSION This invention has been made to overcome the aforementioned drawbacks of poor image resolution in the scanning direction of an optical image scanning system. It is the primary object ofthe present invention to use an improved sampling method for reducing the blurring effect in the scanning direction. In accordance with the invention, during each sampling period only a pixel width of a picture pattern is exposed to the CCD sensor. Hence, the aforementioned interference caused by the forward and backward pixel patterns can be eliminated.

The present invention achieves the new image scanning and sampling method by controlling the operation of a step motor used in the scanning system. The ting of running the step motor and integrating the CCD sensor signal in the image scanning system are careftilly controlled so that the modulation transfer function of the system can be improved.

According to the present invention, the sensor exposure time does not overlap with the time that the step motor moves. In other words, the cells of the CCD sensor is not exposed to the light while the motor is running. When the sensor is exposed to the light, the motor is stopped.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows an ideal output image for one pixel height range after scanning a black and-white bar pattern picture.

FIG. 2 shows the integration intensity of a CCD sensor in accordance with a conventional optical image scanning system.

FIG. 3 shows the ratio of black and white of an output signal for one pixel height range after scanning a black-and-white bar pattern picture in accordance with the conventional method.

FIG. 4 shows the voltage for one pixel height range after scanning a black-and-white bar pattern picture in accordance with the conventional method.

FIG. 5 shows the integration intensity of a CCD sensor for an optical image scanning systems in accordance with the present invention.

FIG. 6 shows the time slots for running a step motor and exposing the sensor for an image scanning system in accordance with the present invention.

FIG. 7 is a flowchart illustrating the scanning process for an imaging scanning system using a step motor.

FIG. 8 is a timing diagram illustrating the control signal of the data writing and the motor control pulses of a step motor in an optical image scanning system according to the prior art.

FIG. 9 is a timing diagram illustrating the control signal of the data writing and the motor control pulses of a step motor in an optical image scanning system according to this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT According to the operation of this invention, the CCD sensor is exposed to the light only when the motor is stopped. With reference to FIG. 5, the intensity of integration for one scanned pixel of CCD is shown. The scanning direction is along the right direction of the horizontal axis. There is only one pixel width seen during each sampling period of CCD sensor integration. Only the self image of the scanned pattern pixel is seen as shown in the figure.

Referring next to FIG. 6, illustrated is the exposure time and the motor stepping time for the image scanning system of this invention while a picture pattern is scanned along the horizontal axis. The periods of motor stepping and the CCD array sensor exposure are separated from each other. As shown in the figure, the CCD sensor is exposed for sensing the reflected light from a pixel pattern after the motor moves and stops at a pixel location that is represented by a rectangular blank.

According to this invention, a CCD sensor having an array of approximately 8000 sensor cells are used in the scanner. A light source in the scanner illuminates a picture pattern to be scanned. The light reflected from the picture pattern is sensed by the CCD sensor array.

Each sensor cell is exposed to the reflected light and generates an electronic signal proportional to the intensity of the reflected light. The electronic signal represents an image pixel corresponding to a pixel of the scanned picture pattern.

The flow chart of the operation ofthe scanner of this invention is shown in FIG. 7. The scanner first starts the scanning process (step 100). A firmware determines and controls whether the digitized data for the current scanned line should be skipped or written into the computer. If the data is to be skipped, a write mask is set to disable the writing of the data into the computer (step 102). In this case, the step motor continues to move (step 104). It then waits for the next synchronization signal for the next scan line (step 106) and determines if the whole input picture has been scanned (step 109). If the whole picture has been scanned,.

the scanning process is terminated (step 110). Otherwise, the operation returns to step 101 and repeats the next scan line.

If the digitized data for the scanned line have to be written into the computer, the write mask is cleared (step 103) to enable the writing ofthe data into the computer. The step motor continues to run (step 105) until the synchronization signal for the next scan line is issued (step 107). The next step is to determine whether the input picture is completely scanned (step 109). If the input picture is completely scanned, the scanning process is terminated (step 110). Otherwise, it returns to step 101 and repeats the next scan line.

FIG. 8 illustrates a timing diagram of the motor control signal and the data writing signal of a conventional optical image scanning system. The CCD sensor and the step motor are synchronized. A firmware in the control circuit of the scanning system controls the writing of the scanned data to a memory buffer. As can be seen from FIG. 8, three types of step motors are given as examples. During a clock cycle of stepping the motor for one pixel distance, a full-step step motor receives a single control pulse, a half-step step motor receives two control pulses and a quarter-step step motor receives four control pulses. In the mean time, the CCD sensor is triggered by a data control clock to sense and integrate the reflected light from the scanned object. As can be seen, the timing pulse for writing the data to the memory buffer occurs at the same time as a first control pulse for stepping the motor occurs. Therefore, the sensor exposure time overlaps with the time that the motor is stepping.

FIG. 9 shows a timing diagram of the motor control signal and data writing signal of the optical image scanning system of this invention. As can be seen, a clock cycle is used for stepping the motor and a clock cycle is used for writing the data. Either one, two, or four control pulses are used for stepping a full-step, a half-step or a quarter-step step motor through one pixel distance in a first clock cycle. During the first clock cycle, the CCD sensor is not triggered to sense and integrate the reflected light signal. During the next clock cycle, the CCD sensor is triggered to integrate the signal and write the scanned data to the memory buffer.

An optical scanning system can be regarded as a transformation system. It transforms an image on a plane into digitized signals. MTF is a function that measures the characteristic of the transformation of the system. 'Therefore, it is often used to determine the quality of the image of an optical scanning system. In general, a black-and-white bar pattern with 200 dpi (dots per inch) is used to test the MTF of an optical scanning system having 400 dpi resolution. The histogram of the gray level of the output image is studied.

The gray level at the 99.7% distribution point is defined as t'a " and the gray level at the 0.3% point distribution point is defined as web". One way to quantify the MTF is using the value of a-b. The higher the value of a-b is, the better the resolution and quality of the optical scanning system. The other way to quantify the MTF is using a normalized value, i.e. (a-b)/(a+b). In this case, the normalized value ofthe MTF is between 0 and 1. When the value is closer to one, the system produces better quality images.

Table 1 shows the values of the normalized MTF measured for a conventional optical scanning system with respect to red, green, or blue color light by scanning a test picture.

The step motor is a half-step step motor. As can be seen, the blue color has highest MTF value followed by the green color and then the red color. Therefore, the image scanned for a blue color has best contrast. Similarly, Table 2 shows the values of the normalized MTF measured for the optical scanning system of this invention with respect to red, green, or blue color light by scanning the same test picture. As can be seen, the values measured for this invention are consistently higher regardless of the light color. The normalized MTF value is increased by approximately 0.05 for each color.

Tablet

I I -I I Color (a-bV(a+b) Normalized MTF Red (98-18)/(98+18) 0.689655 Green (104-15)/(104+15) 0.747899 Blue (108-12)/(108+12) 0.800000 Table 2

Color (a-b)/(a+b) Normalized MTF Red (100-15)/(100+15) 0.739130 Green (106-12)/(106+12) 0.796610 Blue i (110-9)/(110+9) 0.848739 Table 3 shows the values of the normalized MTF measured for a conventional optical scanning system with respect to a red, green, or blue color light by scanning another test picture. The step motor is also a half-step step motor. As can be seen, the blue color has highest MTF value followed by the green color and then the red color. Therefore, the image scanned for a blue color has best contrast. Similarly, Table 4 shows the values of the normalized MTF measured for the optical scanning system of this invention with respect to a red, green, or blue color light by scanning the same test picture. As can be seen, the values measured for this invention are still consistently higher regardless of the light color. The normalized MTF value is increased by about 0.03, 0.04 or 0.04 for a red, green or blue color respectively.

Table 3

Color (a-b)/(a+b) Normalized MTF Red (113-31)/(113+31) 0.569444 Green (121-27)/(121+27) 0.635135 Blue (124-24)/(124+24) 0.675676 Table 4

Color (a-by(a+b) Normalized Mm Red (116-29)/(116+29) O.600000 Green (124-24)/(124+24) 0.675676 Blue (126-21)/(126+21) 0.714286 For an optical image scanning system, the normalized MTF values using the motor control and sensor exposure methods according to the present invention are higher than those using the conventional methods. The higher the value of the MTF, the better the resolution of the output image. The quality of the image of a flat-bed scanner based on the optical image scanning system of the present invention is improved over that of a conventional flat-bed scanner.

Although this invention has been described with a certain degree of particularity, it is to be understood that the present disclosure has been made by way of preferred embodiment only and that numerous changes in the detailed construction and combination and arrangement of parts may be restored to without departing from the spirit and scope of the invention as hereinafter claimed.

Claims (4)

What is claimed is:

1. A method for improving the quality of an output image of an optical image scanning system having a step motor for scanning and a CCD sensor, comprising the steps of: issuing a motor control pulse for stepping said motor through one pixel width; and issuing a sensor control pulse for starting the sensor integration of said CCD sensor; wherein said motor control pulse and said sensor control pulse are issued repeatedly one after the other, each said sensor integration ends when a following motor control pulse is issued and said motor stops before a sensor control pulse is issued, and the output of said sensor integration forms the signal of an image pixel.

2. The method for improving the quality of an output image of an optical image scanning system according to claim 1, wherein issuing said motor control pulse and issuing said sensor control pulse are controlled by a firmware in a control circuit of said scanning system.

3. The method for improving the quality of an output image of an optical image scanning system according to claim 1, wherein said motor control pulse comprises a plurality of sub-pulses for stepping said motor through one pixel width.

4. The method for improving the quality of an output image of an optical image scanning system according to claim 3, wherein issuing said motor control pulse and issuing said sensor control pulse are controlled by a firmware in a control circuit of said scanning system.