WO1998043407A1

WO1998043407A1 - Method for opto-electronic scanning

Info

Publication number: WO1998043407A1
Application number: PCT/DE1998/000445
Authority: WO
Inventors: Jens Flick; Uwe Rudi Albrecht
Original assignee: Heidelberger Druckmaschinen Ag
Priority date: 1997-03-25
Filing date: 1998-02-16
Publication date: 1998-10-01
Also published as: DE19712459A1

Abstract

The invention relates to a method for pixel and line opto-electronic scanning of original images (3) by means of a photodiode line (6). Said photodiode line (6) has a charge/voltage converter (12) for converting the charge packets into voltage values, said charge packets being obtained by converting the scanned image pixel data. Said voltage values are converted into digital image values by means of an A/D converter (17) situated downstream. In order to reduce the scanning time for a coarser scanning fineness, the number of voltage values for analog/digital conversion per scanning line is reduced by grouping together a number of adjacent pixels, the number of pixels grouped together corresponding to a reduction factor. The charge packets of each of the pixels grouped together are added up in the charge/voltage converter (12) and the charge packets which have been added up are converted into the voltage values (U1 + U2) of each of the pixels grouped together. The reduction factor is the quotient from the number of sensing elements (8) of the photodiode line (6) and the smaller number of voltage values required by the coarser fineness.

Description

Method for optoelectronic scanning

The invention relates to the field of electronic reproduction technology and relates to a method for pixel-by-line and line-by-line optoelectronic scanning of image originals.

In electronic reproduction technology, image values are obtained and further processed in scanning devices, also called scanners, by pixel-by-line optoelectronic scanning of image templates to be reproduced.

In a scanner, for example of the flatbed type, the image original to be scanned is arranged on a flat original carrier which moves perpendicular to the direction of the scanning lines relative to a scanning element. The scanning element essentially has a light source for line-by-line illumination of the original image, a photo diode line (CCD line) as an optoelectronic converter and a scanning lens for imaging the scanning lines on the photo diode line.

A photodiode line essentially consists of a number of light-sensitive sensor elements arranged in the direction of the scanning lines, a transfer gate and an analog shift register with a number of memory cells corresponding to the number of sensor elements.

The scanning light modulated with the image content of the individual pixels of a scanning line is stored in the light-sensitive sensor elements of the photodiode line as charge packets. Each time a current scan line is scanned, the charge packets of the current scan lines are transferred from the sensor elements via the transfer gate to the memory cells of the analog shift register. While the next scan line is being scanned, the charge packets are then read out serially from the shift register and converted into an analog voltage, the image signal, in a charge / voltage converter. The image signal is converted into digital image values in a downstream A / D converter, which are then temporarily stored and further processed in a signal processing stage.

The scanning speed with which a scanning line is scanned essentially depends on the specific conversion speed per voltage value of the A / D converter used. An increase in the scanning speed There is a limit to the fact that fast A / D converters with correspondingly high operating frequencies are not available or are very expensive.

The scanning fineness (pixels / cm) in the direction of the scanning lines depends on the number of light-sensitive sensor elements of the photodiode line and on the width of the image to be scanned. The scanning fineness perpendicular to the direction of the scanning lines is determined by the relative speed between the original image and the scanning element. The scanning fineness can be varied by the imaging scale, in which the scanning objective maps the respective original width onto the photodiode line. Another possibility for changing the scanning fineness is to interpolate between stored image values in order to achieve a greater scanning fineness or to combine stored image values in order to achieve a coarser scanning fineness.

Conventional scanners have the disadvantage that the scanning speed per scanning line is constant irrespective of the respective scanning fineness.

It is therefore an object of the present invention to improve the known method for optoelectronic scanning of image templates in such a way that a higher scanning speed and good scanning quality are achieved with a relatively small effort when scanning a template with a coarser scanning fineness.

This object is solved by the features of claim 1.

Advantageous refinements and developments are specified in the subclaims.

The invention is explained in detail below with reference to FIGS. 1 to 3.

Show it:

1 is a basic block diagram of a scanner for optoelectronic scanning of image templates,

Fig. 2 time diagrams according to the prior art and

Fig. 3 timing diagrams for explaining the invention 1 shows a basic block diagram of a compartment bed scanner for point-by-line and line-by-line optoelectronic scanning of black and white images. A light source (1) for see-through scanning or a light source (2) for top-up scanning illuminates line by line an image template (3) arranged on a template carrier (not shown). The illuminated scanning lines (4) are successively imaged on the light-sensitive surface of an optoelectronic converter in the form of a photodiode line (6) (CCD line) by means of a scanning lens (5), the image template (3) being perpendicular to the direction of the scanning lines (4) directed relative movement to the light sources (1; 2) and to the photodiode array (6).

The photodiode line (6) essentially consists of a row of sensor elements (7) aligned in the direction of the scan lines (4) with a number of light-sensitive sensor elements (8) arranged next to one another, a transfer gate (9) and an analog shift register (10) with a number of memory cells (11) corresponding to the number of sensor elements (8). The number of sensor elements (8) determines the number of pixels that can be scanned in a scanning line (4), with each pixel of a scanning line (4) a sensor element (8) of the sensor element series (7) and a corresponding memory cell (11 ) of the shift register (10) is assigned. The photodiode array is, for example, of the KLI 8013 type from Kodak.

The photodiode array (6) may have, for example, 8000 light-sensitive sensor elements (8) and memory cells (11). In this case 8000 pixels per scan line (4) can be scanned.

The imaging scale with which a scanning line (4) is imaged by the scanning objective (5) onto the sensor element row (7) determines the scanning fineness (pixels / cm) in the line direction, while the scanning fineness perpendicular to the line direction is determined by the relative speed between the image template (3) and the photodiode array (6) is determined.

The scanning light modulated with the image information of the individual pixels of a currently scanned scanning line (4) is stored as charge packets Q in each case within an integration time in the light-sensitive sensor elements (8) of the sensor element row (7). Each time after the integration time or after scanning the current scanning line (4), the stored charge packets Q of the individual pixels of the current scanning line are removed from the sensor elements (8) via the Transfer the transfer gate (9) into the assigned memory cells (11) of the shift register (10). During the scanning of the next scanning line (4), the charge packets Q are then read out serially from the shift register (10) and fed to a charge / voltage converter (12) of the photodiode line (6).

To transfer the charge packets Q from the sensor elements (8) of the sensor element row (7) into the memory cells (11) of the shift register (10), the transfer gate (9) is enabled by a transfer clock of a transfer clock sequence TT on a line (13). The charge packets Q are read out from the shift register (10) by means of a shift clock sequence T _s on a line (14).

The charge / voltage converter (12) converts the charge packets Q of the individual pixels into voltage values U which are proportional to the charge packets Q. After converting the charge packet Q of a pixel into the voltage value U, the analog charge / voltage converter (12) is reset by a reset clock of a reset clock sequence TR on a line (15) and thus prepared for the conversion of the charge packet Q of the next pixel.

The voltage values U generated in the charge / voltage converter (12) are converted into digital image values in an A / D converter (17) clocked by a digitizing clock sequence T _D on a line (16), which are then converted into a digital image / D converter (17) downstream signal processing stage (18) are temporarily stored and processed.

Transfer clock sequence T _τ , shift clock sequence T _s , reset clock sequence T _R and digitizing clock sequence T _D for controlling the scanner are generated in a clock generator circuit (19).

Fig. 2 shows timing diagrams for explaining the control of the scanner according to the prior art.

The timing diagram (2A) shows the shift clock sequence Ts, which is fed to the shift register (10).

The timing diagram (2B) shows the reset clock sequence T _R , which resets the analog charge / voltage converter (12). The timing diagram (FIG. 2C) shows the voltage values U which are generated in the analog charge / voltage converter (12) by converting the charge packets Q of the individual pixels according to the prior art.

The clock period of the shift clock sequence Ts is equal to the clock period of the reset clock sequence TR and corresponds to the conversion time tw per voltage value of the A / D converter (17) used. In each clock period of the shift clock sequence T _s , the charge packet Q of a pixel is read out of a memory cell (10) of the shift register (10) and processed further. At the beginning of each clock period of the shift clock sequence Ts, the analog charge / voltage converter (12) is reset by a reset clock of the reset clock sequence TR and is thus prepared for converting the charge packet Q of the subsequent pixel, with the output of the analog charge / voltage converter ( 12) first at the respective black reference level U _ref . lies. At the end of a cycle period of the shift cycle sequence T _s , the voltage values U of the pixels scanned in the image template (3), which represent the scanned image information, appear at the output of the charge / voltage converter (12). These are, for example, the voltage value Ui for the first pixel, the voltage value U ₂ for the second pixel and the voltage value U _n for the nth pixel.

In the assumed example with 8000 sensor elements (8) of the photodiode line (6), 8000 voltage values U per scanning line are to be converted analog / digital in the A / D converter (17). With a typical conversion time of the A / D converter (17) of t = 6 μs per voltage value U, according to the prior art there is a sampling time per scanning line of t_ _A = 8000 x 6μs = 48 ms, regardless of whether the original image (3) is scanned in the row direction with a large or a small scanning fineness.

In the method according to the invention, the scanning time is advantageously shortened in the case of a coarser scanning fineness in the line direction in that the number of voltage values U of the pixels of each scanning line to be converted analog / digital is reduced as a function of the desired coarser scanning fineness by At least two adjacent pixels of a scan line are combined and the charge packets Q of the combined pixels are read out in the charge / voltage converter (12) after being read out from the shift register (10), a voltage value U 'representing the image information of the respectively combined pixels. For this purpose, a reduction factor is first determined as the quotient from the number of maximum possible voltage values U per scanning line, which corresponds to the number of sensor elements of the photodiode line, and from the number of voltage values U 'per scanning line resulting from the desired coarser scanning fineness.

The reduction factor determines the number of neighboring pixels to be combined and thus the number of charge packets Q to be added in each case.

The reciprocal reduction factor is a measure of the shortening of the sampling time according to the method according to the invention compared to the sampling time which results from the prior art. In addition, the method according to the invention advantageously ensures that no image information is lost during the scanning of the original and that scanning light sources with a lower light intensity can be used.

The method according to the invention can be implemented in a simple manner by a modified control when reading the charge packets Q from the shift register (10) and when converting the read charge packets Q into voltage values U 'in the analog charge / voltage converter (12).

The cycle period of the modified shift cycle sequence T's is shortened compared to the shift cycle sequence Ts by the reduction factor or the frequency is increased by the reciprocal reduction factor. The same applies to the transfer clock sequence T'τ.

The clock period of the reset clock sequence T'R remains unchanged compared to the reset clock sequence TR according to the prior art and in turn corresponds to the conversion time tw per voltage value of the A / D converter (17) used.

Between every two reset files of the reset clock sequence T ' _{R there} is a number of clock periods corresponding to the reduction factor of the modified shift clock sequence T's, so that now between two reset files a number of charge packets Q corresponding to the reduction factor is read out of the shift register (10) in the analog charge / Voltage converter (12) is added and the added charge packets Q are converted into a voltage value U 'proportional to the charge addition for the combined neighboring pixels. So that the voltage values U 'formed by the addition of the charge packets Q' do not overdrive the charge / voltage converter (12) of the photodiode line (6), which would lead to falsification of the scanned image information, the integration time of the photodiode line (6), ie the duration of exposure of the scanning light on the sensor elements (8) of the photodiode array (6), reduced in accordance with the reciprocal reduction factor. Due to the shortened readout time due to the shortened residence time of the charges Q in the sensor element row (7) and in the shift register (10) and due to the shortened integration time, the noise of the photodiode array (6) is advantageously reduced.

In the assumed example with 8000 sensor elements (8) of the photodiode array (6) and with 4000 pixels to be scanned corresponding to a coarser scanning fineness by a factor of "2", the reduction factor is "2". In this case, the charge packets Q _n and Q _n + ι of two adjacent pixels are added together and a voltage value U _n + U _{n +} ι = U 'proportional to the charge addition is generated for the respectively combined two adjacent pixels. This means that in the A / D converter (17) only 4000 voltage values U 'per scan line can now be converted analog / digital. With the typical conversion time of the A / D converter (18) of tw = 6 μs per voltage value, a sampling time per scanning line of t _A = 4000 x 6μs = 24 ms results, that is, advantageously, a halving of the sampling time per scanning line.

Fig. 3 shows timing diagrams for explaining the control of the scanner according to the invention for a reduction factor "2".

The modified shift clock sequence T's, which is fed to the shift register (10), is shown in the time diagram (FIG. 3A).

The unchanged reset clock sequence T'R, which is fed to the analog charge / voltage converter (12), is shown in a time diagram (3).

The timing diagram (2C) shows the voltage values U 'which are generated in the analog charge / voltage converter (12) by converting the added charge packets Q of the combined pixels according to the invention.

The clock period of the modified shift clock sequence T's is halved compared to the clock period of the shift clock sequence Ts according to the prior art (FIG. 2; time diagram 2A). The clock period of the reset clock sequence T'R is unchanged from the clock period of the reset clock sequence TR according to the prior art (FIG. 2; time diagram B) and in turn corresponds to the conversion time tw per voltage value of the A / D converter (17) used.

There are two clock periods of the modified shift clock sequence T's between each two reset files of the reset clock sequence T ' _R , so that two charge packets Q _n and Q _n + ι are now read out from the shift register (10) between two reset files, in the analog charge / voltage Converter (12) is added and the added charge packets (Q _n + Qn + i) are converted into a voltage value U _n + U _n +1 = U ' _n / n + l of the two combined neighboring pixels n and n + 1 proportional to the charge addition . For example, the charges Qi and Q2 are added for pixels 1 and 2, and the voltage value U'1 / 2 results, while for pixels 3 and 4, the charges Q3 and Q4 are added by the voltage value U'3 / 4 to obtain.

A corresponding reduction in the scanning time for a coarser scanning fineness perpendicular to the line direction is achieved in a simple manner by changing the relative speed between the image template (3) and the photodiode line (6).

Without reducing the clock period of the shift clock sequence Ts, the scanning time per scanning line is not shortened, but by suppressing reset files of the reset clock sequence TR according to the prior art, a corresponding addition of charge packets with the advantages already mentioned is achieved.

Instead of adding the charge packets from combined neighboring pixels, the corresponding charge packets can also be suppressed in the analog charge / voltage converter (12) by a modified residual clock sequence TR, which leads to a shortening of the sampling time, but disadvantageously to loss of Image information would result.

The scanner described is used to scan black and white originals. The scanning method according to the invention can of course also be used in color scanners for scanning color originals. In a first exemplary embodiment of a color scanner, the white light coming from the light source is broken down into red, green and blue scanning light by means of a rotating color filter wheel, and the color template is sequentially illuminated with the colored light. tet. The scanning light is then converted in a monochrome photodiode line into the image signals for "red", "green" and "blue".

In a second exemplary embodiment of a color scanner, the color template is illuminated with white light and the scanning light coming from the color template is divided into three color components by color division using dichroic color filters and three monochrome photo diode lines for conversion into the image signals for "red", "green" and "Blue" fed.

In a third exemplary embodiment for a color scanner, the color template is also illuminated with white light and the colored scanning light coming from the color template is converted into the image signals for "red", "green" and "blue" in a color-selective photodiode array. Corresponding shift registers and A / D converters are then assigned to the individual color channels during color scanning.

It is within the scope of the invention to design the scanning method according to the invention in such a way that a distinction can be made between scanning lines to be evaluated and scanning lines not to be evaluated and within the scanning lines to be distinguished between pixels to be evaluated and pixels not to be evaluated, so that area-related modulation of the reading speed for the shift register he follows. This is the case, for example, if a two-dimensional photodiode array is used instead of a one-dimensional photodiode array.

The invention is also not limited to the field of electronic reproduction technology, but can be used wherever optoelectronic scanning and analog / digital conversion are coupled to one another.

Claims

claims

1. A method for optoelectronic scanning of templates, in which - a template (3) is scanned pixel by row and line by row using a number of optoelectronic converters (8) arranged in the line direction, which convert the scanned image information of the individual pixels into voltage values (U), the voltage values (U) are read out from the optoelectronic converters (7) and the read out voltage values (U) are converted into digital image values in an A / D converter (17), characterized in that the scanning time is reduced in the line direction a coarser fineness with respect to a normal scanning fineness, the number of voltage values (U) of the pixels to be converted analog / digital in the A / D converter (17) per scanning line is reduced by a reduction factor which is the quotient of the number of maximum possible voltage values (U) and the lower number of voltage values (U ') resulting from the coarser scanning fineness.

2. The method according to claim 1, characterized in that the number of voltage values (U) to be converted analog / digital in the A / D converter (17) is in each case reduced by combining a number corresponding to the reduction factor in a scanning line of adjacent pixels and the voltage values (U '), which each result from the voltage values (U) of the combined pixels, are converted analog / digital.

3. The method according to claim 1 or 2, characterized in that the number of maximum possible voltage values corresponds to the number of optoelectronic transducers (8).

4. The method according to any one of claims 1 to 3, characterized in that - the optoelectronic transducers are the sensor elements (8) of a photodiode line (6) which convert the scanned image information of the individual pixels into electrical charge packets (Q), - The photodiode line (6) has a shift register (10) for temporarily storing the charge packets (Q), a transfer gate (9) for transferring the charge packets (Q) from the sensor elements (8) into the shift register (10) and a charge / voltage converter (12) for converting the charge packets (Q) into voltage values, the charge / voltage converter (12) being reset in each case by a clock of a reset clock sequence (TR),

the charge packets (Q) of the individual pixels are read out serially from the shift register (10) by means of a shift clock sequence (T _s ), - a number corresponding to the reduction factor is combined in one scan line of adjacent pixels,

- The read out charge packets (Q) of the respectively combined pixels in the charge / voltage converter (12) are added up and

- The added charge packets (Q) are converted into the voltage values (U ') relevant for the combined pixels.

5. The method according to claim 4, characterized in that the combination of pixels and the addition of the associated charge packets (Q) is in each case achieved by suppressing a number of clock pulses corresponding to the reduction factor of the reset clock sequence (T _R ) with normal scanning fineness.

6. The method according to claim 4 or 5, characterized in that the clock period of the reset clock sequence (TR) is selected equal to the conversion time of the A / D converter (17).

7. The method according to any one of claims 4 to 6, characterized in that the clock period of the shifting clock sequence (Ts) is shortened by a reduction factor in the case of coarser scanning fineness compared to the clock period in the case of normal scanning fineness.

8. The method according to any one of claims 4 to 7, characterized in that the integration time of the photodiode array (6) is shortened to avoid overdriving the photodiode array (6).

9. The method according to any one of claims 1 to 8, characterized in that the coarser scanning fineness perpendicular to the line direction by changing the Relative speed between the image template (3) and optoelectronic transducers (8) perpendicular to the line direction is reached.

10. The method according to any one of claims 1 to 3, characterized in that the optoelectronic transducers (8) are the sensor elements of a photodiode array for the area-like scanning of the template (3).

11. The method according to any one of claims 1 to 10, characterized in that the method is used in the optoelectronic scanning of color documents.