United States Patent
`Webb et al.
`
`[19]
`
`[54] VARIABLE OPTICAL SAMPLING RATE
`DEPENDENT ON REQUESTED SCAN
`RESOLUTION
`
`[75]
`
`Inventors: Steven L. Webb, Loveland; Greg A.
`Degi, pom Comm, both of C010‘
`
`[73] Assignee: Hewlett-Packard Company, Palo Alto,
`Calif.
`
`[21] APP1- N0-5 3391107
`[22]
`Filed:
`Nov. 14, 1994
`
`
`
`Int. Cl.“
`[51]
`[52] U.S. Cl.
`
`.............................................. H01L 27/00
`............... 250/208.1; 250/234; 348/297;
`358/482
`[58] Field of Search ..................................... 348/362, 294,
`348/297; 358/482, 474, 483; 250/2081,
`234, 235, 236
`
`[56]
`
`References Cited
`
`'
`
`4,591,727
`5,032,712
`5,047,871
`
`U's‘ PATENT DOCUMENTS
`250/578
`5/1986 Gaebelein er al.
`250/208.1
`7/1991 Ito .............. ..
`
`........................... 358/486
`9/1991 Meyer et a1.
`
`|I||||I|I|I|||I|l|||I|IIIIIIIIII||||||l|I|||||IIl|I|I||||||||||||||||||I||I
`US005489772A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,489,772
`Feb. 6, 1996
`
`11/1993 Curbelo et al.
`5,265,039
`8/1994 Boyd 21 al.
`.
`5,336,878
`5,373,372 12/1994 Loewen ................................... 358/486
`OTHER PUBLICATIONS
`
`......................... 364/574
`
`United States Patent Application Ser. No. 08/174,868 fllcd
`Dec. 29, 1993 for Procedure for Reducing Time for Image
`Elements by Combining Chafgc Of Adjacent Pixels H110 3
`New Composite Pixel of Greg A. Degi.
`Primary Examir/er——Edward P. Westin
`Assistant Examz'ner~Stephen Calogero
`[57]
`ABSTRACT
`
`A memod of determjning an exposure time for a photosensor
`based on a desired resolution along a scan direction and 2.
`desired resolution in a cross direction may comprise the
`steps of determining an initial exposure time based on the
`desired resolution in the cross direction; determining a
`minimum resolution in the scan direction based on the initial
`exposure time; comparing the minimum resolution in the
`scan direction to the desired resolution in the scan direction;
`and increasing the initial exposure time if the minimum
`resolution in the scan direction is greater than the desired
`resolution in the scan direction.
`
`20 Claims, 2 Drawing Sheets
`
` DETECT AND STORE USER SELECTED RESOLUTIONS
`
`IN BOTH SCAN AND CROSS DIRECTIONS
`
`
`
`
`
`DETERMINE INITIAL EXPOSURE TIME BASED
`ON SELECTED RESOLUTION IN CROSS DIRECTION
`
`
`
`
`
`
`MINIMUM RESOL.s
`YES
`SELECTED RESOLUTION
`
`
`
`EXPOSURE TIME =
`
`MAX. EXPOSURE TIME
`
`
`
`
`INCREASE
`DETERMINE SCAN
`
`EXPOSURE TIME
`LINE SWEEP RATE
`
`AND INITIATE
`SCANNING
`
`
`
`
`Apple 1048
`
`U.S. Pat. 6,470,399
`
`Apple 1048
`U.S. Pat. 6,470,399
`
`

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`5,489,772
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`.5528
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`.22:
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`~.0~k
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`U.S.Patent
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`Feb.6,1996
`
`Sheet1of2
`
`

`
`U.S. Patent
`
`Feb. 6, 1996
`
`Sheet 2 of 2
`
`5,489,772
`
`54
`
`58
`
`DETECT AND STORE USER SELECTED RESOLUTIONS
`
`IN BOTH SCAN AND CROSS DIRECTIONS
`
`DETERMINE INITIAL EXPOSURE TIME BASED
`ON SELECTED RESOLUTION IN CROSS DIRECTION
`
`DETERMINE MINIMUM RESOLUTION IN SCAN DIRECTION
`
`56
`
`MINIMUM RESOL.é
`SELECTED RESOLUTION
`
`YES
`
`
`
`
`EXPOSURE TIME= _
`MAX. EXPOSURE TIME
`
`
`
` INCR EASE
`LINE SWEEP RATE
`EXPOSURE TIME
`
`DETERMINE SCAN
`
`
`
`AND INITIATE
`
`SCANNING
`
`

`
`1
`VARIABLE OPTICAL SAMPLING RATE
`DEPENDENT ON REQUESTED SCAN
`RESOLUTION
`-
`
`BACKGROUND
`
`The present invention relates to image scanner technology
`in general and more specifically to a method for varying the
`optical sampling rate of an image scanner.
`Optical scanners generate data signals representative of an
`object or document by projecting an image of the object or
`document onto an optical photosensor array. The data sig-
`nals may then be digitized and stored for later use. For
`example,
`the data signals may be used by a personal
`computer to produce an image of the scanned object or
`document on a suitable display device.
`Most optical scanners use illumination and optical sys-
`tems to illuminate the object and focus a small area of the
`illuminated object, usually referred to as a “scan line,” onto
`the optical photosensor array. The entire object is then
`scanned by sweeping the illuminated scan line across the
`entire object, either by moving the object with respect to the
`illumination and optical assemblies or by moving the illu-
`mination and optical assemblies relative to the object.
`A typical scanner optical system will
`include a lens
`assembly to focus the image of the illuminated scan line onto
`the surface of the optical photosensor array. Depending on
`the particular design, the scanner optical system may also
`include a plurality of mirrors to “fold” the path of the light
`beam, thus allowing the optical system to be conveniently
`mounted within a relatively small enclosure. In order to
`allow a smaller photosensor array to be used, most optical
`systems also reduce the size of the image of the scan line that
`is focused onto the surface of the photosensor. For example,
`many optical systems have a lens reduction ratio of about
`8:1, which reduces the size of the image of the scan line by
`a factor of about 8.
`
`While various types of photosensor devices may be used
`in optical scanners, a commonly used sensor is the charge
`coupled device or CCD. As is well-known, a CCD may
`comprise a large number of individual cells or “pixels,” each
`of which collects or builds-up an electrical charge in
`response to exposure to light. Since the size of the accumu-
`lated electrical charge in any given cell or pixel is related to
`the intensity and duration of the light exposure, a CCD may
`be used to detect light and dark spots on an image focused
`thereon. In a typical scanner application, the charge built up
`in each of the CCD cells or pixels is measured and then
`discharged at regular intervals known as exposure times or
`sampling intervals, which may be about 5 milliseconds or so
`for a typical scarmer. Since the charges (i.e., image data) are
`simultaneously collected in the CCD cells during the expo-
`sure time, the CCD also includes an analog shift register to
`convert the simultaneous or parallel data from the CCD cells
`into a sequential or serial data stream. A typical analog shift
`register comprises a plurality of “charge transfer buckets”
`each of which is connected to an individual cell. At the end
`of the exposure time, the charges collected by each of the
`CCD cells are simultaneously transferred to the charge
`transfer buckets, thus preparing the CCD cells for the next
`exposure sequence. The charge in each bucket
`is then
`transferred from bucket to bucket out of the shift register in
`a sequential or “bucket brigade” fashion during the time the
`CCD cells are being exposed to the next scan line. The
`sequentially arranged charges from the CCD cells may then
`be converted, one-by-one, into a digital signal by asuitable
`analog-to-digital converter.
`
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`5,489,772
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`2
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`In most optical scanner applications, each of the indi-
`vidual pixels in the CCD are arranged end-to-end,
`thus
`forming a linear array. Each pixel in the CCD array thus
`corresponds to a related pixel portion of the illuminated scan
`line. The individual pixels in the linear photosensor array are
`generally aligned in the “cross” direction, i.e., perpendicular
`to the direction of movement of the illuminated scan line
`across the object (also known as the “scan direction”). Each
`pixel of the linear photosensor array thus has a length
`measured in the cross direction and a width measured in the
`scan direction. In most CCD arrays the length and width of
`the pixels are equal, typically being about 8 microns or so in
`each dimension.
`
`The resolution in the cross direction is a function of the
`number of individual cells in the CCD. For example, a
`commonly used CCD photosensor array contains a suflicient
`number of individual cells or pixels to allow a resolution in
`the cross direction of about 600 pixels, or dots, per inch
`(dpi), which is referred to herein as the “native resolution in
`the cross direction.”
`
`The resolution in the scan direction is inversely related to
`the product of the scan line sweep rate and the CCD
`exposure time (i.e., the sampling interval). Therefore, the
`resolution in the scan direction may be increased by decreas-
`ing the scan line sweep rate, the CCD exposure time, or both.
`Conversely,
`the resolution in the scan direction may be
`decreased by increasing the scan line sweep rate, the CCD
`exposure time, or both. The “minimum resolution in the scan
`direction” for a given exposure time is that resolution
`achieved when scanning at the maximum scan line sweep
`rate at that exposure time. For example, a maximum scan
`line sweep rate of about 3.33 inches per second and a
`maximum exposure time of about 5 milliseconds will result
`in a minimum resolution in the scan direction of about 60
`dpi.
`The resolution in the cross direction may be decreased
`below the native resolution in the cross direction by using
`any one of a number of pixel dropping algorithms to ignore,
`or drop, data from certain cells in the CCD. For example, the
`resolution in the cross direction in a CCD having a native
`resolution of 600 dpi may be decreased to 300 dpi by
`ignoring or dropping data from every other pixel. Most
`commonly used pixel dropping techniques ignore or drop
`the pixel data after it has been converted into a digital signal
`by the analog-to-digital converter. It is also possible to
`increase the resolution in the cross direction over the native
`resolution in the cross direction by using various data
`interpolation techniques to increase the effective resolution
`in the cross direction. For example, some data interpolation
`techniques can be used to increase the effective resolution in
`the cross direction to 1200 dpi or more with a CCD having
`a native resolution in the cross direction of only 600 dpi.
`As mentioned above, the resolution in the scan direction
`is a function of the scan line sweep rate as well as the CCD
`exposure time. Therefore, the resolution in the scan direction
`can be varied by changing the scan line sweep rate, the CCD
`exposure time, or both. It should be noted that resolution in
`the scan direction corresponding to a given maximum scan
`line sweep rate and CCD exposure time is fixed and repre-
`sents the minimum resolution in the scan direction for that
`exposure time. However, the resolution in the scan direction
`may be further reduced by ignoring or dropping whole lines
`of data. Such line dropping techniques are analogous to the
`pixel dropping techniques described above.
`One problem associated with scanners that drop pixels to
`decrease the resolution in the cross direction, or drop lines
`
`

`
`5,489,772
`
`3
`to decrease the resolution in the scan direction, or both, is
`that the pixel and line dropping processes tend to introduce
`various artifacts and distortions into the image data, such as
`alising and moire patterns.
`Another problem associated with pixel and line dropping
`processes is that the pixel and line dropping functions are
`usually performed after the charge data from the individual
`CCD cells have been converted into digital form. Conse-
`quently, the maximum sampling rate, thus scanning speed, is
`limited to the data conversion rate of the analog to digital
`(A/D) converter. Since most scanners operate at the maxi-
`mum effective sampling rate of the A/D converter,
`the
`scanning rate when scanning at reduced resolution is essen-
`tially the same as when scanning at maximum resolution.
`Put in other words, selecting a decreased resolution will not
`usually result in an increased scan rate.
`Even if faster analog to digital converters are used, there
`is a limit to the maximum scan rate that can be achieved. For
`
`example, the provision of a faster analog-to-digital converter
`will allow faster scan rates at a given resolution only if the
`exposure time (i.e., sampling interval) is decreased. How-
`ever, since the amount of charge produced by a given CCD
`cell is proportional to the exposure time, shorter exposure
`times will result
`in proportionally lower signal
`levels.
`Assuming constant system noise, such lower signal levels
`will yield a lower signal to noise ratio, which noise usually
`appears in the image data as “snow.”
`Therefore, there remains a need for an image scanner that
`can scan at a wide range of resolutions but without the image
`degradation problems, such as alising, and the generation of
`moire patterns, that are typically associated with the line
`dropping processes typically used in currently available
`scanners. Ideally,
`the selection of a decreased scanning
`resolution should also result in a corresponding increase in
`scanning speed, but without the need to resort to expensive,
`high-speed analog-to-digital converters. and without reduc-
`ing the signal-to-noise ratio of the resulting image data
`signal.
`
`SUMMARY OF THE INVENTION
`
`A method of determining an exposure time for a photo-
`sensor based on a desired resolution along a scan direction
`and a desired resolution in a cross direction may comprise
`the steps of determining an initial exposure time based on
`the desired resolution in the cross direction; determining a
`minimum resolution in the scan direction based on the initial
`exposure time; comparing the minimum resolution in the
`scan direction to the desired resolution in the scan direction;
`and increasing the initial exposure time if the minimum
`resolution in the scan direction is greater than the desired
`resolution in the scan direction.
`
`BRIEF DESCRIPTION OF THE DRAWING
`
`Illustrative and presently preferred embodiments of the
`invention are shown in the accompanying drawing in which:
`FIG. 1 is a schematic diagram of a data sampling and
`conversion circuit for selectively combining and digitizing
`pixel charges from a CCD; and
`FIG. 2 is a flow chart of the steps performed by the control
`unit shown in FIG. 1 to determine an appropriate CCD
`exposure time (i.e., sampling rate).
`DETAILED DESCRIPTION OF THE
`INVENTION
`
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`rate dependent on requested scan resolution according to the
`present invention is best seen in FIG. 1. Essentially, the data
`sampling and conversion circuit 10 comprises a conversion
`circuit 12 for converting pixel charges stored in a photosen-
`sor, such as a charge-coupled-device (CCD) 14 into a digital
`data stream 16. As is well-known, CCD 14 includes 11 charge
`transfer buckets Q1, Q2, Q3, through Q,,, which sequentially
`transfer the charges that were simultaneously collected from
`each of the individual pixels in the CCD. A charge-to-
`voltage converter 18 connected to the last charge transfer
`bucket Q, of CCD 14 converts the charge stored in the last
`bucket Q1 into a voltage, which voltage is then converted
`into a digital signal by an analog to digital (A/D) converter
`22. When scanning at certain resolutions, the converter 18
`converts into a voltage only the charge from one of the
`buckets Q, which, of course, corresponds to the charge from
`a single pixel. However, when scanning at other resolutions,
`the voltage converter circuit 18 collects the charge from two
`or more buckets Q before converting the total charge into a
`voltage. A hold circuit 20 may be connected to the voltage
`converter circuit 18 to act as a buffer in the event data are
`shifted out of the last bucket Q1 faster than they can be
`converted into digital signals by the A/D converter 22. A
`control unit 24 connected to the CCD array 14, charge-to-
`voltage converter 18, hold circuit 20, and A/D converter 22
`controls the timing and operation of each circuit. While any
`circuit may be used that accomplishes the functions of the
`data sampling and conversion circuit 10 shown and
`described herein, the data sampling and conversion circuit
`may be of the type disclosed in U.S. patent application Ser.
`No. 08/174,868 of Degi and filed on Dec. 29, l993, which
`is incorporated herein by reference for all that it discloses.
`When scanning at the “native” resolution in the cross
`direction and at the minimum resolution in the scan direc-
`tion, the control unit 24 operates the CCD at a predetermined
`maximum exposure time (i.e.,
`sampling interval) and
`sweeps the scan line over the object at a predetermined
`maximum scan sweep rate. When operating at the maximum
`exposure time and scan line sweep rate, a single scan line
`comprises data from each and every pixel of the CCD 14,
`and the resolution in the scan direction corresponds to the
`minimum resolution in the scan direction. When operating at
`other resolutions, however, the control unit 24 determines
`the optimum exposure time and the number of CCD cells for
`which charge data should be combined in order to maximize
`the scan line sweep rate, but without substantially reducing
`the signal to noise ratio and without having to resort to line
`dropping techniques.
`A significant advantage of the present invention is that it
`allows scanning over a wide range of resolutions, but
`without the need to resort to line dropping processes, which
`can create alising or moire patterns in the image data.
`Moreover, since the data sampling and conversion circuit 10
`may combine the signals from several CCD pixels before
`they are digitized, scanning at certain resolutions may allow
`a proportionate decrease in the number of analog to digital
`conversions that need to be performed by the A/D converter
`22. This excess conversion capacity allows the CCD expo-
`sure time (i.e., sampling interval) to be reduced, which may
`be followed by a corresponding increase in scan sweep
`speed at a given resolution. Unlike prior scanners,
`the
`decreased exposure times made possible by the present
`invention may result in only a slight reduction the signal to
`noise ratio of the CCD, since the signals from several pixels
`are combined when scarming at such decreased exposure
`times.
`
`The data sampling and conversion circuit 10 of an optical
`scanner (not shown) having the variable optical sampling
`
`The details of the data sampling and conversion circuit 10
`are best understood by referring again to FIG. 1. As was
`
`

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`5,489,772
`
`described briefly above, the data sampling and conversion
`circuit 10 includes a conversion circuit 12 connected to the
`last charge transfer bucket Q1 of CCD array 14. Conversion
`circuit 12 includes charge-to-voltage converter circuit 18
`and hold circuit 20 which together convert the charges from
`one or more of the charge transfer buckets Q, thus individual
`cells in the CCD array 14, into an analog voltage which is
`then converted into a digital signal by analog to digital
`converter 22. The charge-to-voltage converter circuit 18
`comprises a capacitor 30 for collecting charge from the last
`charge transfer bucket Q1 and an isolation amplifier 26 to
`isolate capacitor 30 and prevent it from being loaded by the
`hold circuit 20. More specifically,
`the input line 32 of
`isolation amplifier 26 is connected to the last bucket Q1 via
`switch 28. The capacitor 30 is connected between the input
`line 32 and a suitable ground. A second switch 34 is also
`connected to input line 32 and a suitable ground and, when
`closed, discharges capacitor 30, preparing it to receive the
`next charge from charge transfer bucket Q1.
`The hold circuit 20 is similar to the converter circuit 18
`and comprises a capacitor 44 for storing the output voltage
`from isolation amplifier 26. Hold circuit 20 includes its own
`isolation amplifier 36 to isolate capacitor 44 and prevent it
`from being loaded by analog-to-digital (A/D) converter 22.
`More specifically, the input line 38 of isolation amplifier 36
`is connected to the output 46 of amplifier 26 via a third
`switch 40. The capacitor 44 is connected between the input
`line 38 and a suitable ground. The output line 42 of amplifier
`36 is connected to the analog to digital converter 22.
`A control unit 24 connected to the CCD 14, converter
`circuit 18, hold circuit 20, and A/D converter 22 controls the
`operation of the respective components. Initially, switches
`28 and 40 are open and switch 34 is closed, which eliminates
`any charge on capacitor 30. Next, the control unit 24 closes
`switch 28 and opens switch 34, and then shifts the pixel
`charges stored in buckets Q1—Q,, one bucket to the right.
`This places the charge that was in bucket Q1 onto capacitor
`30. The resulting voltage V on capacitor 30 is equal to the
`amount of charge Q from bucket Q1 divided by the capaci-
`tance C of capacitor 30. That is, V=Q/C.
`Once the capacitor 30 is charged, control unit 24 closes
`switch 40. The output voltage from isolation amplifier 26 is
`then duplicated on capacitor 44. Once capacitor 44 is
`charged, the control unit 24 opens switch 40, thus allowing
`the converter circuit to accept another charge. In the mean-
`time, the output voltage of amplifier 36 is converted into a
`digital signal by the A/D converter 22. The digitized data
`from the A/D converter 22 may then be processed as desired,
`ultimately resulting in a digital representation of the source
`image.
`Note that the processing of the charge in the next bucket
`can begin while the charge from the previous bucket is being
`converted into a digital signal. However, the conversion
`process must be completed before switch 40 is again closed.
`Of course, the hold circuit 20 would not be required if the
`A/D converter 22 is fast enough to digitize the voltage
`signals before the charge from the next bucket is shifted into
`capacitor 30.
`When scanning at certain reduced resolutions, the control
`unit 24 controls voltage converter circuit 18 in such a way
`that it combines the charges from two or more pixel buckets
`before sending the signal to hold circuit 20. As an example,
`the charge from the first pixel is clocked into capacitor 30 in
`the manner already described. Then, before closing switch
`40, and without closing switch 34 to discharge capacitor 30,
`the control unit 24 clocks into capacitor 30 the charge from
`
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`the second pixel. When the desired number of pixels have
`been combined (i.e., two or more), the output voltage across
`capacitor 30 is duplicated in the hold circuit 20, which
`voltage is then digitized by A/D converter 22.
`While the data sampling and conversion circuit 10 may be
`operated in the manner described above to accomplish
`scanning at reduced resolutions in the cross direction,
`increased scan sweep rates at such reduced resolutions can
`only be accomplished by decreasing the exposure time (i.e.,
`sampling interval) of the CCD 14. However, if the reduced
`exposure time is not properly selected, the resolution in the
`scan direction may not correspond to the desired resolution
`in the scan direction, thus making it necessary to drop lines
`to achieve the desired resolution. To illustrate the problem,
`consider a scanner having a native resolution in the cross
`direction of 600 dpi and a minimum resolution in the scan
`direction of 60 dpi. Suppose the operator desires to scan at
`300 dpi in the cross direction and 100 dpi in the scan
`direction. If so, the control unit 24 would operate the data
`sampling and conversion circuit 10 so that the charges from
`two pixel elements would be combined together by the
`converter circuit 18. As explained above, combining the
`charges from two pixels in a CCD having a native cross
`resolution of 600 dpi yields an effective resolution in the
`cross direction of 300 dpi. In order to increase the scanning
`speed, the control unit would also shorten the exposure time
`of the CCD array to one half of the maximum exposure time,
`i.e., the exposure time corresponding to the native resolution
`in the cross direction. Of course, so shortening the exposure
`time has the effect of doubling the minimum resolution in
`the scan direction from 60 dpi to 120 dpi. However, since the
`user selected a resolution in the scan direction of only 100
`dpi, the image processing circuit (not shown) would then
`have to resort to line dropping processes to decrease the
`resolution in the scan direction from 120 dpi to 100 dpi. As
`mentioned above, such line dropping processes are undesir-
`able since they can result in alising and moire patterns.
`The present invention avoids the need to resort to line
`dropping processes in cases analogous to the above example
`by directing the control unit 24 to operate according to the
`process illustrated in FIG. 2. In the first step 50, the control
`unit 24 detects and stores in a suitable memory system (not
`shown) the desired (i.e, user selected) resolutions in both the
`cross direction and the scan direction. In process 52, the
`control unit 24 determines an initial exposure time based on
`the selected resolution in the cross direction. For example, if
`the selected resolution in the cross direction is between 1/2
`and twice the native resolution in the cross direction, the
`control unit 24 will set the initial exposure time to be equal
`to the native or maximum exposure time, i.e., that exposure
`time corresponding to the native resolution in the cross
`direction. If the selected resolution in the cross direction is
`between 1/3 and 1/2 of the native resolution in the cross
`direction, the control unit will set the initial exposure time
`to be 1/2 the maximum exposure time. If the selected reso-
`lution is less than 1/3 of the native resolution, the control unit
`24 will set the initial exposure time to be 1/3 the maximum
`exposure time.
`
`the
`Having thus determined an initial exposure time,
`control ur1it then performs step 54 to determine the corre-
`sponding or minimum resolution in the scan direction. As
`used herein,
`the term “minimum resolution in the scan
`direction” refers to that resolution in the scan direction
`resulting from a given scan line sweep rate and CCD
`exposure time. The term “minimum” is used since that
`resolution is the minimum resolution in the scan direction
`
`that can be achieved without resorting to line dropping
`
`

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`7
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`5,489,772
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`processes. The control unit 24 next performs step 56 to
`determine whether the minimum resolution in the scan
`
`direction is less than or equal to the resolution that was
`selected by the user. If so, then the control unit 24 then
`proceeds to step 58 to determine the scan line sweep rate and
`initiate scanning. The scan line sweep rate is determined by
`dividing l by the product of the desired resolution in the
`scan direction and the exposure time. If the minimum
`resolution in the scan direction is greater than the selected
`resolution, then the control unit performs step 59 to deter-
`mine whether the exposure time is equal to the maximum
`exposure time. If it is, the exposure time cannot be further
`increased and the control unit 24 proceeds to step 58. If,
`however, the exposure time is still less than the maximum
`exposure time, the control unit 24 will perform step 60 to
`increase the exposure time by one step, then repeat steps 54
`and 56.
`
`In the embodiment shown and described above, the expo-
`sure time may be one of three times, the exposure time
`corresponding to the native resolution in the cross direction,
`i.e., the maximum exposure time, ‘/2 the maximum exposure
`time, and ‘/3 the maximum exposure time. Thus, if the initial
`exposure time was determined in step 54 to be ‘/3 the
`maximum exposure time, and that initial exposure time did
`not result in a minimum resolution in the scan direction that
`
`was less than the user selected resolution, then step 60 will
`result in the control unit selecting the next highest exposure
`time, i.e., ‘/2 the maximum exposure time. The control unit
`24 then performs step 54 to determine the minimum reso-
`lution in the scan direction by dividing 1 by the product of
`the maximum scan line sweep rate and the new exposure
`time. If the minimum resolution is then less than the user
`selected resolution, the control unit will proceed with the
`scanning operation at the new exposure time. If the num-
`mum resolution is still greater than the user selected reso-
`lution, the control unit 24 again executes steps 59 and 60 to
`increase the exposure time by one step, and repeats steps 54
`and 56.
`
`By way of example, the process shown in FIG. 2 and
`described above yields the exposure times corresponding the
`resolutions shown in Table l. The resolutions shown in
`Table l are based on a scarmer having minimum resolution
`in the scan direction of 60 dpi and a native resolution in the
`cross direction of 600 dpi. The scanner is also capable of
`scanning at resolutions corresponding to 1/2 and 1/3 the native
`resolution of 600 dpi, i.e., 300 dpi and 200 dpi.
`
`TABLE 1
`
`
`Requested Scan
`
`Requested Cross Resolution
`
`Resolution
`1200-301
`300-201
`200-12
`
`
`200 dpi
`300 dpi
`600 dpi
`1200-180
`300 dpi
`300 dpi
`600 dpi
`179-121
`
`
`
`600 dpi 600 dpi120-12 600 dpi
`
`For example, suppose a user desires to scan at a resolution
`in the scan direction of 150 dpi and in the cross direction of
`250 dpi. The control unit 24 will then operate the CCD at an
`effective resolution in the cross direction of 300 dpi. That is,
`the conversion circuit 12 (FIG. 1) will combine the charge
`data from every other CCD pixel and the control unit 24 will
`set the exposure time to ‘/2 the mzurimum exposure time. This
`will result in a resolution in the cross direction of 300 dpi
`and a minimum resolution in the scan direction of 120 dpi.
`The image data processing circuit (not shown) will then drop
`pixels as necessary to achieve the desired resolution in the
`
`10
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`cross direction of 250 dpi. However, since the minimum
`resolution in the scan direction is now 120 dpi, there is no
`need to drop lines, and the desired resolution of 150 dpi in
`the scan direction can be achieved by determining a new
`scan line sweep rate, which is equal
`to l/(the desired
`resolution in the scan direction times the CCD exposure
`time).
`It should kept in mind that while the embodiment shown
`and described herein is capable of selecting between CCD
`exposure times corresponding to one of three (3) discrete
`resolutions,
`i.e., 600 dpi, 300 dpi,‘ and 200 dpi, other
`embodiments are possible that would select between three or
`more exposure times, as would be obvious to persons having
`ordinary skill in the art. For example, the data sampling and
`conversion circuit 10 could be easily adapted to select
`between four (4) exposures times corresponding to one of
`four (4) discrete resolutions. Therefore, the present inven-
`tion should not be regarded as limited to exposure times
`corresponding to only three discrete resolutions.
`It
`is contemplated that
`the inventive concepts herein
`described may be variously otherwise embodied and it is
`intended that the appended claims be construed to include
`alternative embodiments of the invention except insofar as
`limited by the prior art.
`What is claimed is:
`
`1. A method of determining an exposure time for a
`photosensor based on a desired resolution along a scan
`direction and a desired resolution in a cross direction,
`comprising the steps of:
`determining an initial exposure time based on the desired
`resolution in the cross direction;
`determining a minimum resolution in the scan direction
`based on said initial exposure time;
`comparing said minimum resolution in the scan direction
`to the desired resolution in the scan direction; and
`increasing said initial exposure time to an increased
`exposure time and redeterrnining a minimum resolution
`in the scan direction based on said increased exposure
`time and recomparing said minimum resolution in the
`scan direction to the desired resolution in the scan
`direction until either: a) said minimum resolution in the
`scan direction is not greater than the desired resolution
`in the scan direction or b) said increased exposure time
`is equal to a predetermined maximum exposure time.
`2. The method of claim 1, wherein the step of determining
`a minimum resolution in the scan direction based on said
`initial exposure time includes the step of multiplying said
`initial exposure time by a predetermined maximum scan line
`sweep rate.
`3. The method of claim 2, wherein the step of determining
`an initial exposure time based on the desired resolution in
`the cross direction includes the steps of:
`comparing the desired resolution in the cross direction
`with a predetermined native resolution in the cross
`direction;
`
`setting the initial exposure time to be equal to the prede-
`termined maximum exposure time if the desired reso-
`lution in the cross direction is not less than the prede-
`termined native resolution in the cross direction; and
`setting the initial exposure time below the predetermined
`maximum exposure time if the desired resolution in the
`cross direction is less than the predetermined native
`resolution in the cross direction.
`
`4. The method of claim 3, wherein said initial exposure
`time can be selected to be the predetermined maximum
`exposure time divided by an integer.
`
`

`
`9
`
`10
`
`5,489,772
`
`5. The method of claim 4, wherein the step of setting the
`initial exposure time below the predetermined maximum
`exposure time comprises the steps of:
`setting the initial exposure time to be equal to 1/2 the
`predetermined maximum exposure time if the desire