Shading compensation for scanning apparatus

• US 4,314,281 A
• Filed: 10/12/1979
• Issued: 02/02/1982
• Est. Priority Date: 10/12/1979
• Status: Expired due to Term

• Current Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Original Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Inventors: Wiggins, Douglas G., Elie, Pierre Jr.
• Primary Examiner(s): Orsino, Jr., Joseph A.
• Application Number:
  US06/084,035
• Time in Patent Office: 844 Days
• Field of Search: 358/212, 358/213, 358/163, 358/280, 358/282, 358/294, 250/578, 250/211 J, 340/146.3 AG
• US Class Current: 358/406
• CPC Class Codes: H04N 1/401 Compensating positionally unequal response of the pick-up or reproducing head H04N1/403 takes precedence

Abstract

An image input terminal (IIT) with automatic document handler for feeding documents to be scanned into registered position on the IIT platen. The IIT includes a pair of multi-element scanning arrays with cooperating scan lamp and optics on a movable carriage disposed in scanning relationship with the platen. Analog image signals generated by the scanning arrays are initially processed in separate channels and then combined into a serial stream. The stream of analog image signals may optionally be thresholded or screened to provide binary level image signals or converted to multi-bit gray scale.

A control system including microprocessor provides timing and control signals for synchronizing operation of the scanning carriage, document handler, and image signal reading and processing together with electronic crossover between arrays to avoid loss of data, automatic signal gain control, and deletion of bad image signals or pixels.

A defocused calibration strip is scanned to provide shading signals for calibrating the individual scanning elements of the arrays, the shading signals being processed in successive blocks each of which comprises a preset number of shading signals. Following processing, the shading signals are stored in memory and accessed during normal scanning. The shading signals are employed in all operational modes to compensate the image signals produced by the IIT for deficiencies in the scanning components.

Claims

1. A method of compensating for variations in light to which individual image sensors of a scanning array are subjected, the steps comprising:

(a) processing image signals output by said sensors when scanning original images to provide multi-bit digital signals representative of the image gray level in a first operating mode;

(b) processing image signals output by said sensors when scanning original images to provide single bit binary level signals representative of the image in a second operating mode;

(c) producing a separate test signal from each sensor in said array by scanning a test image of predetermined optical density;

(d) segregating said sensors into groups each comprised of a preset number of sensors;

(e) processing said test signals from said sensors in said first operating mode to provide multi-bit test signals;

(f) averaging the test signals from each of the sensors in said sensor groups to provide composite compensating signals for each of said sensor groups;

(g) storing said compensating signals; and

(h) correcting the image signal output of said sensors in both said first and second operating modes with the stored compensating signal for the sensor group within which said sensor is included.

2. The method according to claim 1 including the step of reducing interference with said test signals from dust and defects by defocusing said test image.

3. The method according to claim 1 including the step of modifying said compensating signals by a predetermined constant to offset differences between said first and second operating modes.

4. A method of compensating for variations in light to which individual image sensors of a scanning array are subjected, the steps comprising:

(a) processing image signals output by said sensors when scanning original images to provide multi-bit digital signals representative of the image gray level in a first operating mode;

(b) processing image signals output by said sensors when scanning original images to provide single bit binary level signals representative of the image in a second operating mode;

(c) producing a separate test signal from each sensor in said array by scanning a test image of predetermined optical density;

(d) segregating said sensors into groups each comprised of a preset number of sensors;

(e) processing said test signals from said sensors in said first operating mode;

(f) averaging the test signals from each of the sensors in said sensor groups to provide composite compensating signals for each of said sensor groups;

(g) storing said compensating signals;

(h) correcting the signal output of each sensor with the stored compensating signal for the sensor group within which said sensor is included;

(i) selectively screening or thresholding image signals processed in said second operating mode; and

(j) adjusting the screen and threshold levels with said compensating signals to correct image signals processed in said second operating mode.

5. The method according to claim 4 including the steps of:

(a) converting image signals processed in said first operating mode from analog signals to digital signals; and

(b) adjusting the analog-to-digital signal converting level in response to said compensating signals.

6. In a multi-mode raster scanner having a first image processing mode wherein analog image signals are converted to multi-bit digital signals representative of the image gray level and a second image processing mode wherein analog image signals are converted to single bit binary signals, selector means for selecting one of said first and second image processing modes, a platen for supporting image bearing originals for scanning, and at least one multi-element scanning array movable relative to said platen along a predetermined scan path to scan said platen and any original thereon line by line, the combination of:

(a) a calibrating strip disposed in said scan path, said strip having an image of predetermined optical density, scanning of said strip by said array producing calibration signals;

(b) said selector means routing said calibration signals to said first image processing mode whereby to produce multi-bit digital calibration signals;

(c) means for averaging said calibration signals in blocks, each of said blocks consisting of a predetermined number of said scanning array elements;

(d) a memory for storing said calibration signals;

(e) address means for addressing said memory to access said calibration signals; and

(f) means to combine said calibration signals from said memory with image signals output by said array in each of said first and second image processing modes, said calibration signals being combined with image signals from the blocks of scanning array elements from which said calibration signals are derived.

7. The scanner according to claim 6 in which said combining means includes analog-to-digital converting means for each of said first and second image processing modes.

8. In a raster scanner having a platen for supporting image bearing originals for scanning, at least one array for line scanning an original on said platen, said array being comprised of a plurality of photosensors producing analog image signals representative of the image area scanned, and analog-to-digital converting means for converting said analog image signals to digital image signals, the combination of:

(a) a calibration image, scanning of said calibration image by said array producing analog calibration signals;

(b) means for feeding said analog calibration signals to said analog-to-digital converting means;

said analog-to-digital converting means converting said analog calibration signals to digital calibration signals;

(c) means for averaging groups of said digital calibration signals, each of said groups comprising digital calibration signals from a preset number of said array photosensors;

(d) digital-to-analog conversion means for converting the averaged calibration signals to analog signals;

(e) compensating means for compensating the image signals produced by said array with said averaged calibration signals;

(f) thresholder means for processing said analog image signals produced by said array to provide binary image signals,

(g) selector means for routing said analog image signals produced by said array to said analog-to-digital conversion means to provide digital image signals or to said thresholder means to provide binary image signals selectively, and

(h) means for inputting said averaged calibration signals to said analog-to-digital conversion means or said thresholder means to modify image signals input to said analog-to-digital conversion means and to said thresholder means.

9. The raster scanner according to claim 8 in which said calibration image is out of focus relative to said array to enhance the calibration signals obtained.

10. In a raster scanner having a platen for supporting image bearing originals for scanning, at least one array for line scanning an original on said platen, said array being comprised of a plurality of photosensors and producing analog image signals representative of the image area scanned, and analog-to-digital converting means for converting said analog image signals to digital image signals, the combination of:

(a) a calibration image, scanning of said calibration image by said array producing analog calibration signals;

(b) means for feeding said analog calibration signals to said analog-to-digital converting means;

said analog-to-digital converting means converting said analog calibration signals to digital calibration signals;

(c) means for averaging groups of said digital calibration signals, each of said groups comprising digital calibration signals from a preset number of said array photosensors;

(d) digital-to-analog conversion means for converting the averaged calibration signals to analog signals;

(e) compensating means for compensating the image signals produced by said array with said averaged calibration signals;

(f) thresholder means for processing said analog image signals produced by said array to provide binary image signals,

(g) selector means for routing said analog image signals produced by said array to said analog-to-digital conversion means to provide digital image signals or to said thresholder means to provide binary image signals selectively, and

(h) means for inputting said averaged calibration signals to said analog-to-digital conversion means or said thresholder means to modify image signals input to said analog-to-digital conversion means and to said thresholder means,

said thresholder means including a screen for screening said image signals and a thresholder for thresholding said image signals, and second selector means to route said image signals to one of said screen and thresholder selectively.

11. In a raster scanner having a platen for supporting image bearing originals for scanning, at least one array for line scanning an original on said platen, said array being comprised of a plurality of photosensors and producing analog image signals representative of the image area scanned, and analog-to-digital converting means for converting said analog image signals to digital image signals, the combination of:

(a) a calibration image, scanning of said calibration image by said array producing analog calibration signals;

(b) means for feeding said analog calibration signals to said analog-to-digital converting means;

said analog-to-digital converting means converting said analog calibration signals to digital calibration signals;

(c) means for averaging groups of said digital calibration signals, each of said groups comprising digital calibration signals from a preset number of said array photosensors;

(d) digital-to-analog conversion means for converting the averaged calibration signals to analog signals;

(e) compensating means for compensating the image signals produced by said array with said averaged calibration signals;

(f) thresholder means for processing said analog image signals produced by said array to provide binary image signals,

(g) selector means for routing said analog image signals produced by said array to said analog-to-digital conversion means to provide digital image signals or to said thresholder means to provide binary image signals selectively,

(h) means for inputting said averaged calibration signals to said analog-to-digital conversion means or said thresholder means to modify image signals input to said analog-to-digital conversion means and to said thresholder means;

(i) said thresholder means including digital-to-analog conversion means for converting digital threshold signals to analog threshold signals, said averaged calibration signals being input to said digital-to-analog conversion means to bias said digital-to-analog conversion means and modify the threshold level output by said digital-to-analog conversion means; and

(j) proportionality means for correlating the average calibration signals input to said analog-to-digital conversion means and said digital-to-analog conversion means with the operating characteristics of said analog-to-digital conversion means and said digital-to-analog conversion means.