Method and apparatus for providing sensor compensation

• US 5,285,293 A
• Filed: 09/16/1992
• Issued: 02/08/1994
• Est. Priority Date: 01/25/1990
• Status: Expired due to Term

• Current Assignee: Hewlett-Packard Company (HP Inc.)
• Original Assignee: Hewlett-Packard Company (HP Inc.)
• Inventors: Webb, Steven L., Beeman, Edward S., Gennetten, Kenneth D., Miller, Craig L.
• Primary Examiner(s): Coles, Sr., Edward L.
• Assistant Examiner(s): Stoll, Thomas L.
• Application Number:
  US07/946,256
• Time in Patent Office: 510 Days
• Field of Search: 358/213.15, 358/213.16, 358/445, 358/460, 358/461, 358/464, 358/471, 358/406
• US Class Current: 358/471
• CPC Class Codes: H04N 1/401 Compensating positionally unequal response of the pick-up or reproducing head H04N1/403 takes precedence

Abstract

A document scanner provides compensation for sensitivity nonuniformity that may exist between the individual sensor cells of the sensor array that is used to sense light reflected from a document being scanned. Compensated is provided for both dark (i.e. unilluminated) sensitivity variation and light (i.e. illuminated) sensitivity variation that may exist in the individual sensor cells of the sensor array.

Claims

1. Document scanner apparatus comprising,

platen means for supporting a document to be scanned,

light source means for illuminating a document on said platen means,

light sensitive array means, associated with said platen means and arranged for movement relative to a document, for receiving light from a document being scanned, said array means comprising a plurality of individual sensor cells, each of which provides an analog output signal in response to light received from a document during a scan thereof,

target means, positioned relative to said sensor array means so as to be illuminated by said light source means, for providing a surface of substantially uniform reflectivity,

first means for determining the dark response analog output signal of each of said sensor cells when said array means views said target means in a nonilluminated state, and for providing a dark response digital output signal for each sensor cell as a result thereof, said first means including means responsive to said dark response analog output signal for calculating said dark response digital output signal as first compensation data bits for each of said sensor cells, said first compensation data bits being representative of said dark response analog output signal,

second means for determining the light response analog output signal of each of said sensor cells when said array means views said target means in an illuminated state, and for providing a light response digital output signal as a result thereof, said second means including means responsive to said light response analog output signal for calculating said light response digital output signal as second compensation data bits for each of said sensor cells, said second compensation data bits being representative of said light response analog output signal,

data storage means, comprising a random access memory (RAM),

means for supplying said first and second compensation bits to said data storage means to thereby store a compensation word for each of said sensor cells, and

signal processing means, comprising:

(a) analog to digital convertor means comprising control input means for receiving from said data storage means compensation words corresponding to each of said sensor cells and signal input means for receiving analog output signals from said sensor cells, said control input means comprising negative and positive reference terminals respectively coupled to outputs of first and second digital to analog (D/A) convertors, said first and second D/A convertors arranged in parallel to receive said first and second compensation bits representative of said dark and light responses and output to said negative and positive reference terminals analog versions thereof; and

(b) means for controlling a transfer function of said analog to digital convertor means in accordance with said analog versions of said compensation bits such that;

(i) in response to receiving via said signal input means an analog signal substantially equal in value to or less than said dark response, said analog to digital convertor means will output a digital signal of a first prescribed value, (ii) in response to receiving via said signal input means an analog signal substantially equal in value to or greater than said light response, said analog to digital convertor means will output a digital signal of a second prescribed value, and (iii) in response to receiving via said signal input means an analog signal having a value between said dark and light responses, said analog to digital convertor means will output a digital signal of a value between said first and second prescribed values.

2. The apparatus of claim 1, wherein said platen means is constructed and arranged to support an edge of a document at home position for scanning, and wherein said target means is positioned adjacent said home position.

3. The apparatus of claim 2, comprising means for scanning a first portion of said target means while the same is not illuminated in order to determine said dark response, and means for scanning a second portion of said target means while the same is illuminated in order to determine said light response.

4. The apparatus of claim 3, comprising means for scanning said first portion of said target means prior to the scanning of the second portion of said target means, to thereby provide said dark response signal prior to providing said light response signal.

5. The apparatus of claim 4, wherein scanning of said second portion of said target means while illuminated is substantially immediately followed by scanning a document while the document is illuminated by continuing illumination of the document substantially immediately after determining said light response signal.

6. A method for compensating for any sensitivity nonuniformity that may exist between the individual sensor cells of a document scanner having a light sensitive array comprising a plurality of individual sensor cells, comprising the steps of:

determining the dark response of each of said sensor cells by sensing the output signals of said individual sensor cells as said array views a target when said target is not illuminated, and providing a dark response signal for each individual sensor cell as a result thereof;

determining the light response of each of said sensor cells by sensing the output signals of said individual sensor cells as said array views said target when said target is illuminated, and providing a light response signal as a result thereof;

said steps of determining the dark response and the light response of said sensor cells including the steps of calculating individual compensation data words for each of said sensor cells, each such compensation word having first bits thereof that are representative of said dark response and having second bits thereof that are representative of said light response, and storing said compensation words for use during scanning of a document; and

scanning a document so as to generate analog signals indicative of the light sensed by said sensor cells and sequentially feeding said analog signals along with corresponding compensation words to an analog to digital convertor, and controlling a transfer function of said analog to digital convertor such that;

(i) in response to receiving an analog signal substantially equal in value to or less than said dark response, said analog to digital convertor will output a digital signal of a first prescribed value, (ii) in response to receiving an analog signal substantially equal in value to or greater than said light response, said analog to digital convertor will output a digital signal of a second prescribed value, and (iii) in response to receiving an analog signal having a value between said dark and light responses, said analog to digital convertor will output a digital signal of a value between said first and second prescribed values.

7. The method of claim 6, including the step of determining said dark response and said light response prior to substantially every document scan.

8. The method of claim 7, including the steps of beginning scanning of said target with said target unilluminated in order to determine said dark response, and scanning a portion of said target with said target illuminated in order to determine said light response.

9. The method of claim 8, wherein said target is generally white in color, such that said dark response is representative of the lowest magnitude light expected to be received from said document during scanning thereof, and such that said light response is representative of a known high magnitude light.