SYSTEMS AND METHODS FOR COMPENSATING FOR FIXED PATTERN NOISE

• US 20100237149A1
• Filed: 03/17/2010
• Published: 09/23/2010
• Est. Priority Date: 03/17/2009
• Status: Active Grant

• Current Assignee: Datalogic Adc, Inc. (Datalogic S.p.A.)
• Original Assignee: Datalogic Scanning Incorporated (Datalogic S.p.A.)
• Inventors: Olmstead, Bryan L.
• Granted Patent: US 8,622,302 B2
• Time in Patent Office: Days
• US Class Current: 235/470
• CPC Class Codes: G06K 7/10722 Photodetector array or CCD scanning

Abstract

This disclosure relates generally to systems and methods for estimating and, at least partially, compensating for fixed pattern noise (FPN) in an image sensor. In one configuration, an estimate of the FPN of an image sensor may be obtained by capturing a dark image (either a linear or an area image, depending on the sensor type) using a first exposure time, an illuminated image may be captured using a second exposure time, the second exposure time is greater than the first exposure time, and the dark image may be subtracted from the illuminated image to compensate, at least partially, for FPN. Certain configurations may utilize virtual scan lines. Two or more dark images may also be utilized to estimate FPN in an image sensor.

Claims

1. A method of compensating for fixed pattern noise in an image sensor of an optical data reader, the method comprising:

capturing a dark image comprising minimal scene information and an estimate of fixed pattern noise using an image sensor, the dark image having a corresponding first exposure time;

storing the dark image;

capturing an illuminated image comprising optical data from a read area using the image sensor, the illuminated image having a corresponding second exposure time, wherein the second exposure time exceeds the first exposure time;

subtracting the dark image from the illuminated image to generate a compensated image comprising optical data that is, at least partially, compensated for fixed pattern noise.

2. The method of claim 1, wherein the second exposure time is between 100 and 1,000 times longer than the first exposure time.

3. The method of claim 1, wherein the second exposure time is between 200 and 300 times longer than the first exposure time.

4. The method of claim 1, wherein the first exposure time is between 1 microsecond and 1,000 microseconds.

5. The method of claim 1, wherein the first exposure time is between 5 microsecond and 30 microseconds.

6. The method of claim 1, wherein the optical data reader comprises an optical code reader and wherein capturing the dark image and storing the dark image are performed each time the optical code reader is activated.

7. The method of claim 1, wherein subtracting the dark image from the illuminated image occurs prior to acquisition of a subsequent illuminated image.

8. The method of claim 1, further comprising:

capturing a plurality of illuminated images, the plurality of illuminated images having a corresponding plurality of different exposure times;

subtracting the dark image from each of the plurality of illuminated images to generate a plurality of images, at least partially, compensated for fixed pattern noise.

9. The method of claim 1, further comprising:

capturing a plurality of preliminary dark images; and

averaging the plurality of preliminary dark images in order to generate the dark image.

10. The method of claim 1, further comprising:

detecting a change in temperature; and

based upon detecting the change in temperature, reacquiring the dark image.

11. An optical data reader, comprising:

an image sensor to capture a dark image comprising minimal scene information and an estimate of fixed pattern noise, the dark image having a corresponding first exposure time, and to capture an illuminated image comprising optical data from a read area, the illuminated image having a corresponding second exposure time, and wherein the second exposure time exceeds the first exposure time;

a computer-readable storage medium to store the dark image; and

a fixed pattern noise compensation subsystem to subtract the dark image from the illuminated image to generate a compensated image comprising optical data that is, at least partially, compensated for fixed pattern noise.

12. The optical data reader of claim 11, wherein the fixed pattern noise compensation subsystem comprises a processor;

and

wherein the computer-readable storage medium further stores a fixed pattern noise compensation module executable on the processor.

13. The optical data reader of claim 11, further comprising:

a temperature sensor, and wherein based upon the detection of a change of temperature measured by the temperature sensor, the image sensor recaptures the dark image.

14. The optical data reader of claim 11, wherein the image sensor comprises a linear CMOS imager.

15. The optical data reader of claim 11, wherein the image sensor comprises a two dimensional CMOS imager, and wherein the optical data reader further comprises:

a virtual scan line generation subsystem to generate a virtual scan line array from a two dimensional image of an optical code symbol; and

wherein at least one of the dark image and the illuminated image comprise a virtual scan line array.

16. The optical data reader of claim 11, wherein the optical data reader reads at least one of a barcode, a signature, a fingerprint, a handprint.

17. A method of compensating for fixed pattern noise in an image sensor of an optical data reader, the method comprising:

capturing a two dimensional dark image comprising minimal scene information and an estimate of fixed pattern noise using an image sensor, the dark image having a corresponding first exposure time;

storing a portion of the two dimensional dark image along a pattern of virtual scan lines to produce a dark virtual scan line array;

capturing a two dimensional illuminated image comprising optical data from a read area, the illuminated image having a corresponding second exposure time;

generating from the two dimensional illuminated image an illuminated virtual scan line array along a pattern of virtual scan lines; and

subtracting the dark virtual scan line array from the illuminated virtual scan line array to generate a two dimensional compensated image comprising optical data that is, at least partially, compensated for fixed pattern noise.

18. A method of compensating for fixed pattern noise in an image sensor in an optical data reader, wherein the fixed pattern noise comprises threshold mismatch dark signal non-uniformity and dark current dark signal non-uniformity, the method comprising:

capturing a first dark image comprising minimal scene information and an estimate of threshold mismatch dark signal non-uniformity using an image sensor, the first dark image having a first exposure time;

capturing a second dark image comprising the estimate of threshold mismatch dark signal non-uniformity and an estimate of dark current dark signal non-uniformity using the image sensor, the second dark image having a second exposure time, and wherein the second exposure time is longer than the first exposure time;

capturing an illuminated image comprising optical data from a read area using the image sensor, the illuminated image having a third exposure time;

creating an estimate of the threshold mismatch dark signal non-uniformity using the first dark image;

creating an estimate of the dark current dark signal non-uniformity by subtracting the first dark image from the second dark image;

creating a scaled estimate of the dark signal non-uniformity by multiplying the estimate of the dark current dark signal non-uniformity by a scaling factor based on the first exposure time, the second exposure time, and the third exposure time; and

subtracting the estimate of the threshold mismatch dark signal non-uniformity and the scaled estimate of the dark current dark signal non-uniformity from the illuminated image to generate a compensated image that is, at least partially, compensated for fixed pattern noise.

19. The method of claim 18, wherein the third exposure time is between 300 and 700 times longer than the first exposure time.

20. The method of claim 18, wherein the third exposure time is between 100 and 300 times longer than the second exposure time.

21. The method of claim 18, wherein the scaling factor comprises a ratio of the third exposure time and the difference between the second exposure time and first exposure time.