Optical reader exposure control apparatus comprising illumination level detection circuitry

• US 6,254,003 B1
• Filed: 06/18/1998
• Issued: 07/03/2001
• Est. Priority Date: 12/18/1995
• Status: Expired due to Fees

• Current Assignee: Welch Allyn Data Collection Inc (Welch Allyn Incorporated)
• Original Assignee: Welch Allyn Data Collection Inc (Welch Allyn Incorporated)
• Inventors: Pettinelli, John A., McEnery, Dennis
• Primary Examiner(s): Le, Thien M.
• Assistant Examiner(s): YVEN, JAMES
• Application Number:
  US09/099,604
• Time in Patent Office: 1,111 Days
• Field of Search: 235/454, 235/455, 235/462.06, 235/462.25, 235/462.26, 235/462.41
• US Class Current: 235/454
• CPC Class Codes: G06K 7/10584 Source control
• CPC Class Codes: G06K 7/10752 Exposure time control
• CPC Class Codes: G06K 7/10851 Circuits for pulse shaping, amplifying, eliminating noise signals, checking the function of the sensing device basic electronic circuitry H03

Abstract

An exposure control apparatus for use with optical readers, such as bar code readers, which utilize photosensitive image sensors. An illumination signal generating circuit generates an illumination signal having a magnitude that varies in accordance with the illumination level at the image sensor. A window detecting circuit samples the illumination signal during a predetermined part of each scan to determine whether the illumination signal is within the window, has exited the window, or has re-entered the window. Exposure control circuitry uses the output of the window detecting circuit to control which of a plurality of the subdivisions of the exposure control range of the image sensor will be used. Changes in exposure time are made only between adjacent subdivisions of the exposure control range. Together with a predetermined hysteresis between the exit and re-entry thresholds of the window, the latter changes stabilize the operation of the reader by reducing exposure control “hunting”.

Claims

1. An apparatus for optically scanning encoded data from an optically readable indicia and converting said data into an electrical signal that may be decoded to recover said data, said apparatus being of the type which continues scanning, without substantial interruption, until scanning is discontinued, including, in combination:

a) an image sensor, having a pixel array that includes a plurality of pixels, for receiving an image of said indicia and for generating an image sensor signal that varies in accordance with the intensity of light incident on said pixel array and the exposure time of said sensor, said image sensor having an exposure time that can be varied over a dynamic range;

b) illumination signal generating means responsive to said image sensor signal for generating an illumination signal that varies in accordance with said image sensor signal;

c) exposure control means for controlling the exposure time of said image sensor, including;

(i) means for storing, at a plurality of sequentially selectable storage locations, a plurality of exposure control values that correspond to different respective exposure times for said image sensor, said exposure control values being selected to make available, for exposure control purposes, approximately all of said dynamic range;

(ii) means for establishing a pointer for identifying the storage location that stores the exposure control value to be used during each scan;

(iii) adjusting means for advancing said pointer in a first direction if said illumination signal is greater than a maximum illumination value during a predetermined portion of a scan, and for advancing said pointer in a second, opposite direction if said illumination signal is less than a minimum illumination value during said predetermined portion of a scan;

whereby the magnitude of said illumination signal changes in a stepwise manner until it remains between said maximum and minimum illumination values.

2. The apparatus of claim 1 in which said advancing causes said exposure control value to change from a value that corresponds to one segment of said dynamic range to a value that corresponds to an immediately adjacent segment of said dynamic range.

3. The apparatus of claim 1 in which said maximum illumination value includes first and second component maximum illumination values, said first component maximum illumination value being associated with a determination that an illumination signal is too large, and said second component maximum illumination signal being associated with a determination that said illumination signal is not too large.

4. The apparatus of claim 1 in which said minimum illumination value includes first and second component minimum illumination values, said first component minimum illumination value being associated with a determination that an illumination signal is too small, and said second component minimum illumination signal being associated with a determination that said illumination signal is not too small.

5. The apparatus of claim 3 in which said minimum illumination value includes first and second component minimum illumination values, said first component minimum illumination value being associated with a determination that an illumination signal is too small, and said second component minimum illumination signal being associated with a determination that said illumination signal is not too small.

6. The apparatus of claim 1 in which changes in said pointer are made, if at all, once each time that said illumination signal does not remain between said maximum and minimum illumination values during said predetermined portion of a scan.

7. The apparatus of claim 1 in which said illumination signal varies approximately in accordance with the instantaneous value of said image sensor signal, and in which said predetermined portion of a scan approximates a single time during that scan.

8. The apparatus of claim 1 in which said illumination signal varies in accordance with a time averaged value of said image sensor signal.

9. An apparatus for optically scanning encoded data from an optically readable indicia and converting said data into an electrical signal that may be decoded to recover said data, said apparatus being of the type which scans, without substantial interruption, until scanning is discontinued, including, in combination:

a) an image sensor, having a pixel array that includes a plurality of pixels, for receiving an image of said indicia and for generating an image sensor signal that varies in accordance with the intensity of light incident on said pixel array and the exposure time of said sensor, said exposure time comprising a controllable fraction of a scan period of said image sensor, the range of possible durations for said exposure time defining the dynamic range of said image sensor;

b) an illumination signal generator for generating an illumination signal that varies in accordance with said image sensor signal;

c) exposure control circuitry for controlling the exposure time of said image sensor, including;

(i) means for storing, at a plurality of storage locations, a plurality of different respective exposure times for said scans, each exposure time being associated with a predetermined respective subdivision of said dynamic range;

(ii) means for identifying, for each scan, the storage location containing the exposure time for that scan;

(iii) adjusting means for directing said identifying means to identify a storage location that stores the next lower exposure time subdivision of said dynamic range if said illumination signal is greater than a maximum illumination value during a predetermined portion of a scan, and for directing said identifying means to identify a storage location that stores the next higher exposure time subdivision of said dynamic range if said illumination signal is less than a minimum illumination value during said predetermined portion of a scan;

whereby said illumination signal is made to progressively take on a value which is within an illumination window having boundaries defined by said maximum and minimum illumination values.

10. The apparatus of claim 9 in which said directing means causes said exposure time to change from a value that corresponds to one subdivision of said dynamic range to a value that corresponds to an immediately adjacent subdivision of said dynamic range.

11. The apparatus of claim 9 in which said window has at least one boundary that includes first and second switching thresholds, and in which a predetermined hysteresis exists between the first and second switching thresholds at said at least one boundary.

12. The apparatus of claim 9 in which each boundary of said window includes first and second switching thresholds, and in which a predetermined hysteresis exists between the first and second switching thresholds at each boundary of said window.

13. The apparatus of claim 9 in which said directing takes place, if at all, once each time that said illumination signal does not remain between said maximum and minimum illumination values during said predetermined portion of a scan.

14. The apparatus of claim 9 in which said illumination signal varies approximately in accordance with the instantaneous value of said image sensor signal, and in which said predetermined portion of a scan approximates a single time during that scan.

15. The apparatus of claim 9 in which said illumination signal varies in accordance with a time averaged value of said image sensor signal.

16. An apparatus for optically scanning encoded data from an optically readable indicia and converting said data into an electrical signal that may be decoded to recover said data, said apparatus being of the type which continues scanning, without substantial interruption, until scanning is discontinued, including, in combination:

a) an image sensor, having a pixel array that includes a plurality of pixels, for receiving an image of said indicia and for generating an image sensor signal that varies in accordance with the intensity of light incident on said pixel array and the exposure time of said sensor, said image sensor having an exposure time that can be varied over a dynamic range;

b) illumination signal generating means responsive to said image sensor signal for generating an illumination signal that varies in accordance with said image sensor signal;

c) exposure control means for controlling the exposure time of said image sensor, including;

(i) means for storing, at a plurality of sequentially selectable storage locations, a plurality of exposure control values that correspond to different respective exposure times for said image sensor, said exposure control values being selected to make available, for exposure control purposes, approximately all of said dynamic range;

(ii) a pointer for identifying the storage location that stores the exposure control value to be used during each scan;

(iii) means for establishing a window having upper and lower window exit thresholds defined by first maximum and minimum illumination values, respectively, and having upper and lower window re-entry thresholds defined by second maximum and minimum illumination values, respectively; and

(iv) adjusting means for changing said pointer if said illumination signal is not between said upper and lower exit thresholds during a predetermined portion of a scan, and for not changing said pointer if said illumination signal is between said upper and lower re-entry thresholds during a predetermined portion of a scan;

whereby the magnitude of said illumination signal changes in a stepwise manner until it remains between said upper and lower re-entry thresholds.

17. The apparatus of claim 16 in which changes in said pointer cause said exposure control value to change from a value that corresponds to one segment of said dynamic range to a value that corresponds to an immediately adjacent segment of said dynamic range.

18. The apparatus of claim 16 in which changes in said pointer are made, if at all, once each time that said illumination signal is not between said window exit thresholds during said predetermined portion of a scan.

19. The apparatus of claim 16 in which said illumination signal varies approximately in accordance with the instantaneous value of said image sensor signal, and in which said predetermined portion of a scan approximates a single time during that scan.

20. The apparatus of claim 16 in which said illumination signal varies in accordance with a time averaged value of said image sensor signal.

21. An apparatus for optically scanning encoded data from an optically readable indicia and converting said data into an electrical signal that may be decoded to recover said data, said apparatus being of the type which continues scanning, without substantial interruption, until scanning is discontinued, including, in combination:

a) an image sensor, having a pixel array that includes a plurality of pixels, for receiving an image of said indicia and for generating an image sensor signal that varies in accordance with the intensity of light incident on said pixel array and the exposure time of said sensor, said image sensor having an exposure time that can be varied over a dynamic range;

b) illumination signal generating means responsive to said image sensor signal for generating an illumination signal that varies in accordance with said image sensor signal;

c) exposure control means for controlling the exposure time of said image sensor, including;

(i) means for storing at a plurality of selectable storage locations a plurality of exposure control values that correspond to different respective exposure times for said image sensor, said exposure control values being selected to make available, for exposure control purposes, approximately all of said dynamic range;

(ii) adjusting means for selecting a storage location having an exposure time shorter than the last selected exposure time if said illumination signal is greater than a maximum illumination value during a predetermined portion of a scan, and for selecting a storage location having an exposure time longer than the last selected exposure time if said illumination signal is less than a minimum illumination value during said predetermined portion of a scan, said maximum and minimum illumination values together defining a window of acceptability; and

d) window adjusting means for adjusting said maximum and minimum illumination values in accordance with variations in the operating voltage of said image sensor;

whereby the magnitude of said illumination signal changes in a stepwise manner until it remains between said maximum and minimum illumination values.

22. The apparatus of claim 21 in which said exposure control means further includes window detecting means for indicating whether said illumination signal is within said window.

23. The apparatus of claim 22 in which said window adjusting means includes a window adjusting circuit for generating first and second output voltages each of which varies as an approximately linear function of the operating voltage of said image sensor, and means for connecting said window adjusting circuit to said window detecting means.

24. An apparatus for optically scanning encoded data from an optically readable indicia and converting said data into an electrical signal that may be decoded to recover said data, said apparatus being of the type which continues scanning, without substantial interruption, until scanning is discontinued, including, in combination:

an image sensor, having a pixel array that includes a plurality of pixels, for receiving an image of said indicia and for generating an image sensor signal that varies in accordance with the intensity of light incident on said pixel array and the exposure time of said sensor, said image sensor having an exposure time that can be varied over a dynamic range;

an illumination signal generating or responsive to said image sensor signal for generating an illumination signal that varies in accordance with said image sensor signal;

an exposure controller comprising a plurality of sequentially selectable storage locations storing a plurality of exposure control values that correspond to different respective exposure times for said image sensor, said exposure control values being selected to make available, for exposure control purposes, approximately all of said dynamic range wherein said exposure controller establishes a pointer for identifying the storage location that stores the exposure control value to be used during each scan, said exposure control being adapted to advance said pointer in a first direction if said illumination signal is greater than a maximum illumination value during a predetermined portion of a scan, and to advance said pointer in a second, opposite direction if said illumination signal is less than a minimum illumination value during said predetermined portion of a scan,

whereby the magnitude of said illumination signal changes in a stepwise manner until it remains between said maximum and minimum illumination values.

25. The apparatus of claim 24, in which said advancing causes said exposure control value to change from a value that corresponds to one segment of said dynamic range to a value that corresponds to an immediately adjacent segment of said dynamic range.

26. The apparatus of claim 24, in which said maximum illumination value includes first and second component maximum illumination values, said first component maximum illumination value being associated with a determination that an illumination signal is too large, and said second component maximum illumination signal being associated with a determination that said illumination signal is not too large.

27. The apparatus of claim 24, in which said minimum illumination value includes first and second component minimum illumination values, said first component minimum illumination value being associated with a determination that an illumination signal is too small, and said second component minimum illumination signal being associated with a determination that said illumination signal is not too small.

28. The apparatus of claim 26, in which said minimum illumination value includes first and second component minimum illumination values, said first component minimum illumination value being associated with a determination that an illumination signal is too small, and said second component minimum illumination signal being associated with a determination that said illumination signal is not too small.

29. The apparatus of claim 24, in which changes in said pointer are made, if at all, once each time that said illumination signal does not remain between said maximum and minimum illumination values during said predetermined portion of a scan.

30. The apparatus of claim 24, in which said illumination signal varies approximately in accordance with the instantaneous value of said image sensor signal, and in which said predetermined portion of a scan approximates a single time during that scan.

31. The apparatus of claim 24, in which said illumination signal varies in accordance with a time averaged value of said image sensor signal.