Optical reader with adaptive exposure control

• US 6,155,488 A
• Filed: 08/23/1996
• Issued: 12/05/2000
• Est. Priority Date: 08/25/1995
• Status: Expired due to Term

• Current Assignee: PTO (Safe Fleet Holdings LLC)
• Original Assignee: PTO (Safe Fleet Holdings LLC)
• Inventors: Olmstead, Bryan L., Colley, James E.
• Primary Examiner(s): TREMBLAY, MARK STEPHEN
• Application Number:
  US08/697,408
• Time in Patent Office: 1,565 Days
• Field of Search: 235/455, 235/462.01, 235/462.27, 235/439, 235/440, 235/462.26, 235/462.29
• US Class Current: 235/440
• CPC Class Codes: G06K 7/10752 Exposure time control
• CPC Class Codes: G06K 7/1093 sensing, after transfer of the image of the data-field to an intermediate store, e.g. storage with cathode ray tube
• CPC Class Codes: H04N 25/40 Extracting pixel data from image sensors by controlling scanning circuits, e.g. by modifying the number of pixels sampled or to be sampled
• CPC Class Codes: H04N 25/533 by using differing integration times for different sensor regions
• CPC Class Codes: H04N 25/77 Pixel circuitry, e.g. memories, A/D converters, pixel amplifiers, shared circuits or shared components
• CPC Class Codes: H04N 3/155 Control of the image-sensor operation, e.g. image processing within the image-sensor

Abstract

An optical or symbol reader including CMOS circuitry preferably integrated on a single chip. A CMOS optical reader chip comprises a CMOS imaging array having a plurality of pixels each with a dedicated pixel-site circuit. Charge is accumulated at each pixel location transferred upon demand to a common bus. In a preferred embodiment, exposure time of the imaging array is controlled using a feedback loop. One or more exposure control pixels are positioned adjacent to or within the imaging array and receive light along with the imaging array. The charge of the exposure control pixel or pixels is measured against a threshold level, and the amount of time taken to reach the threshold level determines the time exposure of the pixels of the imaging array. CMOS signal processing circuitry is employed which, in combination with the exposure control circuitry, minimizes time-to-read over a large range of light levels, while performing spatially optimal filtering. Clocking cycles and control signals are time-adjusted in accordance with the varying output frequency of the imaging array so as to provide invariant frequency response by the signal processing circuitry. A multi-dimensional CMOS imaging array is also provided having simultaneous pixel exposure with non-destructive readout of the pixel contents.

Claims

1. An optical reader comprising:

an imaging array comprising a plurality of imaging array pixels;

at least one exposure control pixel distinct from said plurality of imaging array pixels oriented in the same direction as said imaging array pixels, and sharing a same substrate with said imaging array pixels, whereby a level of ambient light is measured and an exposure control pixel voltage output signal in response thereto;

a comparator having a first input connected to said at least one exposure control pixel, and a second input connected to a threshold signal, whereby said comparator changes output states in response to said exposure control pixel voltage output signal exceeding said threshold signal; and

a clock generator responsive to said comparator for regulating an amount of exposure time of said imaging array pixels.

2. The optical reader of claim 1 wherein said clock generator regulates said amount of exposure time of said imaging array pixels by adjusting a duty cycle of imaging array pixel charge time based on an amount of exposure time of said exposure control pixel.

3. The optical reader of claim 1 further comprising a signal processing circuit connected to an output of said imaging array, wherein said clock generator regulates said amount of exposure time of said imaging array pixels by adjusting a clock frequency of a readout clock connected to said imaging array, and wherein said clock generator correspondingly adjusts a frequency of a clock connected to said signal processing circuit.

4. The optical reader circuit of claim 1 wherein said imaging array, exposure control pixel, comparator, and clock generator are fabricated using a CMOS process and combined onto the same CMOS chip.

5. The optical reader circuit of claim 4 further comprising a signal processing circuit connected to said imaging array and a decoder connected to said signal processing circuit, said signal processing circuit and said decoder combined onto said CMOS chip.

6. The optical reader of claim 3, wherein said signal processing circuit comprises either an infinite impulse response (IIR) digital filter or a finite impulse response (FIR) digital filter.

7. The optical reader of claim 3, wherein said signal processing circuit comprises a synchronous transversal analog filter incorporating clocked bucket-brigade delay lines.

8. The optical reader of claim 3, wherein said signal processing circuit comprises a switched capacitor filter.

9. The optical reader of claim 1, further comprising a serial shift register having an input connected to said comparator, said shift register having a plurality of tapped stages;

wherein each of said imaging array pixels is controlled by a reset control signal; and

wherein each reset control signal is generated from a different tapped stage of said shift register.

10. The optical reader of claim 9, wherein each reset control signal is asserted for an amount of time proportional to said amount of exposure time of said exposure control pixel.

11. A method for optical reading, comprising the steps of:

collecting light on a set of imaging pixels and on an exposure control pixel separate and distinct from said set of imaging pixels but oriented in the same direction as said imaging array pixels, and sharing a same substrate with said imaging array pixels;

monitoring an accumulated level of light energy collected by said exposure control pixel;

comparing said accumulated level of light energy to a threshold level; and

terminating the collection of light on said set of imaging pixels in response to said accumulated level of light energy exceeding said threshold level.

12. The method of claim 11, further comprising the steps of:

adjusting a duty cycle of a clock based on an amount of exposure time of said exposure control pixel; and

controlling exposure time of said set of imaging pixels using said clock.

13. The method of claim 11, further comprising the step of sequentially activating light collection at each of said imaging pixels in response to an initial signal, wherein said step of terminating the collection of light on said set of imaging pixels comprises the step of sequentially deactivating light collection at each of said imaging pixels in response to said accumulated level of light energy collected by said exposure control pixel exceeding said threshold level.

14. The method of claim 13, further comprising the step of propagating said initial signal along a shift register to sequentially activate light collection at each of said imaging pixels, and propagating an end-of-exposure time signal along said shift register to sequentially deactivate light collection at each of said imaging pixels.

15. The method of claim 11, further comprising the steps of:

adjusting a clock frequency of a readout clock connected to said imaging array, whereby an amount of exposure time of said imaging pixels is regulated; and

reading out light collection information from said imaging array under control of said readout clock.

16. The method of claim 15, further comprising the step of applying an output signal from said imaging array to a signal processor, said signal processor varying its time domain response in proportion to the adjustment in clock frequency of said readout clock, such that the spatial response of said signal processor is invariant.

17. The method of claim 16, wherein said step of applying an output signal from said imaging array to a signal processor comprises the step of applying the output signal to an infinite impulse response (IIR) or finite impulse response (FIR) digital filter.

18. The method of claim 16, wherein said step of applying an output signal from said imaging array to a signal processor comprises the step of applying the output signal to a synchronous transversal analog filter having clocked bucket-brigade delay lines.

19. The method of claim 16, wherein said step of applying an output signal from said imaging array to a signal processor comprises the step of applying the output signal to a switched capacitor filter.

20. The method of claim 11, wherein said imaging pixels are arranged in a two-dimensional pattern.

21. The method of claim 11, further comprising the step of collecting light on a plurality of exposure control pixels separate and distinct from said set of imaging pixels, said exposure control pixels dispersed throughout the two-dimensional pattern of imaging pixels.

22. The method of claim 11, wherein

said step of collecting light on an exposure control pixel comprises the step of receiving light incident upon a photoelectric diode;

said step of monitoring an accumulated level of light energy collected by said exposure control pixel comprises the step of transferring charge output from said photoelectric diode and storing said charge on a capacitor connected to said photoelectric diode; and

said step of comparing said accumulated level of light energy to a threshold level comprises the step of connecting an output of said capacitor and a reference voltage to inputs of a comparator, and generating a comparison output signal thereby.