Methods, systems and apparatuses using barrier self-calibration for high dynamic range imagers

• US 20090101796A1
• Filed: 10/23/2007
• Published: 04/23/2009
• Est. Priority Date: 10/23/2007
• Status: Active Grant

• Current Assignee: Aptina Imaging Corporation (ON Semiconductor Corporation)
• Original Assignee: Micron Technology, Inc.
• Inventors: Jerdev, Dmitri, Ladd, John, Agranov, Gennadiy A.
• Granted Patent: US 7,920,193 B2
• Time in Patent Office: Days
• US Class Current: 250/206
• CPC Class Codes: H04N 23/70 Circuitry for compensating brightness variation in the scene
• CPC Class Codes: H04N 25/575 with a response composed of multiple slopes
• CPC Class Codes: H04N 25/583 with different integration times
• CPC Class Codes: H04N 25/589 with different integration times, e.g. short and long exposures
• CPC Class Codes: H04N 25/76 Addressed sensors, e.g. MOS or CMOS sensors

Abstract

Methods, systems and apparatuses proving a high dynamic range imager. Multiple photosensor integration periods are used to capture pixel signal information. A transistor gate is used to remove electrons from the photosensor between the two successive integration periods providing a non-linear pixel response characteristic having a knee point. Each pixel is calibrated for the knee point which is used during adjustment of the pixel output signal. Each pixel may also be calibrated with an arbitrary signal response curve for multiple light intensities.

Claims

1. A method of operating an imager, comprising:

during a first operating mode;

accumulating charge with a photosensor of a pixel during a first integration period;

pulsing a transistor of the pixel with a first voltage which is lower than a turn on voltage of the transistor at the end of the first integration period, the transistor being electrically coupled to the photosensor;

continuing to accumulate charge with the photosensor during a second integration period which follows the pulsing; and

reading out accumulated charge in the photosensor after the second integration period; and

during a second operating mode;

filling the photosensor with charge;

pulsing the transistor with the first voltage to remove a portion of the charge and leaving residual charge in the photosensor; and

reading out the residual charge.

2. The method of claim 1, wherein pulsing the transistor in the first operating mode removes a portion of the charge accumulated by the photosensor during the first integration period when the accumulated charge is greater than a barrier potential set by the first voltage and an integration time for these photosensors is equal to the second integration period.

3. The method of claim 1, wherein pulsing the transistor in the first operating mode does not remove a portion of the charge accumulated by the photosensor during the first integration period when the accumulated charge is less than a barrier potential set by the first voltage and an integration time for these photosensors is equal to the first integration period plus the second integration period.

4. The method of claim 1, wherein the transistor is selected from the group consisting of a transfer transistor and an anti-blooming transistor, and

when the transistor is the transfer transistor, the pulsing of the transistor results in a transferring of charges from the photosensor to a storage region, and

when the transistor is an anti-blooming transistor and the pulsing of the transistor results in a draining of charges from the photosensor during an integration period.

5. The method of claim 1, wherein the first operating mode occurs for each image captured by the imager and the second operating mode occurs once for each predetermined number of images captured.

6. The method of claim 1, wherein the read out accumulated charge corresponds to an image signal value and the method further comprises adjusting the image signal value in dependence on a pixel response knee point value for image signals produced by the pixel, the knee point value being determined based on the read out residual charge.

7. The method of claim 6, wherein the pixel response knee point value is determined in accordance with:

KNEEPOINT=CALIB*(Tint_—

1+Tint_—

2)/(Tint_—

1),

wherein KNEEPOINT is the signal value at which the knee point occurs, CALIB is a calibrated value corresponding to the knee point determined based on the read out residual charge, Tint_1 is the first integration period and Tint_2 is the second integration period.

8. The method of claim 1, wherein:

the first operating mode further comprises;

after the first integration period and before the second integration period, accumulating charge with the photosensor of the pixel during a third integration period; and

pulsing the transistor of the pixel with the first voltage at the end of the third integration period; and

the second operating mode further comprises;

before reading out the residual charge, filling the photosensor with charge a second time; and

pulsing the transistor again with the first voltage to remove a portion of the charge and leaving residual charge in the photosensor.

9. A method of operating an imager, the method comprising:

filling a pixel photosensor with stored charges;

removing a portion of the stored charges from the photosensor by lowering to a set level a potential barrier which maintains residual charges within the photosensor; and

reading out a signal representing the residual charges in the photosensor after the portion of stored charges are removed.

10. The method of claim 9, further comprising using the read out signal representing the residual charges to determine a pixel response knee point value for image signals produced by the pixel.

11. The method of claim 9, wherein the pixel comprises a transistor having a gate for transferring charges from the photosensor and wherein the lowering of a potential barrier to a set level comprises applying a voltage to the gate of the transistor which is higher than a turn on threshold voltage of the transistor.

12. The method of claim 10, further comprising:

capturing an image portion with the pixel by accumulating charges during first and second integration periods, the potential barrier being lowered to the set level at the end of the first integration period to begin the second integration period;

reading out an image pixel signal based on charges accumulated in the photosensor at the end of the second integration period; and

adjusting a value of the image pixel signal in dependence on the knee point value.

13. The method of claim 12, wherein an image pixel signal below the knee point value is adjusted differently from an image pixel signal above the knee point value.

14. A method of operating an imager, comprising:

during a calibration mode;

filling a photosensor with stored charges;

removing a portion of the stored charges from the photosensor by lowering to a set level a potential barrier which maintains residual charges within the photosensor; and

reading out a signal representing the residual charges in the photosensor after the portion of stored charges are removed;

repeating the filling, removing and reading out steps for a plurality of light intensities; and

using the read out signals to determine a pixel response curve for image signals produced by the pixel; and

during an image capture mode;

accumulating charge with a photosensor of a pixel;

reading out an image signal based on the accumulated charge in the photosensor; and

adjusting a value of the image signal in dependence on the pixel response curve,

wherein the calibration mode occurs once for each predetermined number of images captures and the image capture mode occurs once for each image captured.

15. An imager, comprising:

a pixel array comprising a plurality of pixels for providing an output signal, each pixel comprising;

a photosensor configured to accumulate charge;

a transistor having a gate from transferring charge from the photosensor, the transistor being electrically coupled to the photosensor; and

a control circuit, the control circuit configured to cause;

during a first operating mode;

the photosensor to accumulate charge during a first integration period;

the transistor to be pulsed with a first voltage which is lower than a turn on voltage of the transistor, at the end of the first integration period;

the photosensor to continue to accumulate charge during a second integration period which follows the pulsing; and

the photosensor to output the accumulated after the second integration period; and

during a second operating mode;

the photosensor to be filled with charge;

the transistor to be pulsed with the first voltage to remove a portion of the charge and leaving residual charge in the photosensor; and

the residual charge to be output by the photosensor.

16. The imager of claim 15, wherein during pulsing of the transistor in the first operating mode:

when the accumulated charge is greater than a barrier potential set by the first voltage, then a portion of the charge accumulated by the photosensor during the first integration period is removed and an integration time for these photosensors is equal to the second integration period; and

when the accumulated charge is less than a barrier potential set by the first voltage, then none of the charge accumulated by the photosensor during the first integration period is removed and an integration time for these photosensors is equal to the first integration period plus the second integration period.

17. The imager of claim 15, wherein the transistor is selected from the group consisting of a transfer transistor and an anti-blooming transistor, and

when the transistor is a transfer transistor, the imager further comprises a storage region and the pulsing of the transistor results in a transfer of charges from the photosensor to the storage region, and

when the transistor is an anti-blooming transistor, the pulsing of the transistor results in a draining of charges from the photosensor during an integration period.

18. The imager of claim 15, wherein control circuit is further configured to cause the first operating mode to occur for each image captured by the imager and to cause the second operating mode to occur once for each predetermined number of images captured.

19. The imager of claim 15, wherein the accumulated charge that is output from the photosensor corresponds to an image signal value and the image processor is further configured to adjust, for each pixel, the image signal value in dependence on a pixel response knee point value for an image signal produced by the pixel, the knee point value being determined based on the output residual charge.

20. The imager of claim 19, wherein the pixel response knee point value is determined in accordance with:

KNEEPOINT=CALIB*(Tint_—

1+Tint_—

2)/(Tint_—

1),

wherein KNEEPOINT is the signal value at which the knee point occurs, CALIB is a calibrated value corresponding to the knee point determined based on the output residual charge, Tint_1 is the first integration period and Tint_2 is the second integration period.

21. An imager comprising:

a pixel array comprising a plurality of pixels for providing an output signal, each pixel comprising;

a photosensor configured to accumulate charge;

a transistor having a gate from transferring charge from the photosensor, the transistor being electrically coupled to the photosensor; and

a control circuit, the control circuit configured to cause;

a pixel photosensor to be filled with charge;

a potential barrier to be lowered to a set level, thereby removing a portion of the charge from the photosensor while maintaining residual charges within the photosensor; and

a signal representing the residual charges in the photosensor after the portion of stored charges are removed to be output.

22. The imager of claim 21, further comprising an image processor, the image processor being configured to use the output signal representing the residual charges to determine a pixel response knee point value for image signals produced by the pixel.

23. The imager of claim 21, wherein the control circuit causes the potential barrier to be lowered to a set level by applying a voltage to the gate of the transistor which is lower than a turn on threshold voltage of the transistor.

24. The imager of claim 22,

wherein the control circuit is further configured to cause:

the photosensor to accumulate charge during first and second integration periods, thereby capturing an image portion, the potential barrier being lowered to the set level at the end of the first integration period to begin the second integration period; and

an image pixel signal based on charges accumulated in the photosensor at the end of the second integration period to be output; and

wherein the image processor is further configured to adjust a value of the image pixel signal in dependence on the knee point value.

25. The imager of claim 24, wherein the image processor is configured to adjust an image pixel signal below the knee point value differently from an image pixel signal above the knee point value.