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Electronic Calibration for Logarithmic CMOS Pixels

Explore an advanced electronic calibration scheme for logarithmic CMOS pixels to reduce fixed pattern noise and improve image quality by comparing residual FPN and correcting FPN error with high precision. The study includes detailed techniques, experimental results, and conclusions on enhancing contrast sensitivity and mitigating gain variations.

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Electronic Calibration for Logarithmic CMOS Pixels

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  1. An Electronic Calibration Scheme for Logarithmic CMOS Pixels Bhaskar Choubey, Satoshi Ayoma*, Stephen Otim, Dileepan Joseph**, Steve Collins, University of Oxford, UK *Now with Renessas Technology Corp, Japan ** Now with University of Alberta, Edmonton, Canada

  2. Outline • Introduction • FPN reduction Techniques • High Gain Readouts to reduce effects of gain variations • Electronic Calibration Scheme • Experimental Results • Conclusion

  3. Introduction • Limited range of CCDs and CMOS APS • Linear high dynamic range pixels require large number of bits with complex circuitry and/or post processing • Logarithmic Pixels • Human Eye like response • Encode Contrast Information

  4. Logarithmic Pixel Circuit

  5. Fixed Pattern Noise • Device Variations cause Fixed Pattern Noise • FPN reduces the contrast sensitivity of the image sensor, resulting in inferior images, compared to linear sensors • Joseph and Collins Model of Logarithmic Pixel

  6. FPN Reduction Methods • In Pixel Techniques • Hot Electron Effects (Ref. Ricquer and others) • Gate Voltage Adjustment with feedback (Ref. Loose and others) • Off Pixel Technique • Subtraction of Uniform Scene Response (Ref. IMS Chips) • Convenience • Error Possibility • Off Chip Technique • On Chip current source for Double Sampling (Ref. Kavadias and others) • Offset only • Remaining FPN ~ 15 % of one decade response

  7. FPN Reduction Methods • Corrected Response • Error in Corrected response • Equivalent Percentage Error

  8. Electronic Calibration • Joseph and Collins used 24 images and an iterative scheme • Computation of thee parameters

  9. Circuit Implementation

  10. The Two Readouts

  11. Residual FPN

  12. Comparison of Residual FPN

  13. Conclusion • Offset only FPN correction can not produce high quality image. • Impact of gain variation can be reduced by having a high gain readout like differential readout. • FPN can be reduced to a contrast sensitivity of <2% by electronic calibration scheme, comparable to the human eye.

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