A 200x100 Array Of Electronically Calibratable Logarithmic CMOS Pixels

1
A 200x100 Array Of Electronically Calibratable
Logarithmic CMOS Pixels

• Bhaskar Choubey
• Satoshi Ayoma
• Dileepan Joseph
• Stephen Otim
• Steve Collins
• University of Oxford, Oxford
• Satoshi Ayoma is currently with Renesses
  Technology Corp, Japan
• Dileepan Joseph is currently with University of
  Alberta, Edmonton, Canada

2
Introduction

• Need for High Dynamic Range Imaging
• Linear Pixel
• CCD
• CMOS Active Pixel Sensor
• Techniques to improve dynamic range lead to
• Low Fill Factor
• Complex In-pixel Circuits
• Large Number of Bits

3
Logarithmic Pixels

• Can capture Wide Dynamic Range
• Encode Contrast Information
• Similar Fill Factor as that of APS
• Randomly Addressable
• Device Variations Cause Fixed Pattern Noise

4
Ways to Reduce FPN

• On-chip Methods
• Electron Injection (Ricquer and coworkers)
• Gate Voltage Adjustment (Loose et.al.)
• Subtracting Reference Response (Kavadias et. al.)
• Off-chip Methods
• Subtracting Reference Response (IMSC chips)
• Model Based Correction (Joseph and Collins)
• Electronic Calibration (Choubey and coworkers)

5
Electronic Calibration

• Log Pixel Response
• Parameter Extraction
• Photocurrent Extraction

6
Chip Design

• 0.35 µ 2-Poly 3-Metal Process
• Each Pixel 10 µ x 10 µ
• Photodiodes used N-diff, P-substrate
• 200 x 100 Pixel array
• Column and Row Scanners

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Experimental Procedure

• All Clocks were generated externally using
  Instrunet data acquisition board and HP Vee
  programming interface
• Analogue values from Pixels were read through
  semiconductor parameter analyzer (Agilent 4155B)
• Slow speed clock was used to remove all transient
  effects
• All electronic experiments were carried out in a
  stable temperature oven to remove temperature
  variability.

8
Experimental Result

• A uniform scene without correction, after
  single parameter correction and after 2 parameter
  correction.

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Experimental Result
10
Experimental Results

• Uncorrected FPN was of the order of 104
• Offset FPN Correction
• FPN is dramatically reduced
• However a low error is only obtained close to the
  calibration point.

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Experimental Results

• Calibration Points
• The two points should be far from dark point to
  remove leakage effects.
• They should be well separated to remove temporal
  noise effects on gain.
• The second point should not be too close to the
  moderate inversion region.
• FPN Correction
• A contrast sensitivity of 2 is obtained for more
  than 6 decades of intensity.

12
Conclusion

• Log Pixels have the ability to match the
  performance of human eye, but are crippled by
  high fixed pattern noise.
• Offset only correction fails to get a high
  quality image.
• Electronic Calibration provides an easy method of
  reducing the FPN caused by variation in all
  parameters.
• The residual error after correction is as low as
  2 and hence matches the human eyes contrast
  sensitivity.