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 Off-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

7. 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.

9. 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.

11. Experimental Results Calibration Points FPN Correction
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 eye’s contrast sensitivity.