A 200x100 Array Of Electronically Calibratable Logarithmic CMOS Pixels

Bhaskar ChoubeySatoshi Ayoma

Dileepan JosephStephen OtimSteve Collins

University of Oxford, OxfordSatoshi Ayoma is currently with Renesses Technology Corp, Japan

Dileepan Joseph is currently with University of Alberta, Edmonton, Canada

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

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

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)

Electronic Calibration

• Log Pixel Response

• Parameter Extraction

• Photocurrent Extraction

)/ln( 21

21

xx

yyb

)ln( cxbay

)ln( 11 xbya

b

ayc dexp

ij

ijijij b

ayx exp

)/ln(exp 2121

11 xx

yy

yyx

ijij

ijij

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

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.

Experimental Result

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

Experimental Result

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.

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.

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.