An Electronic Calibration Scheme for Logarithmic CMOS Pixels
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 Outline Introduction
FPN reduction Techniques High Gain Readouts to reduce effects of gain variations Electronic Calibration Scheme Experimental Results Conclusion 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 Logarithmic Pixel Circuit 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 y a b log (x c) 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 FPN Reduction Methods Corrected Response ycorr b log( xin / xcorr ) Error in Corrected response ycorr b log( xin / xcorr ) Equivalent Percentage Error 100b k log( xin / xcorr )
b Electronic Calibration Joseph and Collins used 24 images and an iterative scheme Computation of thee parameters y2 y1 b log( x2 / x1 ) yd a c exp b
a y1 b log( x1 ) Circuit Implementation The Two Readouts Parameter Mean/SD SF Diff Amp
Offset (V) Mean SD 0.5524 0.0165 1.7430 0.0174 Gain (mV/decade)
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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