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Temperature Dependence of FPN in Logarithmic CMOS Image Sensors Dileepan Joseph¹ and Steve Collins² ¹University of Alberta, Canada ²University of Oxford, UK Outline Image Sensors – CCD versus CMOS – Linear versus logarithmic Logarithmic Imagers – Fixed pattern noise (FPN) – Colour rendition – Temperature dependence Conclusion May 1–3 IMTC 2007 2 Image Sensors A digital camera consists of many components, of which the image sensor is considered principal Typical imagers may be charge coupled device (CCD) sensors or complementary metal-oxidesemiconductor (CMOS) sensors May 1–3 IMTC 2007 3 CCD Image Sensors March photo-generated charge systematically from an array of pixels to an output amplifier Established technology High resolution, high sensitivity, low noise Fabrication process is optimised for imaging May 1–3 IMTC 2007 4 CMOS Image Sensors Work like memory with photosensitive pixels inside each cell Signal processing may be incorporated on the same die as pixels High yield and good video performance May be fabricated by microchip makers May 1–3 IMTC 2007 5 Linear Pixels Linear pixels (either CCD or CMOS type) “count” photons over a discrete period of time They produce a voltage directly proportional to the light intensity Unfortunately, the response may saturate white or black easily May 1–3 IMTC 2007 © IMS Chips http://www.ims-chips.de/ 6 Logarithmic Pixels Logarithmic pixels (CMOS only) measure the “rate” of photon incidence continuously They produce a voltage directly proportional to the logarithm of the light intensity The response is similar to that of human vision May 1–3 IMTC 2007 © IMS Chips http://www.ims-chips.de/ 7 The Problem Logarithmic pixels are great for high dynamic range video but… FPN is worse compared to typical linear pixels Colours are worse than for typical linear pixels Impact of temperature on the image quality is poorly understood May 1–3 IMTC 2007 8 The Solution… IMTC 2001 – We fixed the fixed pattern noise IMTC 2002 – We improved the colour rendition IMTC 2007 – We considered temperature May 1–3 IMTC 2007 9 Fixed Pattern Noise Two photodetectors in the human eye or in a digital camera are not going to be identical A varying response to light stimulus causes “fixed pattern noise” The eye uses motion to factor out the FPN; not practical for cameras May 1–3 IMTC 2007 10 Fixed Pattern Noise Modelling the FPN of logarithmic pixels, we improved calibration Responses to uniform stimuli were used to define corrections Our correction reduced the FPN to the same order as the random temporal noise May 1–3 IMTC 2007 11 Colour Rendition May 1–3 IMTC 2007 Reference We have shown how to render accurate colours with logarithmic pixels A colour mapping was defined using images of a reference chart Perceptual error of the rendered colours was comparable to that of consumer cameras Rendered 12 Temperature Dependence Unlike with humans, digital cameras do not regulate temperature Temperature affects the response of a pixel to a light stimulus A “new” FPN appears when the temperature dependence varies from pixel to pixel May 1–3 IMTC 2007 13 Temperature Dependence The dark response of a pixel depends only on temperature Thus, it may be used to correct FPN due to temperature in the light response We validated this idea by simulation with real mismatch data May 1–3 IMTC 2007 14 Conclusion Logarithmic CMOS image sensors are ideal for capturing high dynamic range video Our research aims to improve the image quality of these cameras from machine grade to consumer grade and better The dark response of the image sensor may be used to correct temperature-dependent fixed pattern noise in the light response Future work will simplify our methods and implement them in a complete prototype May 1–3 IMTC 2007 15 Acknowledgements The authors gratefully acknowledge the support of the Natural Sciences and Engineering Research Council of Canada and the Engineering and Physical Sciences Research Council of the United Kingdom May 1–3 IMTC 2007 16