Temperature dependence of fpn in logarithmic cmos image sensors
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Temperature Dependence of FPN in Logarithmic CMOS Image Sensors. Dileepan Joseph ¹ and Steve Collins ² ¹University of Alberta, Canada ²University of Oxford, UK. Outline. Image Sensors CCD versus CMOS Linear versus logarithmic Logarithmic Imagers Fixed pattern noise (FPN)

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Temperature dependence of fpn in logarithmic cmos image sensors

Temperature Dependence of FPN in Logarithmic CMOS Image Sensors

Dileepan Joseph¹ and Steve Collins²

¹University of Alberta, Canada

²University of Oxford, UK


Outline
Outline Sensors

  • Image Sensors

    • CCD versus CMOS

    • Linear versus logarithmic

  • Logarithmic Imagers

    • Fixed pattern noise (FPN)

    • Colour rendition

    • Temperature dependence

  • Conclusion

IMTC 2007


Image sensors
Image Sensors 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-oxide-semiconductor (CMOS) sensors

IMTC 2007


Ccd image sensors
CCD Image Sensors 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

IMTC 2007


Cmos image sensors
CMOS Image Sensors 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

IMTC 2007


Linear pixels
Linear Pixels Sensors

  • 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

© IMS Chips

http://www.ims-chips.de/

IMTC 2007


Logarithmic pixels
Logarithmic Pixels Sensors

  • 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

© IMS Chips

http://www.ims-chips.de/

IMTC 2007


The problem
The Problem Sensors

  • 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

IMTC 2007


The solution
The Solution… Sensors

  • IMTC 2001

    • We fixed the fixed pattern noise

  • IMTC 2002

    • We improved the colour rendition

  • IMTC 2007

    • We considered temperature

IMTC 2007


Fixed pattern noise
Fixed Pattern Noise Sensors

  • 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

IMTC 2007


Fixed pattern noise1
Fixed Pattern Noise Sensors

  • 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

IMTC 2007


Colour rendition
Colour Rendition Sensors

  • 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

Reference

IMTC 2007


Temperature dependence
Temperature Dependence Sensors

  • 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

IMTC 2007


Temperature dependence1
Temperature Dependence Sensors

  • 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

IMTC 2007


Conclusion
Conclusion Sensors

  • 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

IMTC 2007


Acknowledgements
Acknowledgements Sensors

  • 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

IMTC 2007


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