Transcript Temperature Dependence of Fixed Pattern Noise

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