computational optical imaging - optique numeriquegiana.mmci.uni-saarland.de/website-template/... ·...
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Ivo Ihrke / Winter 2013
Computational Optical Imaging -Optique Numerique
Winter 2013
Ivo Ihrke
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Ivo Ihrke / Winter 2013
Joseph Nicéphore Niépce 1765 - 1833
First photograph
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1824
Exposure time 8-12 hours
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Ivo Ihrke / Winter 2013
Louis Daguerre 1787-1851
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Daguerrotype
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Ivo Ihrke / Winter 2013
Daguerrotype
Interesting panorama 1848
“Cincinnati water front”
Restoration: Tang, X., Ardis, P.A., et. al. “Digital Analysis and Restoration of Daguerreotypes” 2010. Proceedings of the SPIE
http://1848.cincinnatilibrary.org/
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Photovoltaic Effect - 1839
Alexandre-Edmond Becquerel, 1839
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Selenium
First semiconductor
Photoelectric effect
Willoughby Smith (1873)
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Photodiode
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Image Sensors
CCD
CMOS
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Ivo Ihrke / Winter 2013
Image Sensors
Photodetection
CCD vs. CMOS
Sensor performance characteristics
Noise
Color Sensors
Exotic Sensors
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Rays, waves and particles....
When does light behave like rays, waves, or particles?
Today, it's a particle :)
Light
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Photons and electrons
Light: photon
m0 = 0 (massless)
q = 0 (no electric charge)
E = hν = hc/λ
Electric charge: electron
m0 = 9.1 * 10-31 kg
q = –1 e = –1.6 * 10-19 C
E = m0c2 + mv2/2 – eφ + ...
energy of a photon depends
ONLY on the wavelength!
rest
energy
kinetic
energy
potential
energy
Unit of energy:
1 eV = energy required to move 1 electron by 1 V in an electrostatic potential
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Radiant flux [1W]: Φ = E n= hν dN/dt
Measurement: integrate over time Δt
Poisson random process
If I count 1 photon, 100 photons, what's the error (standard deviation)?
σ(N) = √N
Amount of light
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Semiconductors
Band gap in semiconductors:
Diamond (C): 5.47 eV
Gallium arsenide (GaAs): 1.43 eV
Silicon (Si): 1.11 eV
Germanium (Ge): 0.67 eV
Photon energy:
400 nm (violet): 3.1 eV
700 nm (red): 1.77 eV
1100 nm (infrared): 1.12 eV
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Ivo Ihrke / Winter 2013
Photogeneration
Silicon
“Band gap” of 1.124eV between valence bandand conduction band.
Incident photon > 1.124eV (hc/ λ) may be absorbed, causing election to jump to conduction band.
Visible light (λ=400 to 700nm)
λ = 400nm (violet) E = 3.1eV
λ = 700nm (red) E = 1.77eV
λ = 1100nm (infrared), E=1.12eV
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Integration
Measuring a single electron is hard! (small electric charge…)
Fortunately, photoelectrons can be stored.
So integrate the charge over a period of time.
10’s to 1000’s of electrons.
Two fundamental structures…
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Photodetectors
(a) photodiode, (b) photogate
All electrons created in depletion regionare collected, plus some from surrounding region.
image: Theuwissen
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Photodiode in CMOS sensor
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Photodetector Performance Metrics
Pixel size
Fill factor
Full well depth
Spectral quantum efficiency
Sensitivity
(Saving noise & dynamic range for later)
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Pixel Size
Large pixels collect more light.
Typically 3μm-10 μm
20 μm for astronomy
Pixels getting tinier for cell phones, digital cameras
1.4μm x 1.4μm (iPhone 5)
Bottleneck = optical resolution.
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Currently (Aug 2010) Highest-Res Chip
Canon 120 MPixel (13280 x 9184) -experimental
size 29.2 mm x 20.2mm
readout @ 9.5 fps
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Fill Factor
Percentage of pixel area that captures photons.
Typically 25% to 100%
Reduced by non-light gathering components in pixel (see CMOS sensors…)
Can be increased using microlenses:
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Lenslets
Increase effective fill factor by focusing light
Can double or triple fill factor
Changes the cos^4 law - vignetting
image: Kodak application note DS00-001
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Full Well Depth
“Saturation charge” 45 to 100 ke–
depends on the pixel size
Limits dynamic range (more about this later)
Once the well is filled up, it can overflow into neighbouring pixels. This is called blooming.
(Blooming almost irrelevant for CMOS)
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Blooming
http://www.ccd-sensor.de/assets/images/blooming.jpg
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Extra Overflow Drain
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Absorption Coefficients
image: Theuwissen
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Penetration Depth
Wavelength (nm) penetration
depth (μm)
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Spectral quantum efficiency (QE)
source: Kodak KAI-2000m data sheet
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Filtered Spectral Quantum Efficiency
source: Kodak KAF-5101ce data sheet
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Factors for Quantum Efficiency
Color filters
Absorption coefficients & depletion depth
Blue light is absorbed quickly, red wavelengths penetrate more deeply.
Photogate detectors have poor blue response because the gate absorbs blue light, too.
Fill factor
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Extended Sensitivity
blue plus – applies a phosphorescent layer
back illuminated CCDs – decrease thickness
Front side illumination (FSI)
Back side illumination (BSI)
[ViseraTech]
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Back Illuminated CCDs
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Sensitivity
Sensitivity = quantum efficiency * conversion gain
Conversion gain: “how many volts per electron?”.
Depends on device process, topology, etc.
Sensitivity is often expressed as Volts/lux
1 Lux = (1/683)W/m2 at λ = 555nm
1 Lux (or lumens/m2) = 4.09e11 photons/(cm2sec)
Clear sky ~= 1e4 Lux
Room light ~= 10 Lux
Full moon ~= 0.1 Lux
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Ivo Ihrke / Winter 2013
CCD’s vs CMOS Image Sensors
Differ primarily in readout—how the accumulated charge is measured and communicated.
CCDs transfer the collected charge, through capacitors, to one output amplifier
CMOS sensors “read out” the charge or voltage using row and column decoders, like a digital memory (but with analog data).
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CCD Sensor 1969
Willard S. Boyle (left) and George E. Smith (1974), Nobel Prize 2009
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Charge Transfer for CCD’s
image: Theuwissen
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Charge Transfer
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Example:Three Phase CCD’s
image: Theuwissen
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Full Frame CCD
Photogate detector doubles as transfer cap.
Simplest, highest fill factor.
Must transfer quickly (or use mechanical shutter) to avoid corruption by light while shifting charge.
imag
e: C
url
ess
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CCD – Principle of Operation
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Frame Transfer
image: Theuwissen
memory area
is shielded
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Smearing
vertical streak
wikipedia
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Interline CCD
Charge simultaneously shifted to shielded gates.
Provides electronic shutter—snapshot operation
Uses photodiodes (better detectors)
Most common architecture for CCDs
imag
e: T
heu
wis
sen
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Charge Transfer Efficiency
CCD charge transfer efficiency, η, is the fraction of charge transferred from one capacitor to the next.
η must be very close to 1, because charge is transferred up to n+m times (or more for 3-phase…)
For a 1024 x 1024 CCD:
η Fraction at output
0.999 0.1289
0.9999 0.8148
0.99999 0.9797
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Advantages of CCD’s
Advantages:
Optimized photodetectors (high QE, low dark current)
Very low noise.
Single amplifier does not introduce random noise or fixed pattern noise.
Disadvantages
No integrated digital logic
Not programmable (no region of interest)
High power (whole array switching all the time)
Limited frame rate due to charge transfer
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CMOS Sensors (active pixel sensor)
• charge converted to a voltage at the pixel
• pixel amp, column amp, output amp.
bitline
row select
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CMOS Sensors
Image : EE392B, El Gamal
Enables
Region of
Interest (ROI)
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Example CMOS Pixel
Photo sensitive area is reduced by additional circuitry.
Source: Stanford EE392B notes
photo diode
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Ivo Ihrke / Winter 2013
Rolling Shutter
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Ivo Ihrke / Winter 2013
Rolling Shutter Distortion
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Ivo Ihrke / Winter 2013
Interlacing
Compatibility with old TV norms
Two half-frames (odd and even fields at twice the frame rate)
Interlacing + rolling shutter
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Ivo Ihrke / Winter 2013
CMOS Sensors
Advantages
Integrated digital logic
Fast
Mainstream process (cheap)
Lower power
Disadvantages
Noise & quality
Most high quality cameras still use CCDs.
this is changing though Canon 5D mark II has CMOS
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Ivo Ihrke / Winter 2013
CMOS with Integrated Logic
[micro.manget.fsu.edu]
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Ivo Ihrke / Winter 2013
CMOS vs CCD, bottom line
CCD’s transfers charge to a single output amplifier. Inherently low-noise.
CMOS converts charge to voltage at the pixel.
Read out like a digital memory - windowing
Reset noise (can use correlated double sampling CDS)
Fixed pattern noise (device mismatch)