Overview of Scientific Imaging using CCD Arrays
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Transcript of Overview of Scientific Imaging using CCD Arrays
Overview of Scientific Imagingusing CCD Arrays
Jaal Ghandhi
Mechanical EngineeringUniv. of Wisconsin-Madison
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V IVAT
COMBUSTO SE
MP
ER
T -25
•1946 1986 •ERC
Detector Architecture
• Charge-Coupled Device (CCD)• High quantum efficiency• Low noise• High dynamic range• High uniformity
• Photodiode Array• CMOS
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V IVAT
COMBUSTO SE
MP
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T -25
•1946 1986 •ERC
CCD Overview
• Photons incident on silicon form electron hole pairs
• Polysilicon mask is used to create a potential barrier to isolate the charge in a region of space (pixel)
• By modulating the potential the charge can be moved with very high efficiency (CTE > 99.9998%)
• Charge is transferred to the output amplifier where it is digitized
Serial Reg ister
Pixel Array
OutputAmplifier
Serial Reg ister
Pixel Array
OutputAmplifier
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COMBUSTO SE
MP
ER
T -25
•1946 1986 •ERC
CCD Architecture
Serial Register
PixelArray
Serial Register
MaskedStorageArray
Serial Register
PixelArray
ActivePixels
StoragePixels
Full Frame Frame Transfer Interline Transfer
Scientific Imaging
PIV CamerasVideo-rate Imaging
Video-rate Imaging
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COMBUSTO SE
MP
ER
T -25
•1946 1986 •ERC
Microchannel Plate Intensifier
• Gain is controlled by VMCP
• Gating achieved by pulsing VPC
• Intensifier Advantages• Very short gate times possible
(~1ns)• High rejection ratio• Gain aids in raising signal out
of the read-noise limited regime
• Intensifier Disadvantages• Decreased spatial resolution• Limited dynamic range• Amplification of noise• Moderate quantum efficiencies
e- h
e-
Phosphor MCP Photocathode
VpcVMCPVph
e- h
e-
h
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COMBUSTO SE
MP
ER
T -25
•1946 1986 •ERC
Coupling Intensifier to Camera - ICCD
• Lens coupling – not recommended• Limited f-number• Alignment
• Fiber coupling
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V IVAT
COMBUSTO SE
MP
ER
T -25
•1946 1986 •ERC
Electron Multiplying CCD - EMCCD
• By increasing the clocking voltage in a CCD you can create a controlled ionization that generates electrons
• The gain factor is small, ~1.015, so it must be performed serially
• Low noise amplification
Serial Register
PixelArray
Gain RegisterAmplifier
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V IVAT
COMBUSTO SE
MP
ER
T -25
•1946 1986 •ERC
Analysis of SNROptically generated signal
• Photons incident on the detector produce electrons in a probabilistic manner given by the quantum efficiency, = ()
e2V 47-10 Front-illuminated
300 500 900
QE (
%)
60
100
80
40
20
700 1100
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V IVAT
COMBUSTO SE
MP
ER
T -25
•1946 1986 •ERC
Analysis of SNROptically generated signal
e2V 47-10 Back-illuminated
300 500 900
QE (
%)
60
100
80
40
20
700 1100
Uncoated
UV coated
Midbandcoated
FI
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V IVAT
COMBUSTO SE
MP
ER
T -25
•1946 1986 •ERC
Analysis of SNRThermally generated signal
• Thermal oscillations of the silicon lattice can generate electron hole pairs, which is called dark charge
• In principle, this can be subtracted from the signal• Cooling is critical!
-80 -60 -40 20
105
103
101
10-1
-20 0 40 T (C)
Dark
Curr
ent
(e- /p
ixel/s)
e2V 47-10 Back-illuminated
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V IVAT
COMBUSTO SE
MP
ER
T -25
•1946 1986 •ERC
Analysis of SNRTotal signal
• CA/D [counts/e-] – amplifier gain
- quantum efficiency
• Npp – number of photons per pixel
• D – dark charge determined by the dark current and readout + exposure time
• D – mean dark charge obtained with no illumination
• Since the dark noise is (ideally) repeatable
/ , ,A D ppS C N D x y D x y
/A D ppS C N
_
U
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V IVAT
COMBUSTO SE
MP
ER
T -25
•1946 1986 •ERC
Analysis of SNRPhotonic shot noise
• Photon detection in a given area for a given time is probabilistic because the photon flux is not constant, i.e. the arrival time separation is not constant
• Therefore, collecting photons in a given area for a fixed time results in an inherent noise called shot noise.
• Shot noise is described by Poisson statistics• Mean = • Variance =
• Result: The maximum possible signal-to-noise ratio is pp
MAX pp
pp
NSNR N
N
Avg SD2 02 0.8
U
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V IVAT
COMBUSTO SE
MP
ER
T -25
•1946 1986 •ERC
Analysis of SNRRead noise
• There is noise introduced to the signal when the charge is converted to digital counts in the amplifier, termed read noise
• The read noise depends on the frequency (clock speed)
• Result – slow scan cameras 6
4
2
0
Read N
ois
e (
e-
rms)
104
2 4 6 8
105
2 4 6 8
106
Frequency (Hz)e2V 47-10 Back-illuminated
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V IVAT
COMBUSTO SE
MP
ER
T -25
•1946 1986 •ERC
Analysis of SNRDark noise
• The generation of dark charge is probabilistic in nature, and can be described by a Poisson distribution
• Subtracting the mean dark charge, D, from a pixel results in a residual quantity, D(x,y)-D(x,y), which is called dark noise.
_
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V IVAT
COMBUSTO SE
MP
ER
T -25
•1946 1986 •ERC
Analysis of SNRGain noise
• The signal amplification in ICCDs and EMCCDs involves some noise generation.• ICCD: contributes to the shot noise
contribution• EMCCD: contributes to shot noise and dark
noise contributions
U
W
V IVAT
COMBUSTO SE
MP
ER
T -25
•1946 1986 •ERC
Analysis of SNR
CCD ICCD EMCCD
Signal
Shot Noise
Dark Noise
Read Noise
Total Noise
ppN pc ppGNppGN
shot ppN
dark D
read
2shot pc ppG N
dark D 2 2dark G F D
read read
2 2shot pc ppG F N
shot dark read
Npp – number of signal photons - quantum efficiency
G – gain factor (e-/e-) F – noise factor
= F2 – noise factor pc – photocathode
FEMCCD 1.3 FICCD 1.6 ( 2.6)
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V IVAT
COMBUSTO SE
MP
ER
T -25
•1946 1986 •ERC
Slow-scan PerformanceTheoretical
0.1
1
10
100
1000
No
ise
[co
un
ts r
ms]
100
101
102
103
104
Signal [counts]
20 C, 20 kHz -40 C, 20 kHz -40 C, 1 MHz
= 0.9
pc = 0.2
= 2.6
F = 1.3
dark1 = 150
dark2 = 0.02
read1 = 2
read2 = 6
CAD = 4
G = 500
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V IVAT
COMBUSTO SE
MP
ER
T -25
•1946 1986 •ERC
Intensified vs Slow-scan
0.1
1
10
100
1000
SN
R
100
101
102
103
104
Npp
Slow scan ICCD EMCCD slope=1/2
= 0.9
pc = 0.2
= 2.6
F = 1.3
dark1 = 150
dark2 = 0.02
read1 = 2
read2 = 6
CAD = 4
G = 500
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V IVAT
COMBUSTO SE
MP
ER
T -25
•1946 1986 •ERC
Slow-Scan PerformanceMeasured
Apogee AP7 MicroMax
1 10 100 1000 100001
10
100
(Slope = 1/2)
SN
R =
80
SN
R =
40
SN
R =
20
SN
R =
10
SN
R =
5
SN
R =
1
Spatial Phase
Nois
e (C
ounts
)
Average Signal (Counts)
10 100 1000 10000N
pp
1 10 100 1000 100001
10
100
Slope = 1/2
SN
R =
80
SN
R =
40
SN
R =
20
SN
R =
10
SN
R =
5
SN
R =
1
Spatial Phase
Nois
e (C
ounts
)
Average Signal (Counts)
100 1000 10000 100000
Npp
U
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V IVAT
COMBUSTO SE
MP
ER
T -25
•1946 1986 •ERC
Intensified Camera PerformanceMeasured
PI Max IVRC
1 10 100 1000 100001
10
100
1000 Gain = 0.700 Gain = 3.83 Gain = 86.2 Gain = 423 Gain = 1250
Slope = 1/2
SNR = 8
0
SNR = 4
0
SNR = 2
0
SNR = 1
0
SNR = 5
SNR = 1
Nois
e (C
ounts
)
Average Signal (Counts)
100 1000 10000 100000
G*Npp
0.1 1 10 100 10000.1
1
10
Slope = 1/2
Gain = 5.54 Gain = 54.5 Gain = 157 Gain = 436 Gain = 1100
SN
R =
80
SN
R =
40
SN
R =
20
SN
R =
10
SN
R =
5
SN
R =
1
Nois
e (C
ounts
)
Average Signal (Counts)
100 1000 10000 100000
G*Npp
U
W
V IVAT
COMBUSTO SE
MP
ER
T -25
•1946 1986 •ERC
Camera Selection
• For all applications a slow-scan, deeply cooled, back-illuminated CCD is the best choice in terms of SNR and image quality, except when• The signal level is very low, then gain
amplifies the signal above the read noise – EMCCD is best option because of superior image quality
• There is strong luminosity and gating is required – ICCD is required
Scott’s note: all else being equal, cameras with big pixels have an advantage
U
W
V IVAT
COMBUSTO SE
MP
ER
T -25
•1946 1986 •ERC
Case studyResidual gas measurements in an IC engine
1 10 100 1000 100001
10
100
1000 Gain = 0.700 Gain = 3.83 Gain = 86.2 Gain = 423 Gain = 1250
Slope = 1/2
SNR = 8
0
SNR = 4
0
SNR = 2
0
SNR = 1
0
SNR = 5
SNR = 1
Nois
e (C
ounts
)
Average Signal (Counts)
100 1000 10000 100000
G*Npp
1 10 100 1000 100001
10
100
Slope = 1/2
SN
R =
80
SN
R =
40
SN
R =
20
SN
R =
10
SN
R =
5
SN
R =
1
Spatial Phase
Nois
e (C
ounts
)
Average Signal (Counts)
100 1000 10000 100000
Npp
MicroMax PI Max