Overview of Scientific Imagingusing CCD Arrays

Jaal Ghandhi

Mechanical EngineeringUniv. of Wisconsin-Madison

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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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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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V IVAT

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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V IVAT

COMBUSTO SE

MP

ER

T -25

•1946 1986 •ERC

Coupling Intensifier to Camera - ICCD

• Lens coupling – not recommended• Limited f-number• Alignment

• Fiber coupling

U

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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

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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

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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

U

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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

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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

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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