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Overview of Scientific Imaging using CCD Arrays. Jaal Ghandhi Mechanical Engineering Univ. of Wisconsin-Madison. Detector Architecture. Charge-Coupled Device (CCD) High quantum efficiency Low noise High dynamic range High uniformity Photodiode Array CMOS. CCD Overview.

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Overview of Scientific Imaging using CCD Arrays


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overview of scientific imaging using ccd arrays

Overview of Scientific Imagingusing CCD Arrays

Jaal Ghandhi

Mechanical Engineering

Univ. of Wisconsin-Madison

detector architecture
Detector Architecture
  • Charge-Coupled Device (CCD)
    • High quantum efficiency
    • Low noise
    • High dynamic range
    • High uniformity
  • Photodiode Array
  • CMOS
ccd overview
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
ccd architecture
CCD Architecture

Full Frame

Frame Transfer

Interline Transfer

Serial Register

Serial Register

Serial Register

Pixel

Array

Masked

Storage

Array

Pixel

Array

Storage

Pixels

Active

Pixels

Scientific

Imaging

Video-rate

Imaging

PIV Cameras

Video-rate Imaging

microchannel plate intensifier
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

V

V

V

ph

MCP

pc

n

n

h

h

e

e

-

-

e

e

-

-

n

h

Phosphor

MCP

Photocathode

coupling intensifier to camera iccd
Coupling Intensifier to Camera - ICCD
  • Lens coupling – not recommended
    • Limited f-number
    • Alignment
  • Fiber coupling
electron multiplying ccd emccd
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

Gain Register

Amplifier

Pixel

Array

analysis of snr optically generated signal
Photons incident on the detector produce electrons in a probabilistic manner given by the quantum efficiency, = ()Analysis of SNROptically generated signal

100

80

60

40

QE (%)

20

300

500

700

900

1100

e2V 47-10 Front-illuminated

analysis of snr optically generated signal9
Analysis of SNROptically generated signal

100

Midband

coated

UV

coated

80

60

Uncoated

40

QE (%)

20

FI

300

500

700

900

1100

e2V 47-10 Back-illuminated

analysis of snr thermally generated signal

-40

20

-80

-60

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!

105

103

Dark Current (e-/pixel/s)

101

10-1

T (C)

-20

40

0

e2V 47-10 Back-illuminated

analysis of snr total signal
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

_

analysis of snr photonic shot noise
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

Avg SD

2 0

2 0.8

analysis of snr read noise
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

e2V 47-10 Back-illuminated

analysis of snr dark noise
Analysis of SNRDark noise
  • The generation of darkcharge 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.

_

_

analysis of snr gain noise
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
analysis of snr
Analysis of SNR
  • 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)

slow scan performance theoretical

h

=

0

.

9

h

=

0

.

2

p

c

k

=

2

.

6

F

=

1

.

3

d

a

r

k

1

=

1

5

0

d

a

r

k

2

=

0

.

0

2

r

e

a

d

1

=

2

r

e

a

d

2

=

6

C

A

D

=

4

G

=

5

0

0

Slow-scan PerformanceTheoretical
intensified vs slow scan

h

=

0

.

9

h

=

0

.

2

p

c

k

=

2

.

6

F

=

1

.

3

d

a

r

k

1

=

1

5

0

d

a

r

k

2

=

0

.

0

2

r

e

a

d

1

=

2

r

e

a

d

2

=

6

C

A

D

=

4

G

=

5

0

0

Intensified vs Slow-scan
slow scan performance measured
Slow-Scan PerformanceMeasured

Apogee AP7 MicroMax

camera selection
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