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Overview of Scientific Imaging using CCD Arrays

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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  1. Overview of Scientific Imagingusing CCD Arrays Jaal Ghandhi Mechanical Engineering Univ. of Wisconsin-Madison

  2. Detector Architecture • Charge-Coupled Device (CCD) • High quantum efficiency • Low noise • High dynamic range • High uniformity • Photodiode Array • CMOS

  3. 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

  4. 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

  5. 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

  6. Coupling Intensifier to Camera - ICCD • Lens coupling – not recommended • Limited f-number • Alignment • Fiber coupling

  7. 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

  8. 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

  9. 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

  10. -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

  11. 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 _

  12. 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

  13. 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

  14. 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. _ _

  15. 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

  16. 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)

  17. 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

  18. 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

  19. Slow-Scan PerformanceMeasured Apogee AP7 MicroMax

  20. Intensified Camera PerformanceMeasured PI Max IVRC

  21. 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

  22. Case studyResidual gas measurements in an IC engine MicroMax PI Max

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