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 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 > %) Charge is transferred to the output amplifier where it is digitized

CCD Architecture Serial Register Pixel Array Serial Register Masked Storage Array Serial Register Pixel Array Active Pixels Storage Pixels Full FrameFrame Transfer Interline Transfer Scientific Imaging PIV Cameras Video-rate Imaging Video-rate Imaging

Microchannel Plate Intensifier Gain is controlled by V MCP Gating achieved by pulsing V PC 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 - PhosphorMCPPhotocathode V pc V MCP V ph e - h e - h

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

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, ~ , so it must be performed serially Low noise amplification Serial Register Pixel Array Gain Register Amplifier

Analysis of SNR Optically generated signal Photons incident on the detector produce electrons in a probabilistic manner given by the quantum efficiency, = () e2V Front-illuminated QE (%)

Analysis of SNR Optically generated signal e2V Back-illuminated QE (%) Uncoated UV coated Midband coated FI

Analysis of SNR Thermally 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! T (C) Dark Current (e - /pixel/s) e2V Back-illuminated

Analysis of SNR Total signal C A/D [counts/e - ] – amplifier gain  - quantum efficiency N pp – 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 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

Analysis of SNR Read 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 Back-illuminated

Analysis of SNR Dark 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. _ _

Analysis of SNR Gain 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 CCDICCDEMCCD Signal Shot Noise Dark Noise Read Noise Total Noise N pp – number of signal photons - quantum efficiency G – gain factor (e-/e-)F – noise factor  = F 2 – noise factorpc – photocathode F EMCCD  1.3F ICCD  1.6 (  2.6)

Slow-scan Performance Theoretical  = 0.9  pc = 0.2  = 2.6 F = 1.3 dark1 = 150 dark2 = 0.02 read1 = 2 read2 = 6 CAD = 4 G = 500

Intensified vs Slow-scan  = 0.9  pc = 0.2  = 2.6 F = 1.3 dark1 = 150 dark2 = 0.02 read1 = 2 read2 = 6 CAD = 4 G = 500

Slow-Scan Performance Measured Apogee AP7MicroMax

Intensified Camera Performance Measured PI Max IVRC

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

Case study Residual gas measurements in an IC engine MicroMaxPI Max