Overview of Modern Imaging Sensors: Proper Uses for CCD, EMCCD, and CMOS Cameras.

Overview of Modern Imaging Sensors: Proper Uses for CCD, EMCCD, and CMOS Cameras

Charge-Coupled Device (CCD) Introduced in 1969 (Scientific Imaging Standard for 40 years) Electron Multiplying Charge-Coupled Device (EMCCD) Introduced in 2001 (Scientific Low-Light Imaging Standard for 10 years) Scientific Grade Complementary Metal Oxide Semiconductor (CMOS) Introduced in 2010 (Beginning to be Adopted)

Same Basic Pixel Architecture e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- Electrical Connection Potential Well Silicon Silicon Dioxide Polysilicon Gate Incident Light Light when Back-Illuminated Peak Efficiency when Front-Illuminated: ~65% CCD or ~57-65% scientific CMOS Efficiency when Back- Illuminated: ~95%

CCD Collecting Photons: Analogous to Catching Raindrops

CCD Camera Components Photons Electrons Electrons Voltage Serial Register Gain A/D Converter Sensor Custom Electronics Parallel Register Clocking and Other Control Electronics

CCD Image Acquisition Types Frame Transfer CCD Interline CCD Full Frame

Standard Readout Amplifier & ADC Normal Voltage Serial Register High Voltage Serial Register EM Readout Amplifier & ADC EMCCD Cameras Back-illuminated, frame-transfer CCD (for >90% QE) or standard front-illuminated (~65% QE) High Voltage EM register multiplies signal electrons by up to ~1000x BEFORE readout

EMCCD Cameras Normal Voltage Serial Register High Voltage Serial Register Back-illuminated, frame- transfer CCD (for >90% QE) or standard front- illuminated (~65% QE)

Photons Electrons Voltage CMOS Sensor Components Photosensitive Diode Amplifier Photosensitive Diode Amplifier Photosensitive Diode Amplifier Photosensitive Diode Amplifier Row Select Column Select Electronics Built into CMOS Chip

CMOS Sensor Readout Modes Global ShutterRolling Shutter

In rolling shutter mode, each row starts and ends an exposure at a different point in time, not simultaneously as in global shutter mode. If an imaged object is moving, different parts of it will be captured at different times by the rolling shutter, causing possible distortion if object moves quickly enough. The distortion will depend on which direction the sampled object is moving relative to the rolling shutter direction. If rolling shutter captures in this direction, Arrows Indicate Movement Direction Observed object

CMOS Sensor Readout Modes Global Shutter Rolling Shutter (More Common)

How to quantitatively compare performance: Signal-to-Noise Ratio (SNR) Increasing SNR Larger SNR = easier to distinguish image from noise = higher confidence in measurements Noise: Fluctuations in signal that produces uncertainty in the signal

CCD Primary Noise Sources

EMCCD Primary Noise Sources

CMOS Primary Noise Sources

SNR Equation: CCDs

SNR Equation: EMCCDs

SNR Equation: CMOS

CMOS Random Telegraph Noise (Salt-and-Pepper Noise)

CMOS Random Telegraph Noise: Why it Happens Correlated double sampling involves a reference being subtracted from the sample to remove drift. This works great if the reference and sample are at the same signal level but will cause a positive (bright pixel) or negative (dark pixel) whenever the reference is at a different level. a Normal signal Low signal due to e- trapped in pixel defect SHR = Reference signal SHS = Sampled signal

Skewed CMOS Read Noise Compared to Gaussian CCD Read Noise 3% outside of Gaussian fit 40% outside of Gaussian fit The CCD has a read noise distribution close to Gaussian The CMOS read noise distribution is skewed to much larger values due to the noisy pixels (RTN). It is skewed from Gaussian.

Skewed Distribution of Read Noise: How does it behave compared to Gaussian noise? Scientific CMOS ChipBias StackSubtractedCenter Quadrant # Frames AveragedStdev Noise Reduction Expected Noise Reduction%Difference 13.347645 = SQRT(Frame #) 41.6946790.510.501 91.1538240.340.333 160.8872850.270.256 250.7324520.220.209 360.6313510.190.1713 490.5620650.170.1418 Traditional CCD # Frames AveragedStdev Noise Reduction Expected Noise Reduction%Difference 125.051838 = SQRT(Frame #) 412.4962250.4990.5000.2 98.3157340.3320.3330.4 166.2350330.2490.2500.4 254.9850570.1990.2000.5 364.1592810.1660.1670.4 493.5668150.1420.1430.3 Averaging frames is less effective in reducing noise for CMOS. What it means: A researcher will need to acquire more data with a CMOS (compared to CCD) to decrease the error bars by the same amount for low light images.

Binning (in CCDs and EMCCDs) Binning provides the ability to combine pixels into a larger super-pixel before digitization of data. Boosts signal detection capability by increasing effective pixel size Traditional Analog Binning Signal is combined before digitization only 1x RN is applied For 2x2 binning: 4x pixels of signal, 1x RN gives a 4:1 boost in signal to noise ratio

Binning in Scientific CMOS Binning with the scientific CMOS sensor is slightly different when compared to CCDs. In CMOS cameras, binning is applied after readout. So, read noise has already been introduced to each individual pixel before combining in software. Signal is combined after digitization and after RN has been applied to each pixel. Adding 4 pixels together reduces 4x RN to 2x RN. 4x pixels of signal, 2x RN gives only a 2:1 boost in signal to noise ratio

Advantages of conventional CCDs: - Flexibility Conventional CCDs (e.g. ICX 285) allow for greater flexibility and general purpose imaging than sCMOS or EMCCDs. Available in a variety of pixel sizes and formats Available in a wide variety of price points Accessibility of flexible binning Availability of deep-cooled, very low dark current variants

Advantages of EMCCDs: - Low-light fast EMCCDs offer the greatest possible sensitivity, with accessibility of very high speeds Available in back-illuminated frame transfer formats EM multiplication offers the lowest effective read noise Availability of binning Availability of deep-cooled, very low dark current variants Generally at higher price points

Advantages of sCMOS cameras: - high speed, high resolution sCMOS sensors allow for higher speed operation with similar noise performance to conventional CCDs sCMOS readouts allow for high speed operation (>30 fps) with large(er) pixel arrays than EM or conventional CCDs Potentially lower (different ?) noise than comparable ICX285 CCDs Moderate price points Novel technology with room to grow

Summary Conventional CCDs flexible, workhorse Well established. Good combination of speed, sensitivity and resolution EMCCDs low light (fast) Offers the ultimate sensitivity coupled with high speed when needed sCMOS Emerging technology, which offers high speed large format images

Overview of Modern Imaging Sensors: Proper Uses for CCD, EMCCD, and CMOS Cameras.

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