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

2. Charge-Coupled Device (CCD)
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)

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

4. CCD Collecting Photons: Analogous to Catching Raindrops

5. CCD Collecting Photons: Analogous to Catching Raindrops

6. CCD Collecting Photons: Analogous to Catching Raindrops

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

8. CCD Image Acquisition Types
Full Frame
Frame Transfer CCD
Interline CCD

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

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

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

12. CMOS Sensor Readout Modes
Global Shutter
Rolling Shutter

13. Arrows Indicate Movement Direction
Rolling Shutter
If rolling shutter captures in this direction,
Arrows Indicate Movement Direction
Observed object
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.

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

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

16. CCD Primary Noise Sources
SN = Shot (Photon) noise = signal (Physical property of light due to individual photons)
DN = Dark noise = dark current (Temperature dependent)
RN = Read noise (Gaussian distribution of values imparted by single output node/readout amplifier)
-----use common terminology photon noise and signal

17. EMCCD Primary Noise Sources
SN = Shot (Photon) noise = signal (Physical property of light)
DN = Dark noise = dark current (Temperature dependent)
RN = Read noise (Gaussian distribution of values imparted by single output node/readout amplifier)
ENF = Excess noise factor = 1.4 (Caused by probability of creating new electrons due to electron multiplication)
-----use common terminology photon noise and signal

18. CMOS Primary Noise Sources
SN = Shot (Photon) noise = signal (Physical property of light, regardless of sensor)
DN = Dark noise = dark current (Temperature dependent and higher for global shutter)
RN = Read noise (This includes Random Telegraph Noise (RTN), which is non-Gaussian, and depends on multiply column and pixel amplifiers)
-----use common terminology photon noise and signal

19. SNR Equation: CCDs S = Signal = Photon flux * time * QE
DN = Dark noise = dark current
RN = Read noise
SN = Shot (Photon) noise = signal

20. SNR Equation: EMCCDs S = Signal = Photon flux * time * QE
DN = Dark noise = dark current
RN = Read noise
SN = Shot (Photon) noise = signal
ENF = Excess noise factor = 1.4
G = EM Gain factor (Typically up to ~1000x, effective read noise is reduced by this factor)

21. SNR Equation: CMOS S = Signal = Photon flux * time * QE
DN = Dark noise = dark current
RN = Read noise (This includes Random Telegraph Noise (RTN), a significant component of CMOS noise)
SN = Shot (Photon) noise = signal
-----use common terminology photon noise and signal

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

23. CMOS Random Telegraph Noise: Why it Happens
SHR = Reference signal
SHS = Sampled signal
Normal signal
Low signal due to e- trapped in pixel defect
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.

24. Skewed CMOS Read Noise Compared to Gaussian CCD Read Noise
40% outside of Gaussian fit
3% 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.

25. Skewed Distribution of Read Noise: How does it behave compared to Gaussian noise?
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.
Scientific CMOS Chip
Bias Stack
Subtracted
Center Quadrant
# Frames Averaged
Stdev
Noise Reduction
Expected Noise Reduction
%Difference 1
= SQRT(Frame #) 4
0.51
0.50 9
0.34
0.33 3 16
0.27
0.25 6 25
0.22
0.20 36
0.19
0.17 13 49
0.14 18
Traditional CCD
0.499
0.500
0.2
0.332
0.333
0.4
0.249
0.250
0.199
0.200
0.5
0.166
0.167
0.142
0.143
0.3

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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