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Intensified CCDs. seeing the invisible. Intro. The CCD sensor was invented in 1969 by Willard Boyle and George E. Smith of AT&T Bell Labs. Originally intended as a memory device. Terminology. Pixel: Picture Element CCD: Charge-Coupled Device CMOS: Complimentary Metal-Oxide Semiconductor

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intensified ccds

Intensified CCDs

seeing the invisible

intro
Intro
  • The CCD sensor was invented in 1969 by Willard Boyle and George E. Smith of AT&T Bell Labs.
  • Originally intended as a memory device
terminology
Terminology
  • Pixel: Picture Element
  • CCD: Charge-Coupled Device
  • CMOS: Complimentary Metal-Oxide Semiconductor
  • Panchromatic: Sensitivity to a wide range of wavelengths of light.
ccd operation
CCD Operation
  • Photoelectric effect creates an electron-hole pair when light impinges upon a semiconductor
  • Each pixel accumulates a charge
  • When sampled, the “bucket of charge” for each pixel is transported off-chip to on off-chip amplifier
  • A capacitor is used to convert the charge to a voltage. V = q/C
intensified ccd
Intensified CCD
  • Used largely in military and scientific applications.
  • The image-intensifier is added to a CCD to create an intensified CCD.
  • Provides single-photon sensitivity
  • Also enables extremely short exposure times. (down to 200ps)
  • It reduces the shortcomings of a bare CCD
the image intensifier
The Image Intensifier

A: 200V Gating Voltage (variable)

B: 1000V Micro-channel plate

C: 6kV acceleration voltage

A > 0: Shutter is open (gated)

A < 0: Shutter is closed

Phosphor Screen

Micro-channel Plate (MCP)

Photocathode

A

B

C

the image intensifier1
The Image Intensifier

e- s (1000x)

e-

Phosphor Screen

Micro-channel Plate (MCP)

Photocathode

A: 200V Gating Voltage (variable)

B: 1000V Micro-channel plate

C: 6kV acceleration voltage

A > 0: Shutter is open (gated)

A < 0: Shutter is closed

A

B

C

the image intensifier2
The Image Intensifier
  • Converts incoming photons into electrons
  • Acts a shutter (dependant upon bias)
  • Typically made of MgF2 or Quartz
  • Tiny linear channels are angled from parallel by a few degrees
  • Each MCP stage provides a 1000x multiplication in e-
  • 108 multiplication limited due to saturation
  • Converts electrons back into photons
  • Covered with a thin layer of aluminum
  • Different phosphors have different levels of quantum efficiency and different durations of fluorescence.

Phosphor Screen

Micro-channel Plate (MCP)

Photocathode

quantum efficiency
Quantum Efficiency
  • Is officially defined as the percentage of photons hitting a surface that will produce electron-hole pairs
    • Regular photographic film is about 10%
    • Human Eye is about 3%
    • CCDs can have a QE of more than 90% at some wavelengths
  • Useful for rating solar cells
  • Doesn’t account for unwanted recombination in material
quantum efficiency1
Quantum Efficiency

Quantum Efficiency of CCD used in Hubble Space Telescope’s

Wide-Field and Planetary Camera 2

charge transfer efficiency cte
Charge Transfer Efficiency (CTE)
  • When charges are shifted from pixel to pixel it is the loss associated with each shift. A value of 0.999 is actually bad!
  • Most CCDs use 2000-4000 shifts to read a single pixel out.
dark current
Dark Current
  • Thermal excitations can excite electrons into the conduction band
  • This is the reason that most CCDs require extensive cooling (-90 to -40 °C)
read out noise
Read-out Noise
  • Electronic amplifiers are not perfect and introduce their own noise.
  • This determines the “noise floor” of the CCD. It sets the limitation of how faint of an object a CCD can see.
other losses in qe
Other losses in QE
  • Optically insensitive structures for each pixel (absorption loss)
  • Natural reflection of certain wavelengths (reflection loss)
  • Very long and very short wavelengths pass straight through sensor without generating an electron (transmission loss)
summary
Summary
  • CCDs are simple application of the photoelectric effect
  • Intensified CCDs improve the light sensitivity of a bare CCD
  • Intensified CCDs are used in military and scientific application (mostly astronomy)
bibliography
Bibliography
  • http://www.asiimaging.com/pdfs/Comparison_of_CCD_Cameras_to_an_Ideal_Camera.pdf
  • http://www.andor.com/learn/digital_cameras/?docid=326
  • http://www.dalsa.com/dc/documents/Image_Sensor_Architecture_Whitepaper_Digital_Cinema_00218-00_03-70.pdf
  • http://www.dalsa.com/markets/ccd_vs_cmos.asp
  • http://www.iccd-camera.com/technology_main.html
  • http://en.wikipedia.org/wiki/Quantum_efficiency
  • http://wfc3.gsfc.nasa.gov/MARCONI/machines-see.html
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