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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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