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Intensified CCDs

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

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  1. Intensified CCDs seeing the invisible

  2. 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

  3. Terminology • Pixel: Picture Element • CCD: Charge-Coupled Device • CMOS: Complimentary Metal-Oxide Semiconductor • Panchromatic: Sensitivity to a wide range of wavelengths of light.

  4. 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

  5. CCD cell construction

  6. CCD Operation

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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

  12. Quantum Efficiency Quantum Efficiency of CCD used in Hubble Space Telescope’s Wide-Field and Planetary Camera 2

  13. 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.

  14. 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)

  15. 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.

  16. 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)

  17. 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)

  18. 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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