1 / 19

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

hazel
Télécharger la présentation

Intensified CCDs

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related