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Understanding Quantum Efficiency in Light Detectors: From Film to CCD Technology

This piece explores the evolution of light detectors, focusing on quantum efficiency and the transition from traditional film to Charge Coupled Devices (CCDs). Historical insights include an 1845 drawing of the galaxy M51 by the 3rd Earl of Rosse alongside a recent photograph to illustrate advancements. Fine-grain film had a quantum efficiency of only 4%, requiring around 25 photons to develop a grain of silver, posing limits on resolution. In contrast, modern CCDs achieve around 70% quantum efficiency, enabling significant image processing capabilities and wavelength detection beyond the visible spectrum.

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Understanding Quantum Efficiency in Light Detectors: From Film to CCD Technology

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  1. Light Detectors Quantum Efficiency and CCDs

  2. Light Detectors • People are limited – not all of us can draw this well: Left: The 3rd Earl of Rosse’s drawing of galaxy M51 (The Whirlpool), 1845. Right: A photograph of it this year.

  3. Mechanical image collection (from mid-19th Century) • Fine grain film used – enables as many photons to be captured as possible • Approx 25 photons required to deposit a grain of silver on the film – quantum efficiency of 4% Problems: Size of grain could be a limit to resolution if bigger than Rayleigh Criterion for resolution. Poor quantum efficiency Also need very precise mechanism to track image across sky – films are dumb!

  4. Charge Coupled Device (from late 1970s) • Quantum efficiency closer to 70% (just over 1 photon required for a signal in a single pixel) • Can detect wavelengths beyond visible (especially infrared) • Not dumb - computer can do image processing.

  5. How it works (1) Some photons hit the CCD and a small electric charge is created (2) This electric charge is shifted to a ‘store’ (3) The ‘store’ is emptied and a varying voltage is recorded (the potential barrier drops). (4) This is sampled and converted into a digital signal (a number) for processing one pixel

  6. Why should you know about it? • Q.Eff = 70% c.f. 4% for film, 1% for eye (i.e. very sensitive) • Can detect non-visible λ • Versatile • Hubble has 4 CCDs, each with 640,000 pixels – basically a very expensive 2.5 megapixel camera. How many pixels does your camera have? Note: CMOS chips are cheaper and perform a similar (although less well defined) function – you may find one of these in your camera rather than a CCD

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