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Detecting Single Photon Energy

  1. Jul 21, 2008 #1
    I was taught that individual photons have discrete energies (E = hv) according to their wavelength (which is a smooth parameter). Why then do we use filters before the ccd camera in a microscope to detect photons of different wavelengths?

    Isn't there a detector that can tell me the energy (and therefore the wavelength) of a single photon?
  2. jcsd
  3. Jul 21, 2008 #2


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    For high energy photons, yes, detectors can tell the energy of a single photon. However, for photons in the visible range, their energy is so small, that we can barely detect them. In principle, one could, if the detection process is the photo-electric effect (ok), if the photo-electron would start out from a well-defined state of known energy (not ok in solid photocathodes, but in a gas, this can be ok), and if we could measure precisely the energy of the photo-electron (is not unthinkable).
    But this is not the case for a photomultiplier.
    As a CCD is an integrating device which accumulates the charge of many many photons in one pixel, and then measures the total charge, you see that this is even further away from measuring single-photon energies.

    As I said, for X-ray photons, that's not a problem. Cooled germanium crystals can be used to measure the energy of such photons, and that's actually used a lot in spectroscopy. But for visible photons, the energy is too low to do that in practice.
  4. Jul 22, 2008 #3
    There's apparently more to consider. For example, a real detector will have thermal motions of its own, which will cause it to see phase jitter in an arriving pure tone, and hence a line spread. Several basic issues of this kind have been discussed some years ago at the SPIE Nature of Light: What is a Photon conference, as I recently found here.
  5. Jul 23, 2008 #4
    They won't tell you the energy of the incident photon, but avalanche photodiodes (APDs) are capable of detecting single photons with fairly high probability. Cooling an APD to cryogenic temperatures will lower the dark count rate.
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