Photodetectors: PMTs and CCDs
A photomultiplier tube (PMT) turns single photons into a large, measurable pulse of current by knocking one electron loose via the photoelectric effect, then multiplying it through a cascade of dynodes that each release several secondary electrons per electron they absorb. A CCD instead uses the photoelectric effect inside a semiconductor, at millions of independent pixels at once, storing each pixel's accumulated charge and reading it out to build up an image.
Practise this conceptA single photon carries so little energy that the one electron it can knock loose via the photoelectric effect is far too small a signal to measure directly — you need some way to turn one electron into a burst of billions. A photomultiplier tube does exactly that with a chain of electrodes called dynodes, held at successively higher voltages. The photocathode releases one electron per absorbed photon; that electron accelerates toward the first dynode and, on impact, knocks loose several secondary electrons of its own. Each of those accelerates to the next dynode and repeats the process. With ten dynode stages and a modest gain of four secondary electrons per stage, the single original electron becomes roughly a million by the time it reaches the anode — an exponential cascade, not an additive one, because the multiplication compounds at every stage rather than just adding up.
A CCD image sensor uses the same underlying photoelectric effect but spreads it out instead of amplifying it. Each pixel is a tiny region of semiconductor where an absorbed photon frees a charge carrier, and that charge simply accumulates in place for the whole exposure rather than being multiplied. After the exposure, the accumulated charge from each pixel is shifted, row by row, into a readout register that converts it to a digital value — this is the "charge-coupled" part of the name, referring to the way charge is passed from one pixel to the next during readout rather than read directly in place.
Neither device tells you what colour hit it. A photomultiplier's single output current says nothing about the photon's wavelength — only how many photons arrived, weighted by how efficiently the photocathode responds at that wavelength. A CCD pixel's accumulated charge is exactly the same: proportional to how much light hit it, with no colour information in the charge itself. Colour cameras get around this by putting a different colour filter over each pixel (or by taking multiple exposures through different filters), then combining the results — the sensor itself is colour-blind either way.
Key ideas
Requires: The photoelectric effect
Formulas
| Symbol | Name | Unit |
|---|---|---|
| gain | 1 (dimensionless) | |
| number of dynodes | 1 (dimensionless) | |
| gain | 1 (dimensionless) |
Common misconceptions
- Believing a photomultiplier tube's dynode cascade adds a fixed number of extra electrons at each stage, rather than multiplying the electron count by roughly the same factor at every stage — which is why the total gain grows exponentially with the number of dynodes, not linearly.
- Believing a CCD detects light by some fundamentally different process than a photomultiplier tube, rather than both relying on the same photoelectric effect — a CCD just uses it inside a semiconductor at millions of independent locations instead of once, followed by a dynode cascade, at a single location.
- Believing a single pixel in a CCD image directly reports which colour of light hit it, rather than only the total accumulated charge — colour information requires either separate filtered pixels or multiple exposures, because the photoelectric charge a pixel collects doesn't by itself encode the photon's wavelength.
Resources
Part of these UT courses
This concept is mapped as covering part of the following course(s) in the University of Tartu curriculum.
- OpticsLOFY.01.008
- SpectroscopyLTFY.01.014