Three Technologies
Scientists can choose between three different devices for the most advanced photon counting:
Single Photon Counting Modules (SPCMs) – A SPCM counts photons with an avalanche photodiode
(APD). An APD is a solid-state, silicon-based device that converts photons into electrons in a dramatically
different way than an ordinary photodiode. In ordinary photodiodes, the photon/electron ratio is one to
one. In APDs, however, these photon-generated electrons can excite more electrons, which creates an
avalanche effect, resulting in an internal gain that can be adjusted with the bias,
e.g. 1 to 1000, and results in a signal that is proportional to the light’s intensity.
PerkinElmer’s Photon Counting Technologies
The PerkinElmer SPCM uses an APD that is 200 microns in diameter and operates at 20 volts above
breakdown voltage to generate a gain of 108. Since a gain at this level cannot be sustained without
burning the device, the SPCM comes with electronic circuitry that monitors and repeatedly quenches the
current with a TTL pulse. Operation in this photon-counting mode is extremely fast, allowing users to
count a photon every 50 nanoseconds.
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