According to wikipedia, photon counting is a technique in which individual photons are counted using a single-photon detector (SPD). However I do not know why we do photon counting because the terms used are out of my knowledge.
In: 5
Basically, single photon detection allows us to detect very dim light with better resolution and less noise than a more conventional detector. This is good when the light is already really dim (telescopes) or we don’t want to use more light than we absolutely have to (X-rays). There are other applications too but they all boil down to SPDs basically being digital (“photon” or “no-photon”) compared to a more conventional analog detector.
Edit:typos
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