Which photodetector is described as the most common and most sensitive for low light levels?

• A. Barrier Layer Cell
• B. Phototube
• C. Photomultiplier tube
• D. Photodiode

Answer: (C)

The ability to detect very faint light relies on a detector that can massively amplify the tiny signal produced by a photon. The photomultiplier tube achieves this with a cascade of dynodes: the photocathode emits an electron for each incoming photon, and that single electron triggers a chain reaction that releases many more electrons at each stage. This internal multiplication can reach gains of millions, making even a single photon produce a measurable electrical pulse. The result is extremely high sensitivity and a fast, clean signal, which is why PMTs are widely used in low-light scenarios in physics, chemistry, and astronomy.

A photodiode, by contrast, converts light directly into a current without internal gain. In very low light, the signal is so small that it’s hard to distinguish from noise unless you add external amplification, which reduces the advantage of a solid-state, compact device. A phototube is an older vacuum detector with a photocathode and an anode; while it detects light, it generally offers less gain and slower response than a modern photomultiplier tube. Barrier layer cells are another older, less common detector with limited gain.

So, for detecting weak light levels with the highest sensitivity, the photomultiplier tube is the best choice.