A photomultiplier tube is an extremely sensitive light detector that can resolve single photons. It works by multiplying the small current produced by incident light up to 108 times through a process called secondary emission using a photocathode, dynodes, and anode. This allows even tiny and normally undetectable currents to become much larger and measurable. Compared to a phototube, a photomultiplier tube multiplies the electrons emitted from the photocathode, providing much higher gain and allowing it to be used for very low light signals. Photomultiplier tubes cost between $175-300 depending on their specifications and are used in applications requiring high sensitivity light detection.