Photomultiplier tubes offer the ultimate light-detection sensitivity they can sense individual photons. For a single incident photon, PMTs can produce hundreds of millivolts across a 50-W load resistor at the PMT output. A cascade arrangement of about ten dynodes provides electron multiplications of up to one million. The tube multiplies the number of electrons produced by accelerating them into plates-dynodes-that emit a large number of secondary electrons for each electron that hits them.
PMTs are specialized vacuum tubes that incorporate a light-sensitive photocathode that absorbs incoming photons and subsequently emits electrons. Fortunately, there are sensors for almost any application imaginable-from a PMT that produces a large voltage pulse for each photon it detects to cooled thermopiles that absorb kilowatts of power and produce a thermocouple voltage proportional to the optical power absorbed. Light-sensing optionsĪpplications vary widely, ranging from specialized instrumentation that detects individual photons to measurement of the power output of high-speed welding or cutting lasers that produce kilowatts of optical power.
The optimal approach involves consideration of application-specific parameters such as source wavelength, image size, optical power, signal-to-noise ratio, and package constraints. The choices may seem overwhelming and include photodiodes, phototransistors, photodarlingtons, photomultiplier tubes (PMTs), photoresistors, and integrated circuits, in addition to various hybrids and even thermopiles. Many factors are involved in choosing a detector for a given light-sensing application.
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