Which of the following is NOT a major type of photodetector?

The correct answer identifies electrochemical as not being a major type of photodetector. Photodetectors are devices that convert light into an electrical signal, and the three primary types of photodetectors are photoconductive, photoemissive, and photovoltaic.

Photoconductive detectors operate by changing their electrical resistance when exposed to light, which can be measured and is a common mechanism in sensors.

Photoemissive detectors work based on the photoelectric effect, where light photons strike a material and release electrons, thus generating a current that can be measured.

Photovoltaic detectors generate electricity directly from light, utilizing the photovoltaic effect, which is the principle behind solar cells.

Electrochemical processes, while they do involve the interaction of light, typically relate to chemical reactions; they don’t function by converting photons directly into an electrical signal, hence not fitting into the main classifications of photodetectors. Understanding these distinctions helps clarify why electrochemical is the option that does not belong among the major types.