Like other types of photodetectors, PN junction photodiodes are widely used in devices such as photoresistors, photocouplers, and photomultiplier tubes. They can output corresponding analog electrical signals (eg measuring instruments) or switch between different states of digital circuits (eg control switches, digital signal processing) according to the illuminance of the received light.

Photodiodes are also used in consumer electronics such as CD players, smoke detectors, and infrared remote controls that control televisions and air conditioners. For many applications, photodiodes or other photoconductive materials can be used. They can all be used to measure light, often working in camera photometers, automatic brightness adjustment of street lights, etc.

All types of light sensors can be used to detect bursts of light or to detect luminescence within the same circuitry. Photodiodes are often combined with light-emitting devices (usually light-emitting diodes) to form a module, which is often referred to as an optocoupler device. If so, the movement of external mechanical devices (such as optical choppers) can be analyzed by analyzing the received light. Another function of the photodiode is to act as an intermediary between the analog circuit and the digital circuit so that the two circuits can be coupled by the optical signal, which can improve the safety of the circuit.

In scientific research and industry, photodiodes are often used to accurately measure light intensity because of their better linearity than other photoconductive materials.

Photodiodes are also widely used in medical applications such as X-ray computed tomography and pulse detectors.

PIN junction photodiodes are generally not used to measure very low light intensities. Avalanche photodiodes, photocouplers, or photomultiplier tubes can be useful if high-sensitivity detectors are required in low-light situations, such as applications in astronomy, spectroscopy, night vision equipment, and laser rangefinders.