Infrared Sensors

Infrared (IR) sensors respond to electromagnetic radiation in the infrared band, which lies just beyond the red end of the visible spectrum at wavelengths longer than about 700 nanometres. Because all objects above absolute zero emit infrared radiation in proportion to their temperature, IR sensors can detect heat and the presence of warm bodies without any physical contact. They form the basis of motion detectors, non-contact thermometers, remote controls and many industrial measurement systems.

The passive infrared (PIR) sensor is the most familiar type. It contains a pyroelectric element that generates a small charge when the infrared radiation falling on it changes. A segmented Fresnel lens divides the field of view into zones, so that a person moving across the zones produces a changing signal that the electronics interpret as motion. PIR sensors are called passive because they emit nothing themselves; they simply respond to changes in received infrared energy, which makes them efficient and reliable for intruder alarms and automatic lighting.

Other infrared sensing approaches serve different purposes. Thermopiles and bolometers measure absolute temperature for non-contact thermometers and thermal imaging. Active systems pair an infrared emitter, such as an LED, with a photodiode or phototransistor to detect reflected or interrupted beams, a principle used in proximity detection and in IrDA data links. Each approach is matched to the wavelength range and response time the application demands.

Frequently asked questions

Why is a PIR sensor called passive?

Because it emits no radiation of its own; it only senses changes in the infrared energy that objects in its field of view naturally emit.

What is the Fresnel lens for in a motion detector?

It focuses infrared from several discrete zones onto the sensing element, so movement between zones causes the signal changes the detector uses to register motion.

Can infrared sensors measure temperature?

Yes. Thermopile and bolometer sensors convert received infrared radiation into a temperature reading, which is how non-contact thermometers work.