As all objects above absolute zero emit radiation in the infrared part of the electromagnetic spectrum, infrared sensing allows you to "see in the dark". That is, to form images when light in the visible part of the spectrum is scarce or absent.
Infrared imaging is also useful for detecting heat sources even when there is enough visible light. For example, the heat of a body or an object can be measured with an infrared thermometer. As well as many devices used by police, security services and military organizations, such as gun aiming systems, certain vehicles, missiles and orbital systems. All utilize some form of infrared sensing technology.
Infrared detection devices have sensors that detect radiation in the infrared part of the electromagnetic spectrum. That is a wavelength between 700 nm and 1 millimeter. Or a few hundred Gigahertz to over 400 THz in terms of frequency. Just between visible light and microwave radiation.
Then the acquisition chain uses this analog measurement. There are two main ways to take advantage of this. Either the signal is converted by spectrum shift in the visible and returned to the operator as is the case in astronomy (we see red but it is in fact infrared), or the servo system operates directly the signal, as is the case for the guidance of a missile.
Infrared light just beyond the human visual limit (wavelength between 700 nm and 2.5 μm) is conventionally called "near infrared". While the light further from the visible spectrum is divided into mid and far infrared (between 2.5 μm and 1 mm). All objects above absolute zero glow in the far infrared, so no illumination source is needed to view such radiation. On the other hand, for observation in the near infrared, it is also necessary to use specific lighting with a light-emitting diode or a filter bulb.
So why use near infrared? Quite simply because a heat source emits in the near infrared, but also because near infrared radiation sensors are always much less expensive than far passive sensors. Some far infrared sensors used for astronomy are even cooled with liquid nitrogen, in order to avoid disturbances caused by their own radiation.
The age-old television remote control that lets you choose your favorite program contains a near-infrared emitter, usually around 950 nm. It emits a very particular and codified radiation according to the brand and the type of instruction. The television or hi-fi receiver receives the signal and interprets it. This radiation is inexpensive to create since it is cheap light-emitting diodes. However, it is easily disturbed by neon or fluorescent lighting, hence its short range.
Other less common items, such as infrared-sensing automatic trash cans, also use a transmitter and a receiver. With the difference that these are side by side. The transmitter constantly emits the signal, and the receiver detects glare in the infrared spectrum. This corresponds to an object approaching the lid of the dustbin and reflecting the radiation of the infrared emitter. In order to save battery power, infrared emission modulation is generally used.
Non-contact thermometers calculate infrared radiation from the human body or objects. We know that the temperature of a black body can be determined by measuring intensity in the infrared spectrum, thanks to Planck's law. To the factor near the emissivity, allowing conversion between black body radiation (ideal:Planck's law) and gray body radiation (real:measured).
Most of the time, this calculation consists of an approximation by a proportional intensity-temperature relationship. This is to overcome the computational complexity on the one hand, and the inherent glare of the cheap sensor. The sensor is thus calibrated in order to be "effective" in the measurement range expected by the user.
"Heat-sensing" missiles track infrared-radiating gases emitted naturally from jet aircraft engines. They have been commonplace since the 1950s. In addition, since the early 1960s, military infrared imaging satellites have observed Earth to detect infrared emissions from rocket and missile launches.
Infrared imagery is also being explored for landmine detection. Anti-personnel mines are usually buried only a few centimeters below the surface. Thus, the infrared thermal radiation pattern of an area can, under certain conditions, reveal their presence.
Infrared camera systems for "seeing in the dark" are also common. They are composed of a near infrared sensor as well as a powerful infrared light source. They can be mounted on vehicles or in fixed places, to allow night surveillance of a specific area.
But beware for helmets or portable weapons equipped with night vision, it is impossible to use an infrared lighting source, otherwise you will be spotted very easily. Because of this, these systems do not use infrared, but instead passively amplify the visible light already present in a dark scene.