Infrared (IR) light is a form of electromagnetic radiation that occupies the spectrum just beyond what is visible to the human eye, possessing a longer wavelength than red light. Discovered in 1800 by astronomer Sir William Herschel, IR radiation is sometimes referred to as “heat radiation” after he noticed a thermometer placed just outside the red end of a prism’s spectrum registered the highest temperature. It is emitted by any object with a temperature above absolute zero, making it a ubiquitous form of energy in our environment. The ability to sense this invisible energy has led to the development of numerous technologies to detect, measure, and visualize it.
The Invisible Spectrum
The infrared portion of the electromagnetic spectrum is broad, extending from approximately 780 nanometers up to 1 millimeter in wavelength. This vast range is typically segmented into three sub-ranges to categorize their properties and applications. The Near-Infrared (NIR) band is closest to visible light, spanning roughly 0.7 to 2.5 micrometers, and is often used in fiber-optic communications and remote controls.
The Mid-Infrared (MIR) range, from about 2.5 to 12 micrometers, and the Far-Infrared (FIR) or Longwave Infrared (LWIR) range, extending from 12 micrometers to 1 millimeter, contain most thermal energy. Objects near room temperature, including the human body, emit most of their radiation in the FIR/LWIR band. Specialized detection technologies are required for each specific band.
Feeling the Heat: Non-Electronic Detection
The most common way people experience infrared radiation is directly as heat on the skin, a simple, non-electronic form of detection. The absorption of IR energy causes a rise in temperature, triggering the body’s thermal receptors. This perception is largely due to the mid- and far-infrared wavelengths.
Simple instruments like the bulb thermometer used in Herschel’s original experiment also serve as basic IR detectors, relying on a physical change caused by absorbed energy. The absorbed radiation heats the thermometer’s reservoir, causing the liquid inside to expand and rise, providing a measurable indication of thermal energy. Another historical method uses a thermopile, which consists of multiple thermocouples wired in series. This device produces a small voltage proportional to the absorbed heat, translating thermal energy into a rudimentary electrical signal.
The Technology Behind IR Sensors
Modern infrared detection relies on two primary types of electronic sensors to convert IR energy into a usable signal: thermal detectors and quantum detectors. Thermal detectors operate by measuring the change in temperature that occurs when the sensor material absorbs infrared radiation. The most common example is the microbolometer, which uses a tiny element, often made of vanadium oxide or amorphous silicon, whose electrical resistance changes predictably with temperature.
As IR radiation strikes the microbolometer, the element heats up, and the resulting change in resistance is measured by a readout circuit. This sensor is a popular choice for thermal cameras because it is uncooled, operating at room temperature, which lowers the cost and complexity of the device. In contrast, quantum or photonic detectors function by directly measuring the photons striking the semiconductor material.
When an infrared photon is absorbed, it excites an electron to a higher energy level, generating an electrical current. These detectors, such as those made from mercury cadmium telluride (HgCdTe), are much faster and more sensitive than thermal detectors. However, they often require cryogenic cooling to minimize thermal noise that would otherwise overwhelm the faint photon signal.
Visualizing the Invisible: Thermal Imaging
To turn the raw data from an IR sensor into a recognizable image, thermal cameras use a component called a Focal Plane Array (FPA). The FPA is a grid of thousands of individual detector elements, or pixels, arranged at the focal point of the camera’s lens. Each pixel captures the infrared radiation from a specific point in the scene and generates an electrical signal proportional to the IR energy intensity, and therefore the temperature.
The collected electrical signals are then electronically processed and mapped to a visible color scale, a technique known as pseudocolor or false color. Since the human eye cannot perceive infrared light, the camera assigns different colors, such as blue for cooler temperatures and red for hotter temperatures, to represent the thermal data. This visualization allows for practical applications like locating heat loss in buildings, performing non-contact medical diagnostics, or enabling night vision.