Infrared light is a form of electromagnetic radiation, traveling at the speed of light, and is emitted by all objects above absolute zero. Despite its pervasiveness, infrared light is entirely imperceptible to the unaided human eye, meaning it does not “look like” anything in the way that visible colors do. This invisibility is a biological limitation, yet technology allows us to translate this hidden radiation into recognizable images.
The Science of Infrared
Infrared (IR) radiation is situated on the electromagnetic spectrum immediately adjacent to the red end of the visible light range. This positioning means IR waves have a longer wavelength and a lower frequency than red light. The entire infrared spectrum spans from about 700 nanometers (nm) to one millimeter (mm) in wavelength.
Scientists typically divide the infrared spectrum into three main categories based on wavelength and behavior. Near-infrared (NIR) has the shortest wavelengths, closest to visible light, generally ranging from 700 nm to about 2,500 nm. Mid-infrared (MIR) occupies the middle section, and Far-infrared (FIR) has the longest wavelengths, extending up to 100 micrometers or more.
The specific wavelength of the emitted IR depends on the object’s temperature, with warmer objects emitting more radiation. The thermal radiation given off by objects at or near room temperature falls into the far-infrared band, which is strongly associated with heat.
Why It Is Invisible to the Human Eye
The human eye is only capable of detecting a very small segment of the electromagnetic spectrum, known as the visible light spectrum. This narrow band includes wavelengths ranging approximately from 400 nm (violet) to 700 nm (red).
Our vision relies on specialized photoreceptors, specifically the rods and cones, located in the retina. These cells contain light-sensitive proteins tuned to absorb photons within the visible range. When infrared photons strike the retina, their wavelengths are too long and their energy too low to be effectively absorbed and converted into an electrical signal the brain can interpret as light.
While we cannot see infrared light, we perceive the effects of longer far-infrared wavelengths as radiant heat. This is the warmth experienced when standing near a fire or feeling the sun on one’s skin. The sensation of warmth is the energy from the infrared radiation being absorbed by the skin, which is a physiological reaction, not a visual one.
Visualizing the Invisible: Thermal and Night Vision
Specialized technologies are required to translate infrared radiation into a visible image. The two main methods for visualizing the infrared world are thermal imaging and image-intensifying night vision. These distinct technologies operate on different parts of the infrared spectrum and create very different kinds of images.
Thermal imaging, or thermography, works by detecting the heat that all objects emit, focusing on the mid- and far-infrared wavelengths. A thermal camera uses a sensor to measure minute differences in this emitted radiation across a scene. The camera converts these temperature variations into an electronic signal, which is displayed as a visual image called a thermogram. Hotter objects appear in brighter or warmer colors, while cooler objects are represented by darker or cooler colors, creating a pseudocolor display that makes temperature differences visible.
Night vision devices operate on a different principle, primarily using shorter near-infrared (NIR) and residual visible light. This technology gathers faint ambient light from sources like the moon or distant stars. The device amplifies these light photons, including those in the near-infrared band, converting them into a stream of electrons. These electrons are then accelerated onto a phosphor screen, creating the characteristic bright green image that is much brighter than the original scene.
Everyday Examples of Infrared Light
Infrared light is integrated into many common devices and natural occurrences. A common application is the wireless communication used in nearly all television remote controls. When a button is pressed, the remote sends a coded signal using short-wavelength near-infrared light, which is invisible to the user but easily detected by the television receiver.
Longer-wavelength infrared is frequently used in devices designed to generate warmth. Heat lamps, such as those found in saunas or over food service counters, utilize mid- and far-infrared radiation to directly heat objects or people. The heat felt from a toaster or an electric heater is also a direct result of far-infrared emission.
The human body is a constant source of infrared radiation, emitting thermal energy primarily in the far-infrared spectrum. This natural body heat is the signature detected by infrared thermometers used for non-contact temperature measurement. Security cameras and motion detectors also commonly employ near-infrared light for illumination in dark settings.