Infrared

Infrared is the portion of the electromagnetic spectrum with wavelengths longer than visible light but shorter than microwaves, typically from about 700 nanometers to 1 millimeter. All objects above absolute zero emit infrared energy, with the amount and peak wavelength depending on temperature, following Planck’s law. At common temperatures, emission is strongest in the near to mid infrared.

The infrared region is commonly divided into near infrared (roughly 0.75–1.4 μm), mid infrared (about 1.4–5 μm), and far infrared (roughly 5–1000 μm). Some sources refine these ranges further, particularly for thermal infrared applications.

Historically, infrared radiation was discovered by William Herschel in 1800, who found that a thermometer placed beyond the red edge of the visible spectrum rose in temperature, indicating heat beyond visible light. The term infrared was coined to denote this region.

Detection and generation: infrared radiation is produced by warm objects and is detected with photodiodes, thermopiles, bolometers, or cooled detectors such as indium antimonide (InSb) or mercury cadmium telluride (HgCdTe) devices. It is also generated by infrared LEDs and lasers. Common applications include thermal imaging and night vision, infrared spectroscopy, astronomy, environmental sensing, and fiber-optic communications in the infrared range.

Atmospheric transmission varies: absorption by water vapor and carbon dioxide creates atmospheric windows where IR passes more readily, notably around certain portions of the 3–5 μm and 8–14 μm ranges. Safety considerations include the potential for tissue heating from high-intensity IR sources.