Overview
Infrared (IR) radiation is electromagnetic radiation with a wavelength longer than visible red light, felt as heat. This topic covers its parameters, bands and sources, its biological action and its use in diagnosis (thermovision).
Parameters and bands
IR radiation has wavelengths from about 760 nm to 1 mm (frequencies of about 3·10^11 to 4·10^14 Hz). It is divided into three bands:
Properties and sources
IR radiation is reflected, refracted, absorbed and scattered like visible light. Every body above absolute zero emits it, and the emitted power rises with temperature (Stefan–Boltzmann law) while the peak wavelength moves to shorter values (Wien's law). The skin, at about 33 °C, emits mainly at about 9–10 µm.
- Natural: the Sun (about half of its energy), and all warm bodies including the human body.
- Artificial: incandescent lamps, heating elements, IR lamps (e.g. Sollux) and IR lasers and LEDs.
Biological action
IR radiation is absorbed by tissue and converted to heat. It causes local vasodilation (erythema ab igne with repeated exposure), increased circulation and metabolism, and muscle relaxation and pain relief, so it is used in physiotherapy. Excessive exposure causes burns, heat stroke, and damage to the eye: IR-A can injure the retina, and IR-A and IR-B cause cataract (glassblowers' cataract) and IR-C mainly corneal injury. Protection is by goggles, screens and limiting exposure time.