Infrared

• 1. Introduction

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  Infrared is a form of electromagnetic radiation that the human eye cannot see, yet it surrounds us constantly. In 1681, the experimenter Edme Mariotte showed that glass, transparent to sunlight, still blocked radiant heat. It was a clue to something hidden in plain sight. Slightly more than half of the energy the Sun delivers to Earth arrives as infrared. A human body at normal temperature glows in it, radiating chiefly at wavelengths around 10 micrometres. So what exactly lies just beyond the red edge of what we can see? How was an invisible kind of light first caught with a thermometer? And how did this band of the spectrum end up guiding missiles, peering beneath paintings, and letting snakes hunt in the dark?

• 2. A Thermometer In The Dark

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  In 1800, the astronomer Sir William Herschel passed sunlight through a prism and watched a thermometer climb beyond the red part of the spectrum, where no light was visible. He published his results before the Royal Society of London that same year. Surprised by what he had found, he called these invisible rays "Calorific Rays". The word "infrared" itself did not appear until late in the 19th century, built from the Latin prefix infra-, meaning below, because the light sits below red on the spectrum.

  A decade earlier, in 1790, Marc-Auguste Pictet had demonstrated the reflection and focusing of radiant heat with mirrors, in the complete absence of visible light. The discoveries kept coming through the 1800s. In 1830, Leopoldo Nobili built the first thermopile infrared detector. In 1840, John Herschel produced the first thermal image and named it a thermogram. By 1878, Samuel Pierpont Langley had invented the bolometer, a device able to measure tiny temperature fluctuations and so gauge the power of far infrared sources. The trail of names and dates would soon run straight into one of physics' deepest puzzles.

• 3. The Catastrophe Of Hot Bodies

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  Almost all black-body radiation from objects near room temperature falls in the infrared band. Black-body, or thermal, radiation is continuous, meaning it radiates at all wavelengths. The amount peaks at a wavelength inversely proportional to an object's absolute temperature, a relationship known as Wien's displacement law. In 1860, Gustav Kirchhoff formulated the blackbody theorem, and in 1879 the Stefan-Boltzmann law established empirically that the power radiated by a blackbody is proportional to temperature to the fourth power.

  In the 1880s and 1890s, Lord Rayleigh and Wilhelm Wien each solved part of the blackbody equation, but their solutions diverged in different stretches of the spectrum. The failure earned a dramatic name: the "ultraviolet catastrophe and infrared catastrophe". The resolution came in 1901, when Max Planck published the blackbody equation and theorem, solving the problem by quantizing the allowable energy transitions. In 1905, Albert Einstein developed the theory of the photoelectric effect. As a form of electromagnetic radiation, infrared carries energy and momentum, exerts radiation pressure, and behaves as both a wave and a particle, the photon.

• 4. Carving Up The Band

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  There is no universally accepted definition of where infrared begins and ends. By the most common reckoning it stretches from the nominal red edge of the visible spectrum at 780 nanometres out to 1 millimetre, a frequency range from roughly 430 terahertz down to 300 gigahertz. Beyond it lies the microwave portion of the spectrum, and increasingly terahertz radiation is counted as microwave rather than infrared, pushing the edge to 0.1 millimetre.

  Different fields slice the band differently. A commonly used scheme runs from near infrared, abbreviated NIR, up through short-wavelength, mid-wavelength, long-wavelength, and far infrared. The International Commission on Illumination, or CIE, recommends three bands labelled IR-A, IR-B, and IR-C. The ISO 20473 standard names near, mid, and far infrared with its own wavelength cuts. Astronomers use their own divisions, and a photometric system assigns capital letters to spectral regions according to the filters used: I, J, H, and K cover the near-infrared wavelengths, while L, M, N, and Q cover the mid-infrared. The boundary with visible light stays deliberately fuzzy. The eye's sensitivity falls off rapidly but smoothly past about 700 nanometres, so a bright enough near-infrared source can appear as a dull red glow. Intense light at wavelengths as long as 1,050 nanometres has been seen by humans, a quiet hazard since such light may carry a large amount of energy the eye does not register.

• 5. Heat And Its Disguise

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  Infrared is popularly known as "heat radiation", but the label is a coincidence rather than a law. Light of any frequency heats surfaces that absorb it. Infrared light from the Sun accounts for 49% of the heating of Earth, with the rest coming from visible light that is absorbed and then re-radiated at longer wavelengths. Objects at room temperature emit radiation concentrated mostly in the 8 to 25 micrometre band, yet a hot enough body will glow in visible light, and the solar corona radiates all the way up into the X-ray region.

  Sunlight, at an effective temperature of 5,780 kelvin, is slightly more than half infrared. At zenith it delivers just over 1 kilowatt per square metre at sea level. Of that energy, 527 watts is infrared, 445 watts is visible light, and 32 watts is ultraviolet. Nearly all the infrared in sunlight is near infrared, shorter than 4 micrometres. A key concept for reading infrared is emissivity, a surface property describing how an object's thermal emissions deviate from a perfect black body. Two objects at the same physical temperature can show different infrared images if their emissivity differs. The balance between absorbed and emitted infrared has an important effect on Earth's climate, a balance now watched closely from the ground and from space.

• 6. Seeing The Unseen

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  Night-vision equipment turns darkness into sight by converting ambient light photons into electrons, amplifying them, and converting them back into visible light. An infrared light source can boost the available light without revealing an observer, allowing people or animals to be watched undetected. This is distinct from thermography, which builds images from differences in surface temperature. Thermographic cameras detect radiation roughly in the 9,000 to 14,000 nanometre range and turn it into pictures, letting one see an environment with or without any illumination.

  Infrared homing, a passive missile guidance system, tracks the infrared radiated by a hot target, which is why such missiles are called "heat-seekers". In guided missile technology the 3 to 5 micrometre portion of the mid-wavelength band is the atmospheric window where passive heat-seeking seekers are designed to work, homing on a target aircraft's signature, typically the jet engine exhaust plume. Infrared astronomy uses sensor-equipped telescopes to pierce dusty regions of space such as molecular clouds, to find planets, and to view highly red-shifted objects from the early universe. Earth-based infrared telescopes are limited by atmospheric water vapour, so their detectors are chilled with liquid helium and the best location for the work is outer space. Cold, dark molecular clouds glow with radiated heat as embedded stars warm them, and infrared can reveal protostars before they ever emit visible light.

• 7. Beneath The Paint

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  Infrared reflectography can be applied to paintings to reveal hidden layers without damaging them, in particular an artist's underdrawing. Conservators use it to compare the visible paint with the drawing beneath, where alterations made by the original artist are called pentimenti. In general, the more pentimenti a work shows, the more likely it is to be the prime version rather than a copy. Carbon black shows up especially well in reflectograms. Recent infrared-sensitive cameras can uncover entire compositions painted over and lost, as with Picasso's Woman Ironing and Blue Room, where a portrait of a man was found beneath each.

  The same approach serves very old written documents, including the Dead Sea Scrolls, the Roman works in the Villa of the Papyri, and the Silk Road texts found in the Dunhuang Caves, where carbon black ink can show up extremely well. Infrared cleaning works on a different principle, collecting an extra infrared channel during a scan to locate dust and scratches so they can be corrected or filled in. Infrared spectroscopy reaches still smaller, identifying molecules by the vibration of their chemical bonds. Each bond vibrates at a characteristic frequency, and if an oscillation changes the molecule's dipole moment, it absorbs a photon of matching frequency. The technique typically studies organic compounds in the mid-infrared band, from 4,000 to 400 reciprocal centimetres, where a wet sample betrays itself with a broad O-H absorption around 3,200 reciprocal centimetres.

• 8. Creatures That Feel The Heat

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  The pit viper carries a pair of infrared sensory pits on its head, though the exact thermal sensitivity of this detection system remains uncertain. Crotaline and boid snakes use these IR-sensitive pits to identify and capture prey by the heat it gives off. Pythons, some boas, and the common vampire bat, Desmodus rotundus, share the trick, the bat using its pits to find blood-rich regions on a warm-blooded victim. The jewel beetle Melanophila acuminata locates forest fires through infrared pit organs, then lays its eggs on the freshly burnt trees.

  Darkly pigmented butterflies such as Pachliopta aristolochiae and Troides rhadamantus plateni carry thermoreceptors on their wings and antennae that shield them from heat damage while they bask in the sun. Near-infrared vision, long thought impossible because of noise in visual pigments, has been reported in the common carp and in three cichlid species, which use it to capture prey and orient their swimming. Human ambition has pushed even further. On the 14th of February 2013, researchers developed a neural implant that gave rats the ability to sense infrared light, the first time living creatures were granted a wholly new ability rather than a replacement for an existing one.