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Heat a poker and it glows red, then orange, yellow, white. Hot stars are blue-white. The colour depends on the temperature alone, not on what the object is made of.
Wien’s law: the wavelength of the brightest light is 2,898 micrometre-kelvins divided by the temperature. The Sun peaks at 502 nm; you peak at 9.3 µm.
Double the temperature and a surface radiates sixteen times as much. That is why a hot thing loses heat so fast, and why a star’s temperature dominates its brightness.
A 2,700 K filament turns only about 7% of its power into visible light. The rest is infrared. An LED picks its colour without being hot.
Divide 2,898 by the temperature in kelvin and you have the peak wavelength in micrometres: 6,000 K gives 0.48 µm (blue-green), 3,000 K gives 0.97 µm (just past red, in the infrared), 300 K gives 9.7 µm. Cameras that see heat work at 8 to 14 µm for exactly this reason.
Warm white bulbs, the yellowish ones, are 2,700 to 3,000 K; “cool” blue-white ones are 5,000 to 6,500 K. It sounds backwards, but it is the black body’s own order: a candle is cooler than the Sun, and the cooler it is the redder it glows. Lamps are described by the temperature of the black body whose colour they match, their correlated colour temperature, even when, like an LED, they are nothing like one.
Every solid begins to glow a dull red at about the same temperature, whatever it is. An electric hob, a poker and a lump of iron all do it near 525 °C. Below that the radiation is all infrared: hot, but dark.
Betelgeuse looks red because its surface is about 3,600 K; Rigel looks blue-white at about 12,100 K. Astronomers read a star’s temperature from its colour, and its brightness then gives its size: Betelgeuse is cool but radiates tens of thousands of times the Sun’s power because its surface is so large. The table under the picture puts the numbers side by side.
The Sun’s curve is broad: it has plenty of red, orange, yellow, green and blue, and your eyes and brain weigh them together as white. The green peak is only a statement about where one narrow slice of the curve is highest. Seen from space the Sun is white; at sunset the air removes the blue and leaves it red.
This tool is about what heat does to light: the colour, the curve and the power of something at a temperature. The Electromagnetic Spectrum Explorer is about the light itself, from radio to gamma on one axis: wavelength, frequency and photon energy together, what one photon can do and what the atmosphere lets through. Each opens the other at the same place: the link under the spectrum above takes the peak you are looking at there, and its black-body chart opens here at its temperature.
A black body is a perfect radiator. Real surfaces radiate less (their emissivity is below 1), and by a different amount at different wavelengths. Fluorescent tubes and LEDs make light in a few narrow bands, not a smooth curve, and the sky is blue because of scattering, not because it is hot. The colour on your screen depends on its settings, and the brightness of the glow is a picture, not a measurement.
An object that absorbs all the light that falls on it and, when heated, radiates in a way that depends only on its temperature. It is an ideal: soot, the Sun’s surface and the inside of a hot furnace come close. The spectrum it gives out is Planck’s law.
The temperature of the black body whose glow matches the colour of a light. A candle is about 1,850 K, a warm bulb 2,700 K, daylight 6,500 K. For lights that are not black bodies, such as LEDs, it is called the correlated colour temperature: the nearest match.
Hotter atoms jiggle harder and give out photons of higher energy, which are shorter wavelengths. As the temperature rises the whole curve moves to shorter wavelengths and rises steeply, so a hot object first adds red light, then orange, yellow and green, until the mix looks white and then blue-white.
The wavelength at which a black body shines brightest is inversely proportional to its temperature: 2.898 mm·K divided by T. Double the temperature and the peak moves to half the wavelength. It is a consequence of Planck’s law.
The total power a black body radiates from each square metre is σT⁴, with σ = 5.67×10⁻⁸ W/m²K⁴. Because of the fourth power, a surface at 1,200 K radiates about 16 times as much as one at 600 K. A real surface radiates a fraction of that, its emissivity.
A filament has to be very hot to give white light, and a black body at 2,700 K radiates only about 7% of its power as visible light; the rest is infrared. The best a black body can manage is about 47% at 7,000 K, and no material survives that. LEDs avoid the problem by emitting light directly without being hot.
About 525 °C (798 K), the Draper point, as a dull red. By 1,000 °C it is bright orange, by 1,500 °C yellow-white. Below the Draper point objects are hot but dark: their radiation is in the infrared, which a thermal camera sees.
Nearly. Its spectrum is close to that of a black body at 5,772 K, with dips where atoms in its atmosphere absorb light. That temperature is defined so that a black body of the Sun’s size would radiate the power the Sun actually does.
The fraction of a perfect black body’s radiation that a real surface gives out at the same temperature: 1 for a perfect one, near 0.95 for skin, paint and soot, near 0.05 for polished silver. A shiny surface radiates little, which is why a thermos is silvered and why a thermal camera misreads shiny metal.
This page has changed in 13 archived releases, but each of those was a site-wide pass, so none is attributable to this tool on its own and none is listed here. That is not a claim that the tool never changed — a release that reworked many pages at once may well have altered this one too. The changelog has them.