The Planckian Locus: Black-Body Radiation & the Color of White

A black body is an idealized object that absorbs all radiation and reflects nothing; whatever glow it emits comes purely from its temperature. Heat any real object enough and it approaches this behavior: iron glows dull red near 800 K, orange at 1,300, white past 6,000. The crucial fact is universality: the color sequence does not depend on what the object is made of, only on how hot it is. That is why one curve, parameterized by temperature alone, can describe candle flames, tungsten filaments, and stars alike.

By the 1890s physics could not explain its own furnaces. Classical theory predicted that a heated cavity should radiate ever more energy at ever shorter wavelengths (infinite energy in the ultraviolet, the "ultraviolet catastrophe") while the spectra measured at Berlin's Imperial Institute rolled off smoothly, exactly as the plot above does. In October 1900 Max Planck produced a formula that fit the measurements perfectly; by December he had a derivation, at a price he later called "an act of desperation": energy could only be exchanged in discrete packets, E = hν. That assumption, the quantum, cracked classical physics open. Einstein's photon (1905), Bohr's atom, and all of quantum mechanics grew from it.

Planck (1858–1947) was the least likely person to start a revolution: a conservative Berlin professor who, as a student, was famously advised that physics was nearly complete and only details remained. He distrusted his own quantum for years, hoping it would prove a mathematical trick that classical physics would absorb. It didn't; he received the 1918 Nobel Prize for it. His constant, h = 6.626×10⁻³⁴ J·s, is so fundamental that since 2019 it literally defines the kilogram. Every time a colorist dials a white point in kelvins, they are using the temperature scale of radiation physics Planck's law made computable.

B(λ,T) = (2hc²/λ⁵) · 1/(e^(hc/λkT) − 1) Two workhorse consequences fall straight out. Wien's displacement law (λ_max = 2898/T μm·K) says the spectral peak slides toward blue as temperature rises; that is the moving marker on the plot above. The Stefan–Boltzmann law says total radiated power grows as T⁴. For color work the shape matters more than the totals: the visible-band slice of this curve sets the red-to-blue balance we read as "warm" or "cool" white.

Run every temperature's spectrum through the CIE 1931 standard observer and you trace the Planckian locus: the white curve arcing through the diagram above. Candle flames enter at the red end near 1,700 K; tungsten sits at 2,856 K (Illuminant A); daylight passes through 5,000–7,000 K; and the curve flattens into the blue of a clear north sky beyond 10,000 K. Every "white" a display, fixture, or camera can plausibly claim lives on or near this one physics-given curve. It is the spine of white-point colorimetry.

Real sources rarely sit exactly on the locus, so engineers describe a white with two numbers: CCT, the nearest Planckian temperature (found perpendicular to the locus in the 1960 u,v diagram), and Duv, the signed distance above (green) or below (magenta) the curve. Two "6500 K" fixtures can look wildly different if their Duv differs, which is why LED tint is specified as CCT + Duv, and why a probe report gives both. The working unit for corrections is the mired, 10⁶/CCT: equal mired steps look perceptually equal, which is why gels and camera white-balance shifts are labeled in mireds, not kelvins. (Track the MIRED readout as you sweep the slider; kelvins compress at the blue end.)

Real daylight is black-body sunlight filtered and rescattered by the atmosphere, so the CIE daylight (D) locus runs slightly green-side of the Planckian curve; you can see D50 through D93 sitting just above it in the diagram. D65, the broadcast white, corresponds to about 6,504 K; the odd 4 exists because when the radiation constant c₂ was revised in 1968, the D illuminants kept their defined spectra and names, so their effective temperatures drifted. D50 anchors print, D93 is the legacy Japanese broadcast white, and DCI cinema white sits off both curves entirely. Choosing a white point in the Volume Explorer is choosing a spot in exactly this neighborhood.

Every calibration starts on this curve: the target white is defined relative to it, and the probe reports CCT and Duv against it. Bicolor LED fixtures sweep along the locus; wall panels and RGBW fixtures mix to a CCT + tint spec; camera white balance is a traversal of the locus with a green–magenta axis perpendicular to it; "warm" and "cool" in a grade are, quantitatively, mired shifts. Tungsten (3200 K) and daylight (5600 K) balances are just two parking spots on the same 1900-era physics. And a reference monitor holds D65 only because someone measured that it does. Book a calibration →