Color Temperature and White Balance

The everyday words and the physics point in opposite directions, and that is the first thing to get straight. In design we call orange light "warm" and blue light "cool" because of fire and ice. In physics, color temperature is a real temperature - how hot an idealized glowing object is - and hotter objects glow bluer. So the cozy, "warm" tungsten bulb is a relatively cool ~2700 K, while the crisp, "cool" blue of open shade is a much hotter ~9000 K.

Once you accept that the number runs against the feeling, color temperature becomes a precise, single-dial way to describe the color of "white" light - which turns out to be the key to making whites look white in photos, video, print, and on screen.

Color temperature borrows from the physics of a blackbody: an ideal object that glows purely because it is hot. Heat a piece of iron and it goes from dull red, to orange, to yellow-white, to blue-white as it gets hotter - the same progression the Kelvin scale describes. A light source's color temperature is the temperature of the blackbody whose glow matches it. Slide through the range and watch the color of "white" change.

Real light sources cluster at recognizable temperatures. Knowing a few anchors lets you read a room - and set a camera - by eye. Tap any source to send its temperature to the dial above.

Most real lights are not perfect blackbodies, so they do not sit exactly on the blackbody curve. Their color is described by correlated color temperature (CCT): the temperature of the nearest point on the Planckian locus - the curve that traces blackbody colors across the chromaticity diagram. "Nearest" is why a greenish fluorescent and a clean daylight can share a CCT yet still look different.

That leftover difference is the second dial: tint, sometimes called duv - how far the light sits off the locus, toward green or magenta. Color temperature alone is a single axis (warm↔cool); real white balance needs two: temperature and tint. Fluorescents and LEDs often need a magenta nudge to cancel a green cast that no amount of warming or cooling can fix.

White balance is the correction that makes neutral things look neutral under a colored light. The light tints everything - warm light adds orange, cool light adds blue - and white balance applies the opposite per-channel gain to cancel it. It is the camera's version of the eye's automatic chromatic adaptation: your visual system silently does this so a white shirt looks white indoors and out, while a camera must be told how.

A 1000 K change is enormous at the warm end and almost invisible at the cool end - going 2000→3000 K is a dramatic shift, while 9000→10000 K is barely perceptible. Kelvin is not perceptually even. The mired (micro reciprocal degree) fixes this: mired = 1,000,000 / K. Equal steps in mireds look like equal shifts, which is why correction filters and gels are rated in mireds - a given filter makes the same visual change on any source.

Temperature is only one axis of neutralizing light. Real capture and display work also includes tint, filter mired shifts, adaptation strength, target white point, and the color of the object being lit. This lab combines those pieces into one inspection surface.

Screens have a color temperature too - the color they show for "white," called the white point. The near-universal standard is D65 (~6500 K), the white of average daylight, used by sRGB, Rec.709, and Display P3. Print and prepress often work to D50 (~5000 K), a warmer white that better matches viewing prints under standardized light. A monitor running at the wrong white point throws off every color judgment made on it - which is why calibration sets the white point first.

Six questions on the Kelvin scale, CCT, white balance, and mireds. Instant feedback, no scores recorded - a wrong answer comes with a short explanation.

Color temperature ties the physics of light to the perception of white and the calibration of devices. The numbers reflect each article's position in the editorial roadmap.