About white values : temperature

White being perceived differently depending on the environment and the observer, we must, before talking about it, define exactly what it is. Once again, we need an invariant, physical referential, on which everyone can agree, a bit like how Fraunhofer lines can be used as a reference to define precise monochromatic lights*.

Radiation of black bodies * is commonly used to define white.

Black body

A black body is an element which doesn’t necessarily appear black, but which is black in the sense that it absorbs all the electromagnetic rays (and consequently visible light) that it receives; in other words, if the black body were perfectly cold 1, it would be pure black. But every body has some heat, and this heat causes it to emit radiation, some of it in the visible spectrum, which explains why what’s called a black body isn’t seen black.

The consequence is that the light, and therefore the color, of a black body isn’t influenced at all by the light it receives, but is only the result of its heat. Several elements can be considered as black bodies: the embers of a barbecue, fire, a glowing metal, the sun… All these elements are not real black bodies in the physical sense (the radiation they emit isn’t perfectly independent of the radiation they receive) but are an approximation, very close in the case of the sun2.

Black bodies are interesting because the light they emit, and therefore their perceived color, doesn’t depend on the environment in which they are; they form accordingly good objective reference frames. Moreover, we know that their radiation spectrum depends only on their energy, their heat. One can thus associate to a given heat a given spectrum, i.e. a light of a precise composition of monochromatic rays*, resulting in a precise and measurable white*.

Spreadsheet of some colors according to their temperature

Note

Unlike the other incandescent elements in the examples above, the apparent color of the sun isn’t directly the result of its heat even though it’s a black body; indeed, the absorption of rays by the atmosphere changes its color at our level.

Planckian locus

Rather than defining the spectrum of light from black bodies by describing the rays that make it up, we simplify by linking this color, this white, to the temperature of the black body that emitted it; and this measurement is given in Kelvin (and could just as easily be converted into Celsius or Fahreneit degrees). A white light can therefore be directly defined according to this theoretical temperature.

Description Kelvin Celsius Degree Farenheit Degree
Hot lava 1000 K 726,85°C 1340,33°F
Sun at noon 5800 K 5526,85°C 9980,33°F
Cloudy day 7000K 6726,85°C 12140,33°F
Lightning 9000 K 8726,85°C 15 740,33°F

Note that the white temperature reflects its hue and not the actual temperature of the body emitting this light; this temperature is that of the black body that would have emitted radiation of the same color, but not the temperature nor the energy of the element actually seen. These temperature values are given in Kelvin by convention to standardize the description of white.

All these temperatures can be represented in a ramp going from yellow-orange to blue, in a whitish range (these are indeed colors resulting from a complex blend of monochromatic rays[*][glossary.md]* of the visible spectrum; only the proportions change 3).

We call this set of colors distributed in a line the Planckian Locus.

Planckian Locus with temperatures

White balance

Photos of snowy landscape at different times of the day
Under the sun, in the shade or surrounded by clouds, whatever the time of the day, we perceive snow as white *even if in reality the light it reflects is always different.

The brain is constantly making adjustments to compensate for the color of the light illuminating a scene, and always sees the whites… white. The problem arises when capturing colors via a camera to make the same adjustment.

Indeed, a sensor that would receive white at noon under the sun would actually “see” the “true” color, tending more towards yellow, while the same object on a cloudy day would be “seen” by the sensor as much more blue.

An operation of correction of these colors is then carried out to bring back the color of the object to a neutral “white”: that of the device of reproduction. We call this white balance; it’s the process of “erasing” the influence of the light illuminating the scene at the time of the capture.

Thus, this “neutralized” image can be reproduced by the device under standard conditions.

Without this work of “neutralization”, the image is shifted compared to the reference white (the white of the screen or the white of the paper) and appears bluish or orange4:.

Sources & References

  1. Only black holes, which absorb all light by gravitation, can be considered as almost perfect black bodies and don’t emit any radiation themselves; but it’s now known that even black holes emit a very weak radiation, in a somewhat roundabout way, the Hawking radiation. On the other hand, we still don’t really know if this radiation depends on the composition of the black hole (what’s called the information paradox, but it is a completely different subject than color…). ↩

  2. Theoretically, even the sun reflects the rays it receives; but the proportion between the reflected rays and the emitted rays is so insignificant that the sun can be considered a black body. And at on our scale it’s the same for fire, embers, sparks… ↩

  3. The more energy the body has (the hotter it is), the higher the proportion of rays of high frequency* (and low wavelength*) : indeed, the amount of energy carried by light rays depends directly on their frequency. The shorter wavelength rays are those of the blue side of the spectrum. Thus, the hotter the body, the more the proportion of blue rays increases in the emitted blend, the more the color moves away from the orange-red to blue. But one must keep in mind that emitted light remains a combination, and is just a shade of white.
    The discovery of this link between wavelength and energy, and the work on black bodies color by the physicist Max Planck at the end of the XIXth century are at the origin of modern quantum physics, with the discovery by Planck that energy is composed of discrete values (this is the foundation prior needed for the theorization of the photon by Albert Einstein in 1905). ↩

  4. Obviously, white balance can be used for creative work and give a particular atmosphere to a scene, and not to achieve an actual neutral balance… ↩