I.E - About white values : temperature¶

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

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

E.1 - Black body¶

UA black body is an element which does not necessarily appear black, but which is black in the sense that it absorbs all the electromagnetic rays (and thus 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 is called a black body is not seen as black.

The consequence is that the light, and therefore the color, of a black body is not 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 is not 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, does not depend on the environment in which they are; they therefore form good objective reference frames. Moreover, we know that their radiation spectrum depends directly 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*.

E.2 - Planckian locus¶

Rather than defining the spectrum of light from black bodies by describing the rays that make it up, we simplify by simply 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 degrees Celsius or Fahreneit). We can thus define a white light directly according to this theoretical temperature.

Description	Kelvin	Degré Celsius	Degré Farenheit
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

We note that the temperature of white reflects its hue and not the actual temperature of the body that emits 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 seen. We use these temperature values in Kelvin by convention to standardize the description of white.

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

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

E.3 - White balance¶

Under the sun, in the shade or in the clouds, no matter what time of day it is, in reality we perceive snow as white *even though the light it reflects is actually different each time.

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 or a camera to make the same adjustment.

Indeed, a sensor that would receive white at noon in 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: “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.

• To display the image on a screen, the task is to bring the color of the white in the scene (5800 K under the sun or 7000 K under the clouds for example) to that of the white of the screen (in general the one named D65, 6500 K).
• When printing on paper, the white of the scene must be brought back to the white of the paper. Once the image is printed, it can be seen correctly under all possible lighting.

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 orange.

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Sources & References

• Black body on Wikipedia.
• Planckian locus on Wikipedia
• Color temperature on Wikipedia.