What is color?

The word color may seem simple and univocal, but before entering the subject, it’s important to explain what the color(s) word will cover in the following pages.

We will understand by color the result of the interpretation by the brain of the whole range of light that the human eye can perceive, including its nuances of intensity. The term therefore covers the hue as well as the saturation and the luminosity, which are well known terms for people used to handle colors. It’s also implied that there are a certain number of defined colors, that this complete spectrum is divisible into quanta, into bricks. The size of the basic quantum, and incidentally the number of color quanta, the number of different colors, will be discussed later.

Physical color: visible light

What is called color is therefore the perception by human beings of visible light, which is in reality only a small part of the electro-magnetic spectrum, which consists in a large range of frequencies, generally measured in nanometers for visible light.

It is necessary to understand, considering this rainbow spectrum, that in nature, the sources of monochromatic lights are rare, and one perceives essentially a blend of these different frequencies; the colors we’ll discuss here include at the same time these monochromatic sources but also and especially all the possible blends between them, at different intensities. These blends thus represent an infinity of nuances, an infinity of different colors, which it would be impossible to describe all individually.

One naturally try to give oneself some references in all these possible colors, from black to white through gray and in all the shades of the rainbow (i.e. monochromatic lights), red, orange, yellow, green, blue, violet… The problem is then to agree on these names and define precisely for example what is red and what is purple for example.

Historically, a solution came from the discovery of Fraunhofer lines, dark discontinuities in the solar light spectrum, discovered in the early nineteenth century. These lines, caused by the absorption of specific frequencies by the sun’s atmosphere, are visible on precise wavelengths and were one of the first references to define precise colors.

To this problem of reference is added the fact that the definition of color is intrinsically linked to the perception that that one have of light, and this perception obviously differs from one population to another, from one individual to another, and also according to the environment in which color is perceived.

Following light : from emission to reception

Emission, transmission, reflection, reception

A perceived color is the result of a set of light rays emitted (“created”) by a given source (the sun, a screen, a light bulb …), which has passed through one or more media with different properties that have absorbed, deviated or re-emitted part of it 1, which has been reflected by one or more surfaces with different properties that have in turn absorbed, deviated or re-emitted part of it, before finally reaching a sensor: our eye, or a camera…

Diagram from transmission to reception

Breakdown of the received light

In the end, whatever path this set of light rays has followed, we only perceive its arrival state: this is the final color. No matter the path and their origin, all colors are the same thing: groups of light rays.

Color is therefore “just” a blend of a certain number of light rays that have reached a sensor or our eye. Each of these rays, or photons* has a precise wavelength called monochromatic* light, and if colors are seen different from those of the electro-magnetic spectrum, they’re the result of the blend of all these rays.

The quantity of received photons forming this color is what makes the intensity, the luminance*, of the color, and the blend of the various wavelengths which makes the hue*.

Sources and references

  1. We speak in particular of refraction for the deviation of the rays ↩