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Light and Color

V. Lacroix, Light and Color

Light is fundamental in our perception of color.

Sometimes it charms us: in a shop we are seduced by the color of a piece of clothing, and once outside, in daylight, we are disappointed.

The reverse also occurs. A splendid interior decoration in subtle harmonies lit by a large bay window. What a pleasure for the eyes: the colors live and blossom according to the day and the seasons. But in the evening, extra-economical lamp lighting kills all this magic.

What happened? Why these changes?

To answer these questions, a short diversion into the physics of color is essential and will serve as a basis for future articles.

The “white” light

The vision of a rainbow reminds us that daylight is made up of rays of different wavelengths.

The drops act like glass beads that scatter light, like Newton’s prism conceptualize in the animation below. These rays are also represented by a spectrum, in the colors as they appear to us in the rainbow.

All these visible rays, from the shortest wavelengths around 380 nm to the longest around 780 nm, form the “white light”.

Rainbow (Ben_Njeri CC BY-SA 3.0 )
Spectrum of visible light and the various color range according to wavelength (in manometer). V: Violet, B: Blue, G: Green, Y: Yellow, O: Orange, R: Red (source: wiki commons).
Conceptual diagram of the dispersion of daylight in its different rays whose color appears different according to their wavelength (red for long, green for medium and blue-violet for short).

White, did you say white?

White? Would daylight be white? But in the morning, it seems slightly bluish. And then, the Italian light has nothing to do with that of the Lofoten Islands. They certainly don’t have the same color.

In fact, painters testify to the change in the color of light; the observation of color fluctuations is even at the origin of the Impressionist movement. Look at each of Monet’s cathedrals: the stone has not changed, only the light has changed.

Claude Monet, Variations of light on Rouen Cathedral (oil paintings)

In reality, only light in grey weather is “white”. In the paintings above, the central painting reflects the “true” color of the stone. And indirectly, each painting informs us of the color of the light that illuminates the cathedral.
This light, whether morning or evening, is indeed made up of all the waves of the visible range, but in variable proportions.

What does it matter to the colorist, photographer or painter, if not for the pleasure of our eyes?

The spectrum of light

The color of the objects around us depends on the light they receive, as they reflect some of it, so it is important to know the composition of the light.

The illustrations in the section above show which rays constitute light but not to what extent.

Imagine the light as a soup. If you vary the proportion of the ingredients, the soup will have a completely different flavor. Also, if one ingredient dominates, it will tend to impose its taste on the whole soup. The same goes for light. But rather than giving the quantity of ingredients, you note the energy that each ingredient brings.

Therefore, to describe a light source, one provides its spectrum; you can see it as the ingredients of the “light soup”: it gives the energy contribution of each ray according to its wavelength.

At the top, the “ingredients” of the light of a halogen lamp; at the bottom, those of daylight (adapted from the site

The “daylight” in grey day illustrated above is therefore roughly made up of all visible wavelengths contributing an equal share of energy. If we refer to an ideal white, then the shares are exactly the same; we have a perfectly flat spectrum.

The spectra of light

In reality, as Monet points out, light is not white. Even if we would call it white, it doesn’t have the same qualities whether it is from the north, the south, at noon or in the evening. Look at the sky depicted in introduction or the one below; the light is very different whether you look in one or another direction.

V. Lacroix day light within a 180 degrees range

How does this affect colors? The shades of colored objects will be different.

The light that varies the least during the day and the seasons is the light from the sky in the north. This is why painters are advised to have a studio with windows facing north; from one day to the next the color of their paintings remains stable.

The qualities of light result in different spectra as shown in the figure below. Light at sunset has more long waves than light in the northern sky; it will appear redder, or “warmer”.

The Spectra of various “Daylights” (adapted from Bruce MacEvoy; the curves show the relative contribution in energy of the rays with respect to wavelength (in manometer)

Two specific “daylight” spectra are shown in the figure above: D55 and D65. One is referred as daylight illuminant (D65) and the other, noon sunlight illuminant (D55). These are standard illuminants with different color temperatures. This characteristic will be discussed in a future article and in the color challenge #18.

Influence of lighting conditions

Your digital camera guesses the type of lighting you’re in: it does an automatic “white balance”. You can change it, and you’ll get a very different picture, as shown in the two photos below.

Still life in “cold” and “warm” light (CC BY-SA 2.5 Vinciane Lacroix)

In other types of lighting, such as LED or fluorescent lighting, the energy contributions of the waves can be very different. As a result, surfaces receive more or less waves in certain parts of the spectrum, which will change the color.

To find out whether the lighting preserves color well, we talk about its “CRI” (Colour Rendering Index), rated out of 100, which will be discussed in a future article. You should already know that halogen lamps have a CRI of 100, and few LEDs today reach 95.

In the meantime, if the color of an object is important to you, try to see it in daylight or at least in the light conditions in which you will be using it.

Play with lighting conditions

And like artists, painters and photographers, play with the differences in light! Night photos mixing different types of lighting are often interesting. Take the example of photographer Gregory Crewdson, whom I introduced to you in a previous article.

Photography © Gregory Crewdson
Photography © Gregory Crewdson
Photography © Gregory Crewdson

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