Challenge #16 What is the color of the shadow? (I)

Although our attention is mainly captured by the light, the shadow is an essential element that structures the scene we look at. Without it, the world appears flat, formless and roundless. Strong shadows give a graphic touch; soft shadows give a feeling of lightness and tranquillity.

Think of a painting, a picture or even an interior. How much shadow is there? What does it evoke for you? Often, the shadow suggests secrecy, mystery; many expressions testify to this: left in the shadow, to come out of the shadows, etc.

In a garden or a city, do you notice the shadows? In highly urbanized centers, do you feel them crushing you? On the contrary, do you see them animating a landscape, when scattered in small strokes?

But by the way, what is the color of the shadow? The real color or the perceived color? The color of the photographer or the color of the painter?

This question, more complex than it seems, prompts me to write two parts to this challenge. The first is devoted to the scientific aspect. But, as you will read, even a rigorous experiment reveals the subjectivity of our perception.

To find out how the artists approached this subject, see the next article. In the meantime, make your own experiments, observe the shadows, photograph them, find them in architecture, paintings, illustrations or installations. Ask yourself about their visual and emotional impact. And already, create – without inspiration this time, but incorporating the knowledge below.

Own shadow and cast shadow

There are two types of shadow: own shadow and cast shadow. The own shadow of an object is the part that is not directly illuminated. In most cases, as the light is diffuse, this part will be just darker.

The shadow cast – we could say projected – is the area from which it is impossible to see the light source, because of the shielding object. The screen is total if the object is perfectly opaque, and this zone is entirely in darkness if the source is unique. But again, in the presence of diffuse white light, the shadow only darkens the background on which it is projected.

It is less the type of shadow (own or cast) than the nature of the surface that will influence the color.

The own and cast shadow of a pumpkin (V.Lacroix CC-BY-SA 2.5).

Object color

Most of the time, objects scatter the incident light; more precisely, they absorb some of the rays (in the form of heat) and reflect the rest in all directions. However, if the surface is perfectly smooth, like that of a mirror, the rays of light from the source will be reflected in only one direction (which depends on the relative orientation of the ray with respect to the surface). This is known as specular reflection.

Diffuse and specular reflection and absorption of incident light – adapted from Wikipedia

Most of the time, the two phenomena combine. But the composition of the reflected light differs; the one that is sent in all directions depends on the object while the other reproduces the color of the source.

The diffuse reflection determines what painters call the local color of the object: orange for the pumpkin, red for the apple, and finally green for the vase in the figure below. In addition, specular reflection combined with diffuse reflection is marked by a lighter spot, precisely where the light source is reflected. Artists translate this into painting by means of a highlight.

Three objects of different roughness: the smoother the object, the greater the specular reflection. (V.Lacroix CC-BY-SA 2.5)

Also the color variation, via its clarity, gives an indication of the roughness and curvature of the surface. Furthermore, it provides information on the location of the object in relation to the light sources. Therefore, slight irregularities on a surface produce micro-shadows, which can be interpreted as a texture.

The color of the shadow

Objectively, the color of the pumpkin is not the same in the light and the shade. One is orange and the other brown. The same applies to the apple: at the far left, the red is slightly darker, in the highlight it becomes pinkish, and in the shadow it is brown.

At the top, the colors in the illuminated part: pumpkin, apple and vase. At the bottom, the colors in the clean shadow.

However, one property remains relatively stable regardless of the intensity of the lighting: hue. The hue is the “family” to which the color belongs. For convenience, these families are classified on a chromatic disc (read the article on color complements). This arrangement is neither universal nor unique: some families occupy more or less space depending on the model. Below is a color wheel used in the Page software.

The color disc in the Page software; the disc is associated with a brightness indicated by the cursor located in the grey band under the discs. On the right, the brightness is maximum. If you compare the disc to a clock, all the colors along the needle have the same hue (i.e. the colors on the 3 o’clock axis are red).

The graph below shows the variation of the apple’s color along a curve from light to the own shadow zone. If the hue is characterized by the hand of a clock, the hand moves only a few minutes along this path. By way of comparison, the greatest possible difference in hue is 30 minutes, the difference between two complementary hues (in this case, cyan blue). Furthermore, the difference in brightness between the highlight and the shadow itself is around 70, while the largest possible difference is 100, the difference between black and white.

Variation in hue and brightness on the apple (V.Lacroix CC-BY-SA 2.5)

Multiple reflections

Actually, it’s more complicated than that. The color is only constant if the object is isolated. In reality, in addition to diffuse ambient light, the object also receives rays reflected from neighboring objects. In the figure below, note how the shadow of the white cup is colored according to the local color of the object facing it: it is no longer a neutral grey, but a reddish grey on the left and yellowish grey on the right.

Influence of neighboring objects on the color of the white cup’s own shadow. This shadow is grey if the object is isolated (in the middle), in the presence of the apple, it is reddish (left) or yellowish (right) depending on the local color of the apple facing it.

This is why many photographers dress in black; they can work close to their model without fear of changing the scene: neither in terms of brightness nor chromaticity. The photographer Lartigue, for his part, had opted for white: illuminated by the sun, he sent a beautiful diffuse light on his models, while at the same time opening up the shadows.

Multiple reflections are very frequent and can radically transform a room. This is also why you should choose the right colors for clothes worn close to the face: the reflected rays will combine with your skin tone, for better or worse.

Colored lights

What if the light is colored? The reasoning is the same. Obviously the object will look different and, in general, depending on the incident light, will present a color different from its own color, the one it has in daylight. Nevertheless, in both shadow and light – with the exception of the highlight – the hue will remain relatively constant, as in the experiment described above. And, similarly, the hue will be influenced by the reflections of neighboring objects.

And yet, this is not what we perceive. Look at the figure below. We can already deduce the number of spotlights by noting the presence of two shadows. One red and one white spotlight illuminate the clothespin. Based on the shade, can you guess which spotlight is responsible for the shadow on the right? Undeniably it is red, even redder than the background. So it must receive more light from the red spotlight. Therefore it must result from a screen for the white spotlight, which is responsible for the dilution of the red on the white background. The installation diagram confirms this deduction (see below).

And the shadow on the left? For my part, it is certainly not neutral, it seems to me of a grey tending towards blue-green. And for you?

A clothespin lit by two spotlights: one red and one white. What color is the shadow on the left?

Color induction

Where does this blue-green shade come from? The diagram of the experiment indicates that it belongs to the shadow cast by the clothespin illuminated by the red spotlight. The clothespin therefore prevents the red light from reaching the background; the left shadow is therefore only illuminated by the white light spot and the ambient light. It should therefore be neutral or slightly red-tinted.

Diagram of the experience of the clothespin illuminated by a red and a white spotlight. The grey and red squares were respectively taken from the left (screen with red spotlight) and right (screen with white spotlight) shadows.

In fact, when the color is taken from the left shadow, it is grey, not quite neutral, but certainly not blue-green. It is our vision that induces the change in hue.

Benjamin Thompson was the first to note this optical illusion produced by colored lights. Chevreul called this effect simultaneous contrast, but the term induced contrast seems more appropriate. A color – especially if it is intense – induces the complementary hue on its neighbor. Greys are particularly sensitive to chromatic induction as we shall see in Challenge #17. In current case, the very red background induces on the grey of the shadow its complementary hue, a blue-green (cyan).

Shadows in the open air

In the open air, do the shadows also take on the color of the surface on which they are projected?

But first of all, where does the natural light come from? From the sun, you might say, but not only from the sun.

The color of natural light

Before entering the atmosphere, sunlight is white (see this article about light and color) because it is made up equally of all the rays of the visible spectrum. When photons collide with particles in the atmosphere, they are deflected out of their path, especially as their wavelength is short. Clouds, on the other hand, are made up of larger particles that scatter all rays in the same way, so they appear white to us.

The lower the sun is on the horizon, the greater the layer of atmosphere to be crossed, and therefore the more collisions there are. In direct light only the high wavelength rays will remain. This is why sunsets are glowing red.
To simplify, we can say that in broad daylight, direct light, sent in a single direction, is yellow, while indirect light, sent in all directions, is bluish. Hence the blue color of the sky.

To describe this difference in hue, we like to talk about warm light (if it looks more yellow) or cold light (if it looks bluish).

In order to know approximately the color of a light source, it is sufficient to evaluate the respective distribution of rays in three zones of the visible spectrum: short, medium and long wavelengths. The first one is responsible for the impression of blue, the second for green and the third for red. Like the RGB colors of the computer screen, their combination produces color perception.

The color of the shadow outside

For example, an isolated object illuminated by the sun receives warm light directly and cold light indirectly. The shadow (or more precisely its associated object) screens out the sun, so that only the bluer component of the light is present there. This means that shaded surfaces receive more short wavelength rays.

This effect is reinforced by our perception. The yellowish-white wall below is illuminated by a winter morning light.

The color of the shadow outside: 1: the shadow cast by the banister on the wall; 2: wall; 3: shadow cast by the staircase on the bluestone; 4: shadow cast by the bluestone staircase itself.

Here, although it is of a relatively neutral grey (square on the far left), the shadow of the ramp appears bluish, due to the chromatic induction produced by the color of the wall (second square). Moreover, this bluish tint is very real on the shadow of the staircase on the bluestone (third square), an effect reinforced by the chromatic induction. By way of comparison, the square on the far right is practically neutral.

Translucent objects

In the undergrowth the diffuse light is not bluish because it also depends on the color of the light filtered through the leaves. In fact, the leaves are not completely opaque, and if they allow certain wavelengths to pass through in favor of others, the light will be colored. Thus the shadow cast by a leaf will depend not only on the surface on which it is projected but also on the light that the leaf lets through. However, under the foliage, our vision adapts and corrects the dominant, so that unless we pay attention, we are not very sensitive to this change of color.

The leaves are not completely opaque, they allow some light to pass through and their shadows are affected. In these two photographs, they are greenish.

In conclusion

To find out the real color of the shadow, observe it through a hole made in a neutral cardboard to isolate it from the context.

But to understand it, we must first identify the sources of light that illuminate it. Lighting can be direct or indirect.

In the case of a single light source, the shadow will only be illuminated by diffuse light. Otherwise, each source will produce a shadow of the object, itself illuminated by a number of other light sources, depending on their arrangement, as well as by ambient light.

The color of the own shadow of isolated objects illuminated by a white light is almost identical to that of the object in the light, it is just darker; It is the same for the cast shadows of opaque objects: the projected shadow will be the same shade as the surface on which the shadow is drawn, but the color will be darker. However, as soon as the object is surrounded, its surface is indirectly illuminated by the rays reflected by the neighboring objects.

Outdoors, in the open air, the direct light is more or less yellow, orange or red depending on the position of the sun and the layers of atmosphere to be crossed. Diffused light, on the other hand, will have its complementary color, since both combined give white sunlight.

As indoors, the objects project colored rays on top of each other, thus modulating the color of their respective surfaces.

Objects that filter light, such as leaves in the undergrowth, further complicate the problem.

But no matter how complex the scene is, the perceived color will be different from the real color due to the chromatic induction. One color tends to project its complementary hue onto its neighbor. Greys (and therefore shadows) are particularly susceptible to influence.

Go ahead

Many contemporary and non-contemporary artists have an intuitive knowledge of the above-mentioned phenomena. On this basis the creative choices are multiple; one may want to accentuate, cancel or modify them for a personal writing, to transmit an emotion or a message.

In turn, pay attention to all the shadows around you. Observe them. Photograph them. Isolate them from their context to measure how their hue varies with the surrounding color. See how colored objects change the hue of the surfaces next to them. Draw, paint shadows. Use transparent fabrics or colored paper to change the color of your lighting. And above all, have fun!

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