The shadow regions of a subject are often full of subtle variations, but virtually all beginning students make too much of these. It is all too easy to forget that we are looking at a shadow as soon as we begin to observe the intriguing details within them. As always, it is a matter of seeing these variations in the context of the total tone and colour range of the subject.

Sources of illumination in the shadow zone of an object include light reflected from the environment and any secondary light sources. Reflected light from the environment is in turn generally visibly reflected by an object in both a diffuse manner (the conventional reflected light) and a specular manner. Although teaching diagrams conventionally ignore the latter (e.g. see Figure 2.3), all surfaces that are shiny enough to show a strong highlight will be shiny enough to show visible specular reflections of the environment in the shadow zone. The dark shape of the reflected cast shadow in particular is often quite conspicuous, as in Figure 2.4. The diffuse reflection has a variable pattern depending on where the form shadow surface is close to, and faces, a strongly lit part of the environment. In some situations this diffuse reflection may be brightest opposite the direction of the main light source, creating a dark zone, often called a core shadow, adjacent to the entire length of the terminator. In Figure 2.4 on the other hand, the strongly lit tabletop creates a quite different pattern.

The cast shadow of an obliquely illuminated sphere is elliptical in plan view, and remains persistently close to a perfect ellipse in appearance when viewed in almost any perspective. The outer boundary of the cast shadow is a transitional zone called the penumbra, formed where the light source is partly obscured by the sphere. The width of the penumbra, and hence the softness of the shadow edge, depends on the distance between the light rays from either side of the light source (Fig 2.6); it increases with distance from the object casting the shadow, and with the angular or apparent size of the light source.


Figure 2.6. Width of penumbra. The width of the penumbra generally increases with increasing distance from the object casting the shadow.

In most circumstances the darkest part of the shadow zone is the crevice shadow, where two surfaces are in contact (e.g. the sphere and tabletop in Figure 2.4). The darkness of this zone is the result of occlusion - the further we go into the crevice, the less of the environment can contribute light. Both the sphere and the tabletop are darkened in the crevice shadow, although, depending on the direction of observation, the darkened surfaces may not both be visible.

Experiment with the sliders in Figure 2.7 to observe the effect of different combinations of two small and one large or ambient light source. Multiple primary light sources will each create their own pattern of light and shade, and these patterns combine additively in terms of the energy of light coming from each point. Perceived brightness however does not have a linear relationship to light energy. Two equal light sources do not look twice as bright as one, and so if overlaps of shadows get no light, they end up being visually very conspicuous compared to the partly lit areas. Beginning students are always very impressed by these odd-shaped dark overlaps, and copy them faithfully, often making them even darker. In a painting or a very extended drawing an artist may have time to capture the subtle effects of multiple light sources, but otherwise it is usually advisable to simplify by mentally filtering out the other sources, and drawing in relation to one light source.

Figure 2.7. Multiple light sources. Sliders vary the intensity of one large white overhead light source (topmost slider) and two small yellowish oblique light sources. Copyright David Briggs and Ray Kristanto, 2007.



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