Note: 18% gray balls and reflective balls are commonly used as
lighting references on the set. A good VFX supervisor will be sure
to provide these references to the visual effects department where
the CG lighting will take place. The gray ball shows general light
effects on a diffuse, round shape, while the reflective ball works
like a 180-degree mirror, showing you the precise position and
color of the light sources and giving you a rough approximation of
the intensity ratio between light sources.
Here are some of the light types you may encounter, along with some
suggestions on how to start looking at them.
Natural Light
Natural light refers to the light we find in nature — light that is not made
by people. I am going to tell you many times in this section that the best
way to really understand these light types is to go out and study them.
No book, no video, and certainly no CG lights in LightWave will take the
place of observing, experiencing, and understanding for yourself the way
real-world light interacts with things in your environment.
Sunlight
The most obvious natural light source is the sun. Understanding how
sunlight reaches the earth and how it lights objects in your environment
is important to creating realistic sunlight in your CG scene. A common
misconception is that the sun is equivalent to a point source with light
rays radiating outward omnidirectionally. Another misconception is that
the sun is a distant light that is so far away that all its rays are parallel by
the time they reach the earth. The fact is that these are both true in
part, but there are more qualities not yet discussed.
The sun is approximately 149,597,890 kilometers away from the
earth and its diameter is about 100 times that of the earth. The entire
surface of the sun is radiating light in every direction at once. This
means that over the width of the sun’s disc as viewed from the earth,
parallel rays of light are approaching the earth; however, the parallel rays
comprise only a small percentage of all the light reaching the earth from
the sun. There are also rays of light spreading out in all directions from
every point on the surface of the sun and rays of light converging on the
earth and all things on it from many directions. In short, there are many
“rays” of light hitting the earth that are not parallel to each other. The
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59
result of the size differential between the sun and the earth can be
observed during a lunar eclipse when the earth passes directly between
the sun and the moon. The moon first passes into the penumbra, that
area behind the earth where the sun’s light is partially obscured. The
sun is so much larger than the earth that some of the sun’s light man
-
ages to reach behind the earth, creating a partial shadow. The moon then
passes into the umbra, that area where there is no direct sunlight at all.
Let’s look at an illustration of a lunar eclipse to describe this
phenomenon.
If you place a chair in the sunlight, you will notice that the shadow cast
by the chair is sharpest near where the chair is touching the ground and
becomes fuzzier with distance. This is the same effect that is found in
the lunar eclipse. In the case of our chair, the sun is much larger than the
chair, so the converging light rays are able to reach around behind it,
“softening” the shadow. See Figure 3.6.
Skylight
When we say “skylight” we are usually referring to a clear, blue sky.
When the sky is blue, you will notice that shadows have a bluish tint to
them. It does not seem obvious at first, but look closely. If you look out
-
side on a snowy day, it is very obvious. Skylight is a global light source
that has several immediately obvious effects.
Skylight fills the shadows with a blue, low-intensity light. It illumi
-
nates almost every surface except the most obscurely hidden. Skylight
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60
Figure 5.1: This effect occurs for every object lit by direct sunlight,
but the angles are usually so slight as to be imperceptible. The light
rays often appear to be parallel. In many cases, this effect will not
need to be addressed in CG lighting. Sometimes, however,
understanding this effect is critical to your lighting, and more
specifically to your shadows.

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