Rendering a CG Scene - An Introduction to Computer Graphics for Artists - page 226

12.5.5.5 Area Light

An area light is a matrix of point shapes arranged in a line (a linear light source) or
a rectangle. These are used to simulate specifi c shapes, like a fl uorescent bar light or
a group of lights behind a rectangular sheet of plastic.

12.5.5.6 Volume Light

A volume light has a specifi c shape based on a geometric primitive. This can be
non-proportionally scaled to fi ll different spaces, but they cannot be edited as
geometry.

12.5.5.7 Light Shader

A light shader can be applied to any geometry, causing that object to emit light. This
is the most accurate representation of a light source. For instance, a light bulb model
could have the fi lament inside the bulb modeled as geometry. This fi lament could
have a light shader attached to it. The glass shader of the bulb, if realistic, will affect
the light emitted from the fi lament and cause the bulb to glow as might be expected.
If the same thing were attempted with a point light, the bulb would be given a self-
illuminated shader. A self-illuminated shader fi lls the object with a fl at color and
causes it to ignore all lighting effects in the scene so that it doesn't receive shadows.
Then a point or spot light would be positioned just outside of the bulb so that the
geometry wouldn't block the rays it casts. This means that the origin of the light is
incorrect; the light is not interacting with the glass of the bulb, and light is not pro-
jected based on the shape of the bulb or fi lament.

12.5.6 Color Temperature

Traditionally, warm colors are red, yellow, orange, and their derivatives. Cool colors
are green, blue, violet, and their derivatives. These color temperature associations
are the opposite of what they should be, because red is actually the coolest visible
color temperature, and blue is the hottest (Table 12.1 ). The origin of this error may
be that until the modern era, extremely hot blue light could not easily be produced,
but the red of a common fl ame was easily found, and the blue of ice may have been
a reference for “cool” colors. Regardless of the origin, the temperature of a wave-
length determines the color we see.
Table 12.1 provides a crude scale of light temperature equivalents.
Every temperature corresponds to a specifi c color, starting with orange, and
moving up to blue. At lower temperature levels, color is red, and when it is higher
than the highest listed, it becomes ultraviolet.