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AutoCAD and Its Applications—Advanced
Copyright Goodheart-Willcox Co., Inc.
A weblight is a directed light that represents real-world distribution of light. The
illumination is based on photometric data that can be entered for each light. The light
from a standard weblight is directed toward a target. A free weblight is the same as a
standard weblight, but without a target point.
Properties of Lights Properties of Lights
Several factors affect how a light illuminates an object. These include the angle
of incidence, refl ectivity of the object’s surface, and the distance that the light is from
the object. In addition, the ability to cast shadows is a property of light. Shadows are
discussed in detail later in this chapter.
Angle of Incidence
AutoCAD renders the faces of a model based on the angle at which light rays
strike the faces. This angle is called the angle of incidence. See Figure 17-1. A face
that is perpendicular to light rays receives the most light. As the angle of incidence
decreases, the amount of light striking the face also decreases.
Refl ectivity
The angle at which light rays are refl ected off a surface is called the angle of refl ection.
The angle of refl ection is always equal to the angle of incidence. Refer to Figure 17-1.
The “brightness” of light refl ected from an object is actually the number of light
rays that reach your eyes. A surface that refl ects a bright light, such as a mirror, is
refl ecting most of the light rays that strike it. The amount of refl ection you see is called
the highlight. The highlight is determined by the angle from the viewpoint relative to
the angle of incidence. Refer to Figure 17-1.
The surface quality of the object affects how light is refl ected. A smooth surface
has a high specular factor. The specular factor indicates the number of light rays that
have the same angle of refl ection. Surfaces that are not smooth have a low specular
factor. These surfaces are called matte. Matte surfaces diffuse, or “spread out,” the
light as it strikes the surface. This means that few of the light rays have the same angle
of refl ection. Figure 17-2 illustrates the difference between matte and high specular
fi nishes. Surfaces can also vary in roughness. Roughness is a measure of the polish on
a surface. This also affects how diffused the refl ected light is.
Figure 17-1.
The amount of
reflection, or
highlight, you see
depends on the
angle from which
you view the object.
Angle of incidence
Angle of
reflection
Object
Light
rays
Does not
appear
bright
Appears
brighter
Appears
brightest