Copyright Goodheart-Willcox Co., Inc. 482 AutoCAD and Its Applications—Advanced 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. 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 18-1. 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 18-1. 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 18-1. 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 18-2 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 18-1. 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