OpenGL Programming Guide

Chapter 6Lighting

Defining Material Properties

Emission

By specifying an RGBA color for GL_EMISSION, you can make an object appear to be giving off light of that color. Since most real-world objects (except lights) don't emit light, you'll probably use this feature mostly to simulate lamps and other light sources in a scene. In Figure J-21 , the spheres in the fourth column have a greenish value for GL_EMISSION:

GLfloat mat_emission[] = {0.3, 0.2, 0.2, 0.0};
glMaterialfv(GL_FRONT, GL_EMISSION, mat_emission);

Notice that the spheres appear to be slightly glowing; however, they're not actually acting as light sources. You would need to create a light source and position it at the same location as the sphere to create that effect.

OpenGL Programming Guide