IN THE SOLUTIONS TO DIRECT VOLUME RENDERING we have seen so far,radiant energy has only been emitted by the voxels themselves. The emission-absorption model we used is based on the assumption of a dense cloud of particles where each particle simultaneously emits and absorbs light. We did not account for illumination effects caused by external light sources. There was no interaction between light coming in from outside the volume and the volume itself. The only exception was the intensity of the background light, which is absorbed to some degree and thus attenuated by the volume before it reaches the eye. Light energy also traveled along linear rays only. We did not allow for reflection, refraction, and scattering of light. Such illumination effects, however, add a great deal of realism to the resulting images. Surfaces properties, such as roughness, are very hard to recognize without external lighting. Illumination effects provide additional depth cues and greatly enhance the perception of small-scale spatial structures, which is of great importance for scientific visualization.