In volume rendering, most optical models currently in use are based on the assumptions that a volumetric object is a collection of particles and that the macro behavior of particles can be predicted based on the behavior of each individual particle. However, such models are not capable to characterize the collective optical effect of a collection of particles which dominates the appearance of the boundaries of dense objects. In this article, we propose a generalized optical model that mixes particle element and surface element together to characterize both the behavior of individual particles and the collective effect of particles. The new model provides a more powerful and flexible way to properly render volume data sets of different kinds of materials. By adjusting the property of surface and particle contents, different visual effects can be generated. It also provides a more rational basis for surface shading, so the problem of normal based shading in homogeneous regions encountered in conventional volume rendering can be easily avoided.