Our new method for optical modeling puts ray optics on a more solid foundation. This method not only delivers higher accuracy, but also offers estimates of its own errors. The conceptual framework is fully consistent with intuitive interpretations of rays and avoids the ad hoc leaps of standard ray-based modeling. These include problems in such areas as propagation, refraction, reflection, and diffraction. The model's higher accuracy also means that more applications now fall within the sope of ray-based system analysis. This is demonstrated via a simple example involving a waveguide with a smoothly varying refractive index. In particular, a low-order waveguide mode is modeled as it propagates to, and interacts with, a flat interface between the waveguide and a homogeneous medium.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.