Book excerpt: Geometrical Optics and Optical Design is an up-to-date introductory treatment of geometrical optics which is intended to lead students toward the modern practices of computer-aided optical design. The principles of Gaussian optics and first-order layout and design are emphasized, based on the tracing of two paraxial rays and the associated optical invariant. The radiometry of lens systems is seen to rest on the same concepts. Third-order aberration theory is developed in detail. Complete examples of third-order design are provided, together with software tools that allow students to follow the examples in detail or to develop other examples independently. Several problems at the end of each chapter allow students to practice and extend the concepts taught.