This book is an up to date, introductory treatment of geometrical optics, from the point of view of the future optics practitioner. 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 the reader to follow the examples in detail, or to develop other examples...