An optical system is specified in OSLO as a series of surfaces that a ray intersects consecutively in passing through the system. Light enters the system traveling from left to right. The object surface is given the number 0. In a sequential system, surfaces are numbered in the order in which a ray or its extension (in the case of a virtual surface) would intercept them. The highest numbered surface is called the image surface, whether or not there is an image formed on it. The correct ordering of the surfaces is essential for lens input specification to OSLO.
Quantities that pertain to a surface, such as curvatures, aspheric coefficients, etc., carry the number of the surface. Quantities such as refractive indices, thicknesses, etc., that pertain to the space between two surfaces, are assigned to the lower-numbered surface.
Nominally refracting surfaces having no refractive index change have no effect on ray trajectories, and are called dummy surfaces. A dummy surface is often used to keep track of ray data, or to establish a base coordinate system for other surfaces.
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