Semi-Diameters

The size of each surface is described by the semi-diameter setting if it is non-rotationally symmetric and the clear semi-diameter setting if it is rotationally symmetric. The default setting is the radial distance to the aperture required to pass all real rays without clipping any of the rays. Typing a value for the clear semi-diameter or semi-diameter column results in the character "U" being displayed next to the value. The "U" indicates that the clear semi-diameter or semi-diameter is user defined. When a user-defined clear semi-diameter or semi-diameter is placed on a surface with refractive power (which is done by typing in a value in the appropriate column), and no surface aperture has been defined, OpticStudio automatically applies a "floating" aperture to the surface. A floating aperture is a circular aperture whose radial maximum coordinate is always equal to the clear semi-diameter or semi-diameter of the surface.

For more information on surface aperture types, see the Aperture section of the Surface Properties (in the Lens Data Editor).

Clear semi-diameters on any surface for axial symmetric systems are computed very accurately, as long as the surface does not lie within the caustic of the ray bundle (note this usually occurs at or near the image surface). OpticStudio estimates clear semi-diameters for axial systems by tracing a few marginal pupil rays. For non-axial systems, OpticStudio estimates the required clear semi-diameter or semi-diameters using either a fixed number of rays or by an iterative technique, which is slower but more accurate.

See the explanation of "Fast Semi-Diameters" in Advanced Options (system explorer) for details. It is important to note that the "automatic" clear semi-diameter or semi-diameter computed by OpticStudio is an estimate, although it is generally a very good one.

Some surfaces may become so large in aperture that the surface z coordinate becomes multiple valued; for example, a very deep ellipse may have more than one z coordinate for the same x and y coordinates on the surface. For the case of spherical surfaces, this condition is called "hyperhemispheric" and OpticStudio uses this term even if the surface is not a sphere. Hyperhemispheric surfaces are denoted by an asterisk "*" in the semi-diameter column. The indicated semi-diameter is of the outer edge of the surface, which will have a smaller radial aperture than the maximum radial aperture.