Fast Semi-Diameters, clear semi-diameter or semi-diameters, what are the differences in the optical design? we will share more!

OpticStudio computes “automatic” clear semi-diameter or semi-diameters to estimate the clear aperture required on each surface to pass all rays at all field points and wavelengths. For axial systems, this computation can be done accurately by tracing just two rays, the top and bottom marginal rays, for each field and wavelength. By default, for axial systems OpticStudio will trace just these two rays in the “true” tangential plane of the vignetted pupil for each field and wavelength, then use the radial coordinates of each ray at each surface to determine the clear semi-diameter or semidiameter required.

For some non-axial systems, the resulting estimate is not accurate enough. This typically includes systems which have tight edge and clear aperture constraints. For these systems, there is no general way of accurately computing the clear semi-diameter or semi-diameters other than by tracing a large number of rays in the vignetted pupil. For such systems, Fast Semi-Diameters should be checked off!

OpticStudio will iteratively trace as many marginal rays around the perimeter of the pupil as required to determine the clear semi-diameter or semi-diameter for each surface until the values converge to within 0.01% (5 significant figures) of the previous estimate. However, clear semi-diameter or semi-diameters may not be calculated accurately for surfaces with hard boundaries when the illuminated area is very large compared to the span of the surface.

The specific rays traced and the number of iterations used to calculate the automatic semidiameter values depends on the pupil size and position within the system. The semi-diameter value calculated for a particular surface may change if other surfaces are added, deleted, or modified in such a way as to modify the system pupil.

Iteration can be slow, especially during optimization, because OpticStudio needs to update the clear semi-diameter or semi-diameters frequently. Thus, there is a trade-off between speed and accuracy when calculating semi-diameters. For axial systems, OpticStudio will use the slower iterative algorithm only if the “Fast Semi-Diameters” option is checked “off”.

Selected (On) If the “Fast Semi- Diameters” option is checked “on”, OpticStudio will trace only as many marginal rays as required to estimate the automatic clear semi-diameter or semidiameters to equal or better than 0.01%. The algorithm starts by tracing 2, then 4, then 8 rays, then 16 and so on, until the clear semi-diameter or semi-diameter values converge to within 0.01% of the previous estimate.

This option is designed mainly for axial systems. Caution must be exercised when using this option for non-axial systems as the tracing of marginal rays may not be sufficient to properly estimate automatic semi-diameters. If the semi-diameter or clear semi-diameter values changes dramatically when unchecking the “Fast Semi-Diameters” option, it should be left unchecked.

Not Selected (Off) If the “Fast Semi-Diameters” option is checked “off”, OpticStudio will trace at least 32 rays around the vignetted pupil at each field and wavelength, and more if required to estimate the automatic clear semi-diameter or semi-diameters to within 0.01%. This approach is slower, but also safer for ensuring accurate calculations of clear semi-diameter or semi-diameter values in non-axial systems. Both methods used by OpticStudio to determine clear semi-diameter or semi-diameters are accurate only for surfaces whose maximum size is set by the radial coordinate of a marginal ray. The methods do not work for surfaces that lie in a caustic. For surfaces in a caustic, the exact ray-based clear semi-diameter or semi-diameter could be determined using a spot diagram or footprint analysis, however this level of precision is likely never required since diffraction effects, which the rays do not model, would be significant.