Overview: Vignetting Factors

Question

"What are "vignetting factors" and how are they used?"

Synopsis

Overview: Vignetting Factors

Solution

"Please see the video on the subject if there are some questions as to the use of this term.

In systems with large fields of view, the off-axis pupil is usually vignetted compared to the on-axis pupil. When performing system performance calculations (i.e. transverse ray plots, ...etc.), vignetting is handled by either setting the ray aiming mode (see the "General Conditions Data Editor") or setting the appropriate apertures to be "checked" (the "check" flag blocks rays outside the aperture).

However during optimization, if aperture checking were allowed, then variations in the lens system may cause different numbers of rays to be traced successfully from cycle to cycle. Depending on how the error function is assembled, the "adding" or "dropping" of individual rays from one optimization cycle to the next, would probably wreck havoc on the optimization algorithm. For this reason, aperture "checking" is ignored during optimization (see the chart on the last page of Chapter 4 in the OSLO 6.1 Optics Reference Manual).

As an alternative to "checking" apertures during optimization, vignetting factors compress or elongate the system pupil in the +Y, -Y, +X & -X directions. This effectively changes the ordinarily circular system pupil into a different elliptical size and shape for each off-axis field point. Note that all the same rays defined by the user are still utilized, however, the ray set is compressed or stretched according to the vignetting factors. Note also that some ray aiming modes automatically fit off-axis pupils to the +Y, -Y, +X, and -X edges of the aperture stop (potentially resulting in an elliptical pupil for off-axis field points), so vignetting factors may not be required for these modes (see a discussion of this in the OSLO on-line help item ""Contents>>Lens Data Entry>>System Data>>General Conditions>>Ray Aiming Modes"").

To manually set vignetting factors, or to view the current vignetting factors, go to the OLSO menu item ""Optimize>>Error Function Tables>>Field Point Set"". Make sure that the ""Advanced"" button is selected. For an explanation of the various items in the Field Points Set Data Editor, click on the yellow question mark (?) under the command line while the Data Editor window is still open. This activates OSLO's context-sensitive help and will take you to the appropriate page in the OSLO on-line help system.

To automatically set vignetting factors, use the menu item ""Optimize>>Support Routines>>Vignetting>>Vignetting Analysis"".

Note that individual field points can only be defined in OSLO Standard and Premium. Therefore the concept of setting vignetting factors is only valid in these OSLO editions."