Vignetting factors describe how to distort the the pupil to mimic the effects of vignetting by apertures in the system, without having to introduce any apertures into the model. The way this works by distorting the momentum distribution of the ray set for a given point source from a circle to a displaced ellipse, as shown below. Each defined point source (field point) has its own set of vignetting factors that specify how to distort the ray set.
To set vignetting factors:
- Select from the menu System > Source > Vignetting factors
- Enter vignetting factors in the table
- Closing the vignetting factors window (if desired)
The vignetting factors in LensForge specify directly the extents of the vignetted pupil in terms of the original pupil coordinates. This is specified by four quantities: PXA, PXB, PYA, and PYB. The vignetted pupil extends from PXA to PXB in the x-direction and from PYA to PYB in the y-direction. The unvignetted pupil has PXA=-1, PXB=+1, PYA=-1, PYB=+1. The transformation of the pupil coordinates of a circular ray set (PX,PY) to the elliptical ray set (PX',PY') is according to the following formula:
PX' = (PXB - PXA)*PX/2 + (PXB + PXA)/2 PY' = (PYB - PYA)*PY/2 + (PYB + PYA)/2
Definition of vignetting factors in LensForge