Reference Rays, Vignetting, and Apertures

Basic Definitions for CODE V

Vignetting

Vignetting refers to the clipping of portions of a ray bundle by apertures on surfaces other than the aperture stop surface. Vignetting is considered to be positive if the ray bundle is reduced in size, negative if the bundle expands ("pupil expansion"). For a more complete discussion of vignetting, refer to Chapter 2A, "Entering/Changing Data - Specification Data - Vignetting" and Chapter 2B, "Operations on Lens Data."

Reference Rays

Reference rays are a special set of real rays that CODE V traces and maintains automatically whenever the lens is changed. CODE V maintains a set of reference rays for every user-defined field point and for every zoom position, and for some calculations, these rays are used to represent the entire bundle of rays from a particular field point. CODE V uses the reference ray coordinates at each surface to determine default apertures (i.e., apertures the program assumes to apply at each surface in the absence of user-defined apertures). Reference ray information is also used to calculate edge thicknesses and for ray based constraints in optimization. Note that the tracing of reference rays is NOT affected by apertures. This means that reference rays cannot be blocked (although they can fail in several ways, such as missing a surface or encountering total internal reflection [TIR]).

For a general field point (asymmetric, i.e., containing both X and Y components), five reference rays are defined (see figure). For symmetric field points, CODE V saves time by not tracing equivalent rays from the same field point (it still "knows" the coordinates of these rays, however). The chief ray (R l ) is considered to be most representative ray of the ray bundle from a particular field point, while the marginal rays (R2 - R5) define the "envelope" of the bundle.

As shown in the figure below, reference rays are based on pupil definition and vignetting factors. The relationships between reference rays, vignetting factors, and apertures are explained in the following section.

Apertures

Apertures define the optically relevant extent of each optical surface. In image evaluation options (such as MTF), apertures directly limit the bundles of light used in the calculation. Apertures in CODE V can be circular, elliptical, or rectangular. More complex shapes can be constructed with combinations of these basic forms, including decentered or rotated apertures and central obscurations. Apertures and their applications are discussed in detail in Chapter 2A, "Entering/Changing Data - Element Structure - Apertures."

Default Apertures

Every surface has at least one aperture. For any surface without a user-defined aperture, CODE V defines a default aperture. A default aperture is always a centered, circular aperture whose size is determined from the reference rays. When one or more user-defined apertures are placed on a surface, these are used instead of the default aperture. The CA command allows you to control which apertures are used by CODE V

See Chapter 2A, "Entering/Changing Data - Element Structure - Apertures" for more information on the CA command.

Relationships Between Reference Rays, Vignetting Factors, and Apertures

The relationships between reference rays, vignetting factors, and apertures are very important. Vignetting factors and pupil specification (EPD, FNO, NAO, or NA) are used in some diagnostic analysis options and in AUT (optimization) to determine the extents of rays bundles from each defined field point. Surface apertures are ignored by these options. Image evaluation options, such as MTF and SPO, trace grids of rays to determine the size and shape of each field bundle more precisely. Every such ray is checked against apertures at every surface to determine whether it is passed or not. Vignetting factors are not directly used by these options.

By using these two approaches to ray bundle modeling, CODE V can achieve speed and accuracy while giving the user great flexibility in defining a lens model. There is a risk of inconsistency in this dual approach, however, since the user is free to specify surface apertures and vignetting factors independently. It is possible to specify completely inconsistent ray bundles for the same lens.

To help with this problem, CODE V includes a special command (SET) that can be used in several forms to assure that system aperture, vignetting factors, and surface apertures are all internally consistent. This is very easy to do, but does require your attention - the SET command is not used automatically. You must remember to use it whenever the lens is changed in a way that can affect ray apertures or vignetting. The SET command and its application to apertures and vignetting are discussed in detail in Chapter 2B, "Operations on Lens Data."


Excerpted from the CODE V Reference Manual. (c) Copyright 2002 by Optical Research Associates. Excerpted by permission of Optical Research Associates. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from Optical Research Associates.

Maintained by John Loomis, last updated 24 June 1999