How to model a simple telecentric system

A telecentric system is one where the pupil is at infinity. As a result, the ray angles of the chief rays are 0 degrees in the telecentric space. When the entrance pupil is at infinity, the chief ray angles in the object space is 0 degree. In other words, the chief rays in the object space are parallel to the optical axis. This case is often referred to as object space telecentric. In this article, we will discuss how to set up a simple object space telecentric system in CODE V.

System setup:

Pupil: If the system is telecentric in object space, then the Telecentric in Object Space flag (Lens > system Data menu, Pupil commands in the System Data window) should be selected. When this flag is selected, CODE V will send out chief rays from all object points to be initially parallel to the optical axis.

Set pupil type to be Object Numerical Aperture (NAO). The NAO value controls the angular spread from the object points.

Field:

The Telecentric in Object Space flag works best when the field points are specified with object heights. You can set the Field Type at System Data Fields/Vignetting.

Stop location:

Now the chief in our system often will not hit the center of stop surface. We will find the appropriate stop location using Automatic Design. Right click the thickness of Surface 2 and select vary. Click Optimization > Automatic Design. In the Error Function Setup page choose Error Function Content to be Constraints only solution.

In the Specific Constraints tab, click Insert Specific Constraint. Insert Real Ray Trace Data > Local Y Surface Coordinate for Surface Stop, Field F2, Ray 1-Chief Ray, Constraint Mode: Minimize, Constraint Target: 0. Click Insert and Close. In the Automatic Design window, click OK.?

Now you should have the stop located at where the chief ray of Field 2 is 0.

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Aspheric surfaces

To design the system so that the chief ray of Field 3 also passes through the center of the stop and improve image quality. We can introduce aspheric surfaces and add Real Ray Trace constraints for F3 Chief ray to be 0 at the Stop surface. You can vary the image defocus as well. This time, since we also want to improve image performance, we can choose Error Function Type: CODE V error function only.

Image simulation

To visualize the system image performance, we can use Image Simulation (Analysis > Diffraction > 2D Image Simulation).?

From the image simulation result (Figure 1), we can see that the images have constant magnification. In other words, that the apparent size of the image does not change with varying object distance. Because of this property, object-space telecentric lenses are often desired for machine vision and metrology applications.?

Learn more about CODE V: https://www.synopsys.com/optical-solutions/codev.html .