How to Create Complex Lens Geometries for Ray Optics Simulations

Stuart Barnes May 7, 2018
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The Ray Optics Module extends the modeling capabilities of the COMSOL Multiphysics® software to include ray tracing simulation. This module makes it possible to accomplish advanced thermal, structural, and other studies of complex optical systems in an integrated software environment. The first step in a successful simulation is the creation of the model geometry. This blog post examines how to create a complex lens geometry, using the Petzval lens as an example.

Defining the Optical Prescription of a Complex Lens System

An example of a moderately complex optical system is a Petzval lens with a field-flattening lens. A multielement lens system with a focal length of 100 mm and a focal ratio of approximately f/2.4 is shown in the figure below.

A schematic of a Petzval lens.
A Petzval lens that includes a field-flattening lens.

The optical prescription of this camera lens (as described in Ref. 1) is given in the following table. The geometry consists of two doublet lenses together with a field-flattening lens. Altogether, the rays refract across eight different surfaces. Two additional surfaces for the aperture stop and image plane are also shown.

Surface Index
Element Number
Name
Radius of Curvature (mm)
Thickness (mm)
Clear Radius (mm)
Material
0
N/A
Object
N/A
N/A
1
1
Lens 1
99.56266
13.00000
28.478
S-BK7
2
2
Lens 2
-86.84002
4.00000
26.276
S-BASF12
3
N/A
N/A
-1187.63858
40.00000
22.020
N/A
4
3
Stop
40.00000
16.631
N/A
5
4
Lens 3
57.47191
12.00000
20.543
S-SK2
6
5
Lens 4
-54.61865
3.00000
20.074
S-SF5
7
N/A
N/A
-614.68633
46.82210
16.492
N/A
8
6
Lens 5
-38.17110
2.00000
17.297
S-SF5
9
N/A
N/A
1.87179
18.940
N/A
10
N/A
Image
N/A
17.904
N/A

Now, before we can perform a ray optics simulation with COMSOL Multiphysics and the add-on Ray Optics Module, it is necessary to create the model geometry. In the following section, we discuss some options for doing this.

Creating the Model Geometry for a Petzval Lens

There are a variety of ways to create a model geometry in COMSOL Multiphysics. One possibility is to use combinations of built-in geometry primitives such as spheres, cylinders, polygons, and line segments. You can combine these simple geometric entities into more specialized shapes using Boolean, partition, transformation, and conversion operations.

A screenshot of the various options in COMSOL Multiphysics® for lens geometries.
A screenshot showing the Boolean and partition operations.

Various geometry primitives (left) and Boolean and partition operations (right).

It will quickly become apparent that the creation of even the simplest optical geometry (such as a single lens) requires a significant number of operations. The full model geometry might require the same sequence of operations to be repeated several times with different numeric values. For example, different lenses might require the same steps but with different surface radii of curvature. To avoid setting up the same geometry sequence several times in a row, you can define some Geometry Parts. These parts are parameterized subsequences that can be called as if they are geometry primitives. By this means, each repetition of a lens can be accomplished using a Part Instance node, where the input parameters are the relevant details from the optical prescription.

An example of a user-defined geometry part can be seen below. In this case, the part defines a single lens with arbitrary front and rear radii of curvature as well as various definitions for the full, surface, and clear aperture diameters.

A screenshot of the COMSOL Multiphysics® GUI showing an example of a user-defined geometry part.
An example of a user-defined geometry part.

The Ray Optics Module includes a set of predefined parts for some of the most common shapes used in optical simulation, including a lens similar to the one shown above. These are available in the Part Libraries. The Part Library available in the Ray Optics Module is discussed in the following section.

Using the Part Library in the Ray Optics Module

A variety of 2D and 3D parts have been created for use in the Ray Optics Module. One of the more advanced examples is the Spherical General Lens 3D (shown below in the left image). This part can be used to create a wide variety of lens forms, including any combination of concave and convex lens surfaces. Indeed, it is flexible enough that it can be used for all of the lenses in both a Petzval lens as well as a double Gauss lens. The Circular Planar Annulus part is also shown below. This part is a convenient way to create aperture stops and to define the image plane.

A screenshot of a Part Library window in COMSOL Multiphysics®.
A screenshot showing an option selected in the Ray Optics Module Part Library.

Examples of parts in the Part Library in the Ray Optics Module: Spherical General Lens 3D (left) and Circular Planar Annulus (right).

Parts found in the Ray Optics Module’s Part Library often contain one or more variants. These variants are intended to accommodate the use of the same basic part in a variety of ways. For example, the Spherical General Lens 3D can be specified either in terms of the clear aperture diameter or in terms of the clear aperture fraction. Other parts have variations that permit specifying either the central thickness or the edge thickness. It might also be possible to give the effective focal length (and refractive index) in place of the radius of curvature.

Loading the Optical Prescription Parameters into COMSOL Multiphysics®

We now have almost everything in place to create the Petzval lens model geometry. We proceed by inserting instances of the Spherical General Lens 3D and subsequently entering the geometry parameters (that is, the radii of curvatures, central thickness, and diameters) directly from the optical prescription table. However, this process can be simplified by reformatting the lens prescription so that it can be loaded in its entirety (for example, from a text file) into the Global Definitions node of the current model. That way, if you wish to adjust the optical prescription later, you can make all of the changes in one place. The optical prescription parameters for the Petzval lens can be seen below.

A screenshot of the Settings window for optical prescription parameters in a Petzval lens.
The optical prescription parameters for the Petzval lens model.

Having already loaded the two geometry parts from the Part Library (that is, the Spherical General Lens 3D and the Circular Planar Annulus parts), we insert the first element of the Petzval lens (“Lens 1″). The optical prescription parameters we defined above can be used as input parameters for the Part Instance node. Each lens parameter has been given a unique name (such as “Tc_1″, “R1_1″, “R2_1″, etc.) so that these values can be entered directly into the Expression fields. This process can also be simplified by loading a text file that contains definitions of the relevant expressions for each lens element.

A screenshot showing the Settings window for the first element of the lens model geometry.
The first element (Lens 1) of the Petzval lens model geometry.

Note that in addition to allowing definitions of the surface clear diameters, the Spherical General Lens 3D part has parameters that can be used to specify the physical diameters of each surface as well as the overall lens diameter. This capability makes it possible to construct a geometry that closely matches the geometry that will be manufactured. For example, this makes it easier to set up a high-fidelity model that includes the structural and thermal effects on the lenses and the surrounding barrel.

Next, as shown below, we insert a second instance of the Spherical General Lens 3D, which is used to define “Lens 2″. This element must, of course, be placed in the correct absolute location within the model geometry. A standard optical prescription usually specifies the distance from the entrance surface of the current lens to the exit surface of the preceding lens. Therefore, the definition of the Spherical General Lens 3D part includes work planes that locate the entrance and exit vertices of each lens surface. Each subsequent lens part instance can then be placed with respect to the work plane located on the exit vertex of the preceding lens. As demonstrated below, “T_1″ is the distance between the exit surface of Lens 1 and the entrance surface of Lens 2. (In this case, the separation happens to be 0. Therefore, a doublet lens is created, but the same principle applies for any given separation.)

A screenshot showing the second element of the Petzval lens geometry in a Settings window.
The second element (Lens 2) of the Petzval lens model geometry.

Putting It All Together: Ray Tracing the Petzval Lens

Along with the lens elements, additional surfaces can be added to the Petzval lens model geometry that define the aperture stop and the image plane. For convenience, the complete model geometry can be saved as a geometry sequence so that it can be used in other COMSOL Multiphysics simulations. The full model (neglecting stray light apertures) is shown in the figure below. Here, we see that a unique feature of the Ray Optics Module is its ability to perform a ray trace on a fully meshed geometry. The same mesh can be used to solve for other quantities like temperature and structural displacement in the lens geometry, streamlining the setup of bidirectionally coupled multiphysics models.

An image of the surface mesh elements in the complete Petzval lens model.
The completed model geometry for the Petzval lens, showing the surface mesh elements.

Other features can be built into the definition of the parts. For example, work planes have also been defined in the Spherical General Lens 3D part and indicate the location of the front and rear edges of the lens. These work planes can be used to precisely locate apertures for blocking stray light or to locate mechanical mounting features. In addition, selections have been defined, separating the clear apertures, obstructions, and edges of each lens part. These selections can be used to quickly define the various physics features required for a geometrical optics simulation, as seen in the figures below.

An image highlighting the selections used to define clear apertures in a Petzval lens.
An image highlighting the obstructions in a lens.

The selections used in the Petzval lens to define the clear apertures (left) and the obstructions (right).

For this ray tracing example, we generated results via a grid-based ray trace at a single wavelength (550 nm) and field angle (on-axis). The results shown below make use of custom color expressions, added during postprocessing for better visualization. The color expression of the left plot (the ray trace) is based on the ray locations on the image plane. The color expression of the right plot (the spot diagram) is based on the ray locations at the entrance pupil. Each of these color expressions makes it possible to easily visualize the relative contribution of the rays at the entrance pupil to the final image quality.

A ray trace for the Petzval lens model.
A plot of a spot diagram in the COMSOL® software.

Ray trace (left) and spot diagram (right) of the Petzval lens.

Next Steps

The example featured above demonstrates how to create an optical geometry prior to performing a ray tracing simulation. To try this model yourself, click the button below. This takes you to the Application Gallery, where you can download the MPH-file if you have a COMSOL Access account and valid software license.

Learn More About Modeling Lenses

Reference

  1. M.J. Kidger, Fundamental Optical Design, SPIE Press, 2001.

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