Stray Light Analysis of the Cooke Triplet Lens Using Rayzen Software

I. Analysis Workflow

Overview of the analysis methodology and operational process for stray light analysis.

II. Model Construction

Schematic of the model.

Components	Location	Material	Center Thickness	Diameter	Surface Name	Optical Property	Radius	Clear Diameter	Mechanic Diameter
Lens_01	0.00	SK16	2.00	15.600	FrontSurface	fresnel	21.48138	15.600	15.600
					RearSurface	fresnel	-124.100	15.600	15.600
Lens_02	7.26	F4	1.25	10.566	FrontSurface	fresnel	-19.1000	10.000	10.566
					RearSurface	fresnel	22.00000	10.280	10.566
Lens_03	13.2	SK16	2.25	14.542	FrontSurface	fresnel	328.9000	14.542	14.542
					RearSurface	fresnel	-16.7000	14.542	14.542

Geometric parameters of the model.

III. Control Conditions

Ray Number	Preview Ray Number	Max Intersections per Ray	Max Segments per Ray	Relative Ray Power Threshold
10,000	100	10,000	100,000	1e-7

Simulation control conditions.

IV. Excitation Conditions

Source Type	Surface Source
Power	1.0W
Energy Distribution	Aim Region
Shape	Circular, Radius=8.3mm
Location	-22.00
Spatial Distribution	Uniform
Angular Distribution	Uniform
Angle Boundaries	Collimated along +z axis
Spectrum	Blackbody Temperature 6500K，Discrete，Spectrum Range 360nm ~ 830nm
Receiver	Surface Receiver，Circular，Radius=20.0mm

Light source and receiver parameter settings.

V. Processing and Analysis of Simulation Results

Enabling Path Information

Before running the simulation, in ForwardSimulation  Data Collection   Using Predefined Analysis Group , check Ray Path Analysis. Rayzen will collect and record relevant information during tracing to support path analysis.

Enable Path Information

Using power splitting with a low threshold of 1e-7, 100 preview rays split into 200,626 rays hitting the receiver. As shown, one randomly selected ray forms 62 optical paths in split mode, with three paths reaching the receiver.

Preview Ray and Optical Path Rays

Finding Ghost Image Paths

This example does not consider the effects of aperture and surface cleanliness and only analyzes ghost images caused by Fresnel reflections on uncoated smooth surfaces. As shown, after running the simulation, add the settings for receiver mesh size and resolution, and click  Run Mesh Analysis  to view the total illuminance of all optical paths.

Click  Run Ray Path Analysis  to view the list of optical paths and detailed information for each path. In the scene, under the Show Rays button, select Show Filtered Rays and select the corresponding analysis results. Now, by freely selecting or deselecting paths in the path list, you can dynamically view the light path trajectory for each path in the scene.

As shown, 10,000 rays form a total of 212 optical paths. Clicking the  Relative Power  header sorts the paths by relative power, revealing the path with the highest relative power as the main path of interest.

With the selected path checked, click  Update  to view the illuminance image formed by rays in this path on the receiver. As shown below, 5,492 rays in the main path contribute 0.38477 W of power, while all 200,626 rays across all paths contribute 0.38881 W.

By sorting by relative power, examine the illuminance of the second highest path. As shown, the rays on the receiver in this path are evenly distributed without forming an image, indicating this is not the ghost image path we are looking for.

Ghost images form from secondary reflections between mirror surfaces. Therefore, the maximum illuminance of each path can be calculated and sorted to locate ghost paths. As shown, click  Compute Filtered Mesh Data  to calculate the illuminance for each ray path, then sort by maximum illuminance and check the second path.

The rays are focused on the receiver, though fewer preview rays are available. Increase the number of preview rays in ForwardSimulation Preview Ray Path Number to 20, rerun the simulation, and examine the optical paths.

As shown, rays in this path form an image in front of the image plane, identifying it as a primary ghost image path. The maximum illuminance is 2.3‰ of the main path, with an energy contribution of 8.7‰. Although the path does not exhibit typical aberration trajectories, chromatic aberration may be present, which can be verified in spectral distribution analysis. For stray light reduction, a broadband anti-reflective coating should be considered.

After identifying this ghost path, other ghost paths can be checked. By selecting all paths except the main path and the identified ghost path (the top two paths in maximum illuminance), the illuminance distribution on the receiver shows 210 paths with evenly distributed rays and no distinct image, contributing 9.2‰ of the main path energy.

Reducing Stray Light

The identified ghost path, as shown, originates from the first reflection on the final surface of the main path. To reduce this reflection, apply a coating by changing the optical property from Fresnel reflection to ideal 100% transmission to simulate the coating effect. Rerun the simulation.

As shown, after coating, the number of optical paths reduces from 212 to 97, consistent with the exponential relationship between path count and surface count (100% transmission effectively blocks all paths that could reflect from this surface). Checking the illuminance distribution of all paths excluding the main path shows uniformly distributed rays with no distinct image, contributing 7.5‰ of the main path energy, indicating that the ghost image has been eliminated.

VI. Conclusion

This document outlines the complete simulation and analysis process for modeling and reducing stray light in Rayzen. The integration of functional modules within the software helps efficiently identify and analyze stray light sources and design solutions for stray light reduction. One key method for reducing stray light is coating design, and Rayzen supports the design and application of coatings, which will be covered in a dedicated topic with additional case studies.