This article describes how to perform stray light analysis for a lens system using the Ghost Focus Generator in Ansys Zemax OpticStudio. Ghost reflections, caused by unintended light reflections at lens surfaces, can result in spurious images or reduced contrast in imaging systems. These effects are particularly significant in high-power laser systems, where concentrated reflections may damage optics. Properly analyzing ghost reflections is essential for minimizing stray light and improving the overall optical system performance. This article explains how to use the Ghost Focus Generator to simulate and analyze single and double-bounce ghost reflections and provides guidance on interpreting the results for effective system design.
Authored By Sean Lin, Wilson Chen
Introduction
The Ghost Focus Generator simplifies the modeling of ghost reflections by converting refractive surfaces to reflective ones, enabling the tracing of light paths that form ghost images. It supports both single and double bounce reflection analysis, calculates key parameters of ghost systems such as ray heights and focal distances, and generates detailed lens files of each ghost system for further exploration. While powerful, this tool has some limitations, when working with systems containing coordinate breaks or special surface types. For comprehensive stray light analysis, alternative tools, such as non-sequential ray tracing, can complement the Ghost Focus Generator.. This article outlines the Ghost Focus Generator workflow, highlights practical considerations, and discusses strategies for optimizing lens designs to mitigate ghost effects.
Optimized system
Some of the primary considerations in an optical imaging system are factors such as system size, focal length, aberration content and image evaluation criteria such as MTF... Only after achieving these objectives does stray light become a focus of attention. Therefore, before utilizing the Ghost Focus Generator for quick stray light analysis, we first select an optimized imaging system as the foundation. For this workflow, we will refer to the article below and use the optimized cellphone lens described within to perform stray light analysis:
Ghost Focus Generator
First, we will use the Ghost Focus Generator tool, which helps identify single-bounce or double-bounce ghost reflections and saves the corresponding files for further analysis.
This tool will output a list of all ghost reflections and generate lens files for each type of ghost system. However, for more complex systems, many ghost reflections will be calculated which will result in many files to be saved. To streamline the process, we can use an API script to filter out the most impactful ghost reflections. The link below contains a prepared API script that runs the Ghost Focus Generator in Double Bounce mode and sorts the surface combinations in order of which contributes to the most critical Pupil and Image Ghosts:
We first execute the Ghost Focus Generator using the API and then sort the results by impact severity for pupil ghost and image ghost, as shown in the diagram below. A pupil ghost is a ghost image formed by reflected light focusing near the pupil plane. Pupil ghosts typically affect light uniformity and pupil imaging. In contrast, an image ghost is an unintended virtual image formed by reflected light focusing near the image plane. Image ghosts directly impact image quality. Therefore, our priority here is to address the image ghosts.
Next, we focus on the first Image Ghost target. We will again use the Ghost Focus Generator tool and add an AR coating to the “Ghost Reflector Coating”setting. By applying a coating to optical surfaces for ghosting analysis, it is possible to simulate the reflective properties of the surfaces involved in generating ghost reflections. We will use the tool to save the double-bounce file between the seventh and sixth surfaces and analyze the ghost's illuminance using Geometric Image Analysis.
From the analysis image below, we find that when the light source energy is 1 watt, the maximum ghost intensity at the image plane is 0.022 watts per square millimeter. The next step is to attempt optimization to reduce the ghost intensity.
Optimization
Returning to the original lens file, we can now use the GPIM operand in the Merit Function for optimization. GPIM is utilized to calculate and control the positions of ghost pupils and ghost images relative to the image plane. The target value is set to 0 in order to push the ghost image distance towards infinity to minimize its impact on the image plane.
During this round of optimization, we will optimize the GPIM operand in addition to the original merit function used to design this system. This ensures that improving stray light does not significantly alter the system or degrade image quality. After optimization, we can compare the MTF of both cases to verify if the performance remains within the original standards. It can be observed that the structure and MTF before and after optimization show no significant differences and remain within the original specifications.
Improve results
We replicate the previous steps by using the Ghost Focus Generator tool to generate the ghost image file for the seventh and sixth surfaces of this optimized system. Then, we perform Geometric Image Analysis with the same settings to observe the results. The optimized structure successfully disperses the ghost's focal point, reducing the maximum energy to 8.77E-04, significantly mitigating the ghost's impact.
Finally, we applied the same verification method across different fields of view and found that the optimized system effectively reduced stray light across all fields.
Conclusion
In this study, we focused on analyzing and optimizing stray light in an optical imaging system. Using the Ghost Focus Generator, we identified significant ghost reflections, prioritized improving Image Ghosts, and saved double-bounce files for analysis. By employing tools like Geometric Image Analysis and the GPIM operand, we optimized the system to minimize ghost intensity without compromising the original optical performance, as confirmed by MTF comparisons. The optimization successfully dispersed ghost focal points and significantly reduced maximum energy levels. Validation across different fields of view further demonstrated that stray light was effectively suppressed throughout the system, ensuring improved image quality and consistency.
With this method, we can effectively eliminate double-bounce ghosts directly in sequential mode during the design phase. This represents one of the critical steps required to fully assess stray light within a system. Further non-sequential analysis is essential to evaluate the impact of additional effects, such as multiple bounces, total internal reflection (TIR), and stray light caused by mechanical components or external environmental factors. Resources like Stray Light Analysis – Smartphone Camera – Ansys Optics highlight the importance of a comprehensive approach to stray light analysis, ensuring that all potential stray light sources are addressed for optimal system performance.
By integrating these additional analyses, designers can achieve a holistic understanding of stray light behavior, enabling the development of optical systems that are robust against a wide range of stray light sources while maintaining superior imaging performance.
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