Analysis of stray light in optical systems

Abstract: Stray light is the general term for all abnormal transmission light in optical systems, and the impact of stray light on the performance of optical systems varies depending on the system. Therefore, in modern optical design, stray light analysis has become an important link in optical design work. The causes of stray light are complex. The stray light caused by leakage light and residual reflection on the transmission surface is discussed. For the stray light caused by leakage light, a high-density sampling analysis method is given. For the stray light caused by residual reflection, a light optical model with energy factor and a data structure of light Binary tree are established, which can ensure the calculation accuracy and reduce the calculation time. We conducted leakage stray light analysis and paraxial and actual light analysis of residual reflected stray light on the optical surface of a Cassegrain optical system, and obtained measures to reduce stray light, achieving the purpose of stray light analysis.


Stray light is a general term for abnormal transmission light in optical systems, which is generated from residual reflections on leakage and transmission optical surfaces, as well as residual reflections on non optical surfaces such as the inner wall of the mirror tube, as well as scattered light caused by quality issues on optical surfaces. Infrared optical systems also have stray light generated by the system's own thermal radiation. For imaging optical systems, stray light can increase noise on the image plane, especially the stray light convergence points near the image plane that can have a serious impact on imaging, known as "ghost images". The article will discuss the modeling and analysis of residual reflected stray light on leaking and transmitting optical surfaces.
1.2 Residual reflected stray light on the transmission surface of the optical system

No matter how the transmission surface of an optical system is treated, there will always be some light reflected, some of which will form ghost images on the image surface after multiple reflections. Especially for some complex optical systems, when imaging a relatively bright light source, this phenomenon is particularly serious, and a series of large and small bright rings or dots can appear in the image. To simulate the generation and transmission of stray light, each transmission surface can be used as a partially transmitted or partially reflected surface. The transmission of transmitted and reflected light on each surface can be studied separately, and the energy distribution of each beam at a specified position can be calculated. For imaging optical systems, the energy distribution on the image surface needs to be studied. Currently, many optical design and analysis software have ghost image analysis functionality. The Monte Carlo method is usually used for stray light analysis. This method randomly emits many rays at a certain point, and when a ray reaches a transmission surface, its transmission or reflection probability is determined by the transmittance. After tracing a large number of rays, the number and distribution of rays reaching the image surface can be calculated to understand the energy distribution of stray light on the image surface, This method must trace a considerable amount of light, otherwise the results will not have statistical significance, especially when the system is complex and the number of faces is large, the number of light lines will inevitably increase in Geometric series. The issue of computational complexity is very prominent. For this purpose, a dedicated stray light analysis software was designed using ray optics methods. To avoid duplicate calculations, the following processing methods were used:

(1) Using the ray optics method with energy factor When using the ray optics method, each ray can be regarded as a small beam representing a certain space Solid angle. The ray transmission itself is the transmission of light energy. Therefore, each ray is transmitted with an energy factor, indicating the size of the energy carried by the ray. When transmitting, the energy factor changes with the loss of light energy, which can describe the change of light energy. After adopting this method, stray light analysis can be directly and orderly conducted without using Monte Carlo method. When encountering a transmission surface, the original light is divided into two, one continues to transmit while the other reflects. When processing, simply multiply the light energy factor by transmittance to obtain the new energy factor of the transmitted light, and multiply by reflectance to obtain the new energy factor of the reflected light.

(2) The ray Binary tree method aims at the fact that one ray becomes two. If each ray has to start tracing from the beginning, there will be a lot of repetitive work. Therefore, the binary tree data structure is used to describe the relationship between the ray splitting in two on the transmission surface


1. Stray light analysis method

1.1 Leakage stray light refers to the situation where some light does not directly enter the optical system according to the set beam limitations (field of view, aperture, vignetting). This often occurs in the optical path of catadioptric Catoptrics optical systems, so this kind of stray light should be specially analyzed in such optical systems. For example, in a typical Cassegrain system (as shown in Figure l), due to the presence of a through hole in the middle of the primary mirror, without special aperture treatment, light will directly enter the optical system without passing through the reflection of the primary and secondary mirrors, and reach the image surface to form a ghost image.

In order to eliminate this kind of light leakage, it is usually adopted to add a Lens hood and a special cylindrical impurity elimination diaphragm on the primary and secondary mirrors. In order to completely eliminate the leakage and stray light while ensuring the passage of normal beams, it is necessary to conduct a very accurate analysis of the leakage light. Currently, commonly used optical design software (such as Zemax) can be used, which requires a slight modification of the optical system model, changing the primary and secondary mirrors to specific apertures, and conducting dense sampling of the field of view. This method is quite cumbersome, especially after adding cylindrical apertures, which makes modeling difficult. Therefore, the author has designed a dedicated analysis software to conveniently input the position, size, and length of each aperture (cylindrical apertures), set the field of view and high-density sampling of aperture. The software can directly display the distribution of light spots on the image plane and the direction of light reaching the image plane. Under the guidance of this analysis, Designers can easily determine the size, position, and length of each cylindrical aperture.
When the light reaches a transmission surface, a Binary tree node is established on the surface, recording the coordinates of the light on the surface, the direction cosine of the incident light, and the energy factor of the light. The transmission part and the reflection part of the light can be described by two branches of the Binary tree respectively. During the calculation, the light first follows one branch to the end, and then returns to the node of a surface, taking this as the starting point, and then follows another path until all branches are traced. It can be seen from this that the Binary tree data structure stores the basic information of the light on each side during the light tracing process, avoiding repeated calculations, and recursive processing makes the software simple and reliable. A Binary tree is built with a ray tracing. Although the optical system has a large number of faces, it takes up a large amount of memory, but because of its dynamic storage characteristics, once a ray is traced and the results are recorded, the Binary tree can be deleted, Release the occupied memory for the next ray to use. If we study the energy distribution on the image plane, we can divide the grid on the image plane in advance. When a light ray reaches the image plane, it is determined where it falls and the light rays in each grid are weighted and stacked. The weight is the energy factor, and reproducing the values of all grids is the energy distribution on the image plane.


2 Analysis examples

Reflex systems are widely used

As an example of analysis, the long focal Cassegrain refractive and reflective photography objective may have light leakage in this system, which means that light directly enters the system and reaches the image plane without passing through the primary and secondary mirrors. At the same time, due to the presence of a refractive lens, there may also be residual reflected stray light on the transmission surface.

Light leakage analysis The analyzed telephoto Cassegrain system is equipped with a Lens hood, but no other anti leakage diaphragm treatment is done. The light leakage of the system can be obtained through analysis. After the primary and secondary mirrors of the system are equipped with anti leakage light shields, the light leakage is greatly reduced


The above analysis indicates that paraxial analysis provides the most severe ghost images and their causes, which can be addressed by modifying relevant parameters or coating methods. The actual light analysis can obtain the energy distribution and relative illumination of stray light on the image surface, providing very intuitive results. The analysis of light leakage can guide designers to improve the shading tube to ensure the elimination of light leakage with minimal light blocking. The results indicate that the analysis of stray light in modern optical systems is very necessary, otherwise it may have a serious impact on imaging. In some strong laser optical systems, it is necessary to conduct a comprehensive analysis of the stray light and ghost images of the entire system, because if ghost points appear near the surface of optical components, powerful energy may damage the components and cause system damage. In the analysis of leakage stray light and residual reflection stray light on the optical transmission surface in the article, self-developed stray light analysis software was used. This software can also analyze the impact of thermal radiation on the infrared optical system itself, which has been introduced in other articles.