# Introduction to stray light analysis - Part 2

In optical design, even if the optical performance is well designed, there may be some unwanted light on the image plane. How to find out the path of these unwanted light and set the absorption or block this light in the path is the most critical part of stray light analysis. This article is Part 2 of the three articles, which is introducing how to identify the stray light path by Path Analysis tool and filter string syntax.

Authored By Michael Cheng, Yihua Hsiao

## Introduction

The fundamental of how to observe stay light has been introduced in Part 1 of this series. The most important purpose for the stray light analysis is to find the "source" caused the stray light and try to resolve it. Therefore, in this article, we will continue to work with the same example system we prepared in the Part 1. This Part 2 article will introduce how to identify the specific light path for the stray light, and how to resolve the stray light by coating on the specific position of lens.

## Using filter strings to identify stray light paths

Now let's find out the causes of this stray light and explore ways to reduce it. The figure below shows the unintended reflected light (ghost light) on the image surface. To isolate these specific rays, we can use the Filter String function in OpticStudio (the green box in the figure below).

Next, we will use a technique to separate the light with stronger energy that is incident on the image plane (detector) from the ghost light. This technique is achieved by setting the Minimum Relative Ray Intensity, as shown in the red box below, where you can specify the minimum value of the light energy to be traced. The value entered represents the ratio of the light relative to the light source. The default is 1.000E-6, which means that the ray will always be traced until the value is less than 1.000E-6 times of the initial energy. This is a simple way to filter out the weak ghost rays and remain only strong ghost rays. We will introduce Path Analysis feature, which is a better way to identify the strongest stay light path.

Enter a value of 0.005, which will cause OpticStudio to stop tracing rays when their energy is less than 0.005 times the original energy.

Update the layout window multiple times and you will see the most significant ghost ray paths.

## Apply Coating to the lens

The layout plot indicates that the most significant ghost ray paths include a reflection from the curved face from one or other of the central negative elements (objects 6 and 10). In order to reduce energy in these ghost paths, we can use a coating on the lenses. Apply the coating AR on face 2 of object 6 and face 1 of object 12 and we will investigate the effects of this change.

After Ray Trace again, you can see that the surrounding ghost light is greatly reduced. It's good to compare the final picture in the Part 1 article, most of the snowflakes-like sky-blue points are disappeared here. This process could be repeated to understand the effect of coating other lens surfaces.

## Analyze light in a specific area (using Filter String)

After these initial steps to eliminate stray light, we find that there is still unexplained stray light on the detector, mainly in an arc at the bottom-center of the image. Suppose we want to know from where the arc of the stray light is coming. What should we do?

Here we use the Filter String again. In OpticStudio, the function of the Filter String is to use the characteristics of the light to filter the light. In the Help file, you can view nearly 100 instructions. As in the previous demonstration, we can also use logical symbols, such as "&, |, ^", etc., to combine multiple filter conditions.

We will use a combination of four instructions to filter only those rays incident one the specific area of the detector highlighted above.

Run the Ray Trace again, and set the option to Save Rays.

OpticStudio will store all the details of the ray trace to a Zemax Ray Database (.ZRD) file.

In the Layout plot we can read the saved ZRD file , and enter the same Filter String.

The extra {#50} in front of the Filter String means we want to display only the first 50 rays that satisfy the filter. This will prevent the layout from becoming too cluttered. From the Layout, you can see all the paths to the specified area on the image plane. But there is a problem. There are too many possible paths. Based on experience, we know that it is impossible for all paths to be strong. In these paths, it is likely that one or two of them are the main contributors of stray light. We should focus on the paths that contribute the most energy.

Open the Analyze…Ray Trace Analysis…Path Analysis tool. This will enable us to identify on the most important paths. Note that we can enter the Filter String that we used before into this tool.

After analysis, you can see that almost all energy is concentrated on the path of 3 > 6 > 17. Let's go back to Layout and see which path it is. Displaying the first path found in the advanced path analysis tool into the Filter String is easy. Add a &_1 to the end of the original Filter String, and you can see the following picture.

Aha! The analysis found that this path generated by the reflection on the side of the lens. In fact, this will not happen once we add the lens housing. This path also explains why the stray light we see is an arc. The following figure shows the results of the analysis using the second, third, and fourth paths. As before, we just add _2, _3, and _4 to the end of the Filter String.

Previous Article: Introduction to stray light analysis - Part 1

Next Article: Introduction to stray light analysis - Part 3

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