May 06, 2021

Introduction to stray light analysis

Introduction to stray light analysis

Even if the optical performance of a system is well designed, there may be some unwanted ‘stray’ light on the image plane. In general, stray light means light that does not enter the system through a designed path before reaching the image plane and degrading the intended image. Before physical prototyping, it is often essential to investigate the effects of this stray light on performance. Finding out and blocking the paths taken by this unwanted light is the most critical part of stray light analysis. This blog post introduces how to prepare for stray light analysis by using the Convert to NSC Group tool to convert a sequential system to non-sequential mode.

In photography, a common source of stray light is a vital light source (such as the Sun) outside the field of view. Light from such sources can reach the image plane via scattering from the system's mechanical or optical components. Alternatively, light from a source inside the field of view can undergo multiple secondary reflections at lens surfaces before focused on the image surface. 

Preparing the system

At first, we introduce how to prepare an example system for stray light analysis. OpticStudio users can follow along by opening the built-in sample file Samples\Sequential\Objectives\Double Gauss 28-degree field.zmx.

We will use several OpticStudio tools to prepare the file for stray light analysis. First, use the Coat Surfaces tool to remove the coatings from all surfaces. We will find which coating works better later. 

Design Lockdown tool

Next, execute the Design Lockdown tool. This will adjust the system settings so that the lens meets the actual conditions and the analysis results are more correct. For a more detailed explanation, refer to the description of the Help file.

Critical Rayset Generator

Before converting to Non-Sequential Mode, we can export a key ray set from Sequential Mode, consisting of the chief ray and a series of marginal rays. This allows us to examine the critical rayset in the Non-Sequential Mode directly; otherwise, the rayset could only be calculated in the Sequential Mode. The method is as follows:

Convert to Non-Sequential

The most convenient way to analyze stray light in OpticStudio is in Non-Sequential Mode, and the Convert to NSC Group tool enables conversion from the sequential mode in just one step. 

Converting from Sequential Mode to Non-Sequential Mode is described in detail in the knowledgebase article 'Converting Sequential Surfaces to Non-Sequential Objects.'

Click File…Convert to NSC Group, leave all settings at their default values, and click OK

The system has been changed to pure Non-Sequential Mode. All the lenses have been converted from surfaces to object. We can also see that several light sources and detectors have been included to represent the field points and image locations from the Sequential system. This system is the same as the original Sequential Mode, except a built-in Non-Sequential Mode. 

One helpful feature of Non-Sequential Mode for stray light analysis is that the rays can split. Open Analyze...NSC 3D Layout window and check Split NSC Rays. We can observe partial reflection, partial transmission, and multiple reflections on each lens surfaces' rays. This is what we can't see in the Sequential Mode.

We can also see with the NSC Shaded Model: 

Check the status of the critical rayset.

Open the Analyze…Critical Ray Tracer tool, and you will see that the chief and marginal rays of each field can normally pass through the Non-Sequential system. When mechanical components are designed and added to the system, as imported the CAD files or native OpticStudio objects, it will be necessary to use the tool again to ensure that they do not block the critical rayset.

Required settings for stray light analysis

Now we have successfully prepared our system, and we can move onto the primary stray light analysis. Before starting the stray light analysis, it is necessary to adjust some essential settings. 

The first is to adjust the Maximum Intersections Per Rays and Maximum Segments Per Rays to the maximum number (4000 and 2000000 respectively). In the stray light analysis, sometimes the light we want to analyze will reflect and scatter repeatedly. If the settings for the maximum number of segments or intersections are insufficient, it may not be possible to analyze all conditions.  

We have reduced the number of Analysis Rays to 5000. When analyzing stray light, usually each ray splits into many child rays. Ray tracing speed maybe ten or more times slower than without splitting. For this demonstration, we control the number of rays to 5000 per source to keep ray trace times down.

The final step is to set the number of pixels on the detector to 150x150. The fewer pixels also means the fewer essential # Analysis Rays is needed. 

Preliminary ray trace results

We can now look at the preliminary ray trace results. Click Ray Trace, set the operation as shown below. As shown in the figure below, check "Use Polarization" and "Split NSC Rays" when tracing.

After the ray trace is completed, open Analyze…Detector Viewer. The location of this tool is as follows.

Set as in the following figure. 

You can see the stray light, which are sky-blue points like snowflakes shown in the below picture, in this system due to multiple reflections. This result agrees with what we saw earlier in the 3D layout when Split NSC Rays enabled. We can observe numerous reflections only in the Non-Sequential Mode since Split NSC Rays can be performed.

This blog has introduced stray light and the preparation work before stray light analyses, including converting sequential mode to non-sequential mode, Design Lockdown Tool, Critical Rayset Generator, and ray tracing.  To learn more, you will find the following articles on MyZemax.comPart 2, which will introduce how to use filter string to identify specific stray light and use path analysis to find out the strongest stray light path. And Part 3 is an example to explain how to analyze stray light from mechanical components by importing CAD.

Blog Author:

Kerry Herbert
Marketing Manager
Zemax, LLC

Knowledge Base Article Authors:

Michael Cheng
Principal Optical Engineer
Zemax, LLC

Yihua Hsiao
Optical Engineer
Zemax, LLC




 

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