Introduction to stray light analysis - Part 3

This article describes how to analyze stray light from mechanical components. OpticStudio can import the CAD parts into the simulation model as a single object and apply the optical properties for it. By utilizing the Path Analysis tool, mechanical components causing the stray light can be easily identified. An optical engineer can request a mechanical engineer to modify the design of the components during the simulation process. This article is part 3 of  3 articles introducing stray light analysis with OpticStudio.

Authored By Takashi Ishikawa

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Introduction

This article introduces the method to analyze stray light from mechanical components. Rays hitting the lens barrel and the housing holding the optical components are scattered and can reach the detectors. The surfaces of objects illuminated by ambient light could be the sources of stray light. We have to pay special attention to the scattered light generated from surfaces that can be seen from the detector when looking through the optical system from the image plane, because the scattered light could reach the detector with only one-time scattering. For more details, see the HANDBOOK OF OPTICS [1].

OpticStudio model with CAD files

OpticStudio can import CAD components into the simulation model as STEP/IGES/SAT and STL files or Autodesk Inventor/Creo Parametric/SolidWorks files with a dynamic link (for the details of the dynamic link, see "Using the OpticStudio Dynamic CAD link").Therefore, mechanical components designed by the mechanical engineers can be loaded into OpticStudio, and the optical engineer can analyze the scattered light from these components using the functions of OpticStudio. This powerful capability is explored in the following example.

 
The following images show an opto-mechanical system, including mechanical components designed by CAD software. You can download the sample file from the Download section. Lens spacers are in between the lenses, pressure/retaining rings hold the lenses, and a lens mount covers the entire the opto-mechanical system. In general, well-designed opto-mechanical system stray light due to interference with the mechanical components in large part do not occur. This is because the mechanical engineer designs the geometry of the mechanical components so that those components don’t block the designed ray paths. However, interactions between mechanical components and light from outside the intended field of view can cause contamination of the image. The following example shows how to identify these effects.

 

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Applying optical properties to mechanical components

The mechanical components in this system include CAD parts and annulus that is the stop aperture of the optical system. In this example, we’ll apply identical reflection and scattering characteristics for all mechanical components.

  • Reflectance: 5 %
    • Specular reflectance: 0.5 %
    • Scattering reflectance: 4.5 %
  • Scatter Model: Lambertian

To make all surfaces of a CAD object as a single face, set Object properties…CAD…Surface Mode…Use single surface. The same setting can be applied to more than one object simultaneously by selecting multiple lines in Non-Sequential Component Editor (NSCE).


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In Object Properties…Coat/Scatter is set in the following image.


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I.95 coating applies the optical specification that 95 % of intensity of the ray transmits and 5 % reflects (Note, as the material of the mechanical component is set to MIRROR in the NSCE no light can transmit, so effectively 95% is absorbed and 5% reflected) Scatter Fraction 0.9 means the 90 % of reflected ray is scattered according to Scatter Model definition. Therefore, if the rays hit the surface, 5% of the incident power is reflected, and 0.05 x 0.9 = 0.045 (4.5 %) of rays are converted to Lambertian distribution.

 

Stray light due to mechanical components

Add a Source Ellipse at the entrance aperture of the opto-mechanical system as a source of stray light. The position and the diameter of the source are the same as the phisical entrance aperture. In this sample, Z-position is 4.0 e-3 and X/Y Half Width is 7.0 e-3. About the angular distribution, the Lambertian light source is one of candidate models because the stray light source is required to emit rays to omnidirectionally to simulated light which can enter the system from outside the field of view. Set the Cosine Exponent of Source Ellipse as to 1 to make the angular distribution Lambertian.


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Below is the display layout of the system. Note that # Layout Rays of source 1,3,4,5 is set to zero. Since the object number of the stray light detector is 15, a Filter String of H15 is applied. The rays displayed are rays passing through the optical system or scattered by the mechanical components, then reaching the detector.


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Execute the ray trace to get the ZRD file. In this case, the # Analysis Rays is 1E+6. Confirm the # Analysis Rays parameter of the objects 1, 3, 4 and 5 is set to zero because we are interested only in the rays from the Source Ellipse we recently added. Check Save Rays and use Filter Strings of H15 to pick up the rays hitting the detector. Click Clear & Trace.


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When the ray trace is complete, click Exit to close the Ray Trace Control window and open Analyze…Raytrace Analysis…Path Analysis.

 

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Path 1 is the designed optical path which propagates through the lenses in sequence and hits the detector without scattering from mechanical components. In paths 2 and 3, the rays are scattered by mechanical components (object 20 and 21) marked by red rectangles. These are the pressure/retaining rings. Also, in path 4, rays hit the lens mount, object 19, before reaching the detector. These paths have the only one scatter event. On the other hand, rays following paths 7 and 8 reach the detector after two scattering instances from mechanical components (at objects 20 and 21). Consequently, the energy in paths with multiple scattering (8 or 9) is quite low compared with the paths with a single scatter event (2 or 3) and the rays in these paths do not significantly reduce the image quality.


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The following figure shows the rays of path 2 by using filter strings described in Part 2 of this series of articles.


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The output from the Path Analysis tool allows us the identify the most significant stray light paths. We have an insight about which parts of the system (in this case objects 20 and 21) need modifications to their geometry and surface characteristics to reduce the effect of the scattered light from the mechanical components. In the Premium edition of OpticStudio, you can not only identify the stray light source by using Path Analysis, but you can also directly modify the geometry of CAD part: Autodesk Inventor/Creo Parametric/SolidWorks through the dynamic link between OpticStudio and CAD software. The functions of OpticStudio enable optical engineers to verify how the mechanical components affect the stray light and image quality by themselves.

References

HANDBOOK OF OPTICS Volume I, Chapter 38 – Control of Stray Light by Robert P. Breault. (HANDBOOK OF OPTICS Volume I, 2nd edition, Optical Society of America, McGraw-Hill Professional, 1994)

Previous Article: Introduction to stray light analysis - Part 2

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