how does Zemax handle ray tracing when the feature size is comparable to a wavelength?

  • 7 December 2020
  • 3 replies
  • 50 views
X

Dear,


I'm using the non-sequential mode of zemax to do ray tracing, and I have a question:


I have a cylinder cavity in my simulation (non-sequential mode), and there is a random roughness(with rms amplitude of 5micrometer) on the reflective sidewall of the cylinder, I'm using a gold coating on the inner walls of the cylinder, and I'm using a wavelength of 4.3um.


question is: how does zemax handle ray tracing with rays incident on the structure (sidewall roughness in my case) with a feature size that is comparable to the wavelength(5um and 4.3um)? I assume the rays are specularly reflected locally? is that correct?


Another question: how does a Lambertian scatter (in the non-sequential mode) on a reflective surface work? I understand the directions of the reflected rays follow a lambertian distribution, but how about the intensity of reflected rays? are the reflectance of the gold coating when applied a lambertian scatter and  when no scatterer is applied (specular reflection) different?


Thank you in advance


 

3 replies

Csilla Timar-Fulep
Rank
Userlevel 5
Badge +2

Hi Xiaoning,


Thanks for your question here!


How did you model the surface roughness? Did you use a grid sag to model it? If the surface roughness is not much smaller than the wavelength, that sounds like a good solution. If this is the case, the surface normal is always calculated at the ray incident, and the rays are reflected based on this direction, so in my understanding this means it is calculated locally. IS this what you were looking for?


Regarding the reflectance when scattering is applied on reflective surfaces, the Scatter fraction parameter defines the fraction of the energy that goes into the scattered rays, after first accounting for attenuation by the coating. So the reflectance is applied in the same way whether scattering is applied or not, and if scattering is applied then reflectance is accounted for before taking scattering into account.


You may find more information about this in the Help system at:


The Setup Tab > Editors Group (Setup Tab) > Non-sequential Component Editor > Object Properties (non-sequential component editor) > Coat/Scatter > Fraction to Scatter and Number of Scatter Rays


For further references, please check out the following knowledgebase article:


How to model a partially reflective and partially scattering surface · MyZemax


I hope this helps, but if you have further questions, please let us know and we will be happy to help!


Best,


Csilla

Csilla Timar-Fulep
X

Hi Csilla,


For the surface roughness, I imported a CAD object into zemax, with the roughness on the sidewall of the cylinder shown in the attached figure 'rough.PNG', you are saying that if the roughness is comparable to the wavelength, Zemax uses rays to handle it (just as if the roughness is much larger than a wavelength), so diffraction is not considered here, right?


Regarding the reflectance when scattering is applied on reflective surfaces, if I have the same incident angles of the ray for both cases (with/without a lamertian scattering), the reflectance for both cases should be the same right? regardless the direction of the reflected ray(specular or scattered)


 


Thanks!

rough.png
Hui Chen
Rank
Userlevel 4
Badge +1

Hi Jia,


Yes, your understanding is correct. In this case OpticStudio uses rays to model the interaction between the beam and the CAD object (cylinder with rough wall), so no diffraction effect is considered here.


As for the energy splitting on a reflective surface with or without scattering effect, you can specify the energy that goes into the scattered rays by the Scattering Fraction setting. 



For example in the case above, (the rectangular object is a Mirror object), with the scattering settings entered, if I launch 1 ray of 1 watts, 0.5 watts will go to the specular reflected ray; and the rest 0.5 watts will go to the scattered rays. Since I set # of Scattered rays to 3, each of the scattered ray will carry 0.5/3=0.167 watts. And the distribution of the scattered rays will follow the scattering profile you defined, in this case Lambertian. 


Let us know if you have any other questions.


Best regards,


Hui

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