Relative Illumination

  • 24 February 2021
  • 2 replies
  • 883 views
K
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  • KEn
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  • 3 replies

I have a question about the correct way to simulate relative illumination in an imaging lens. The lens is designed to work with apertures between the lens elements which introduce some vignetting towards the edge of the field. The apertures are defined explicitly as 'Circular Apertures' at each surface in addition to a user defined 'Semi-Diameter' at the same value. The lens semi-diameters are defined in the same way using surface 'Circular Apertures' in addition to setting the same aperture semi-diameter as user-defined in the 'Semi-Diameter' column. Think of a kind of mobile phone lens -- the basic structure is similar as is the maximum field angle of about +/-37deg.


The company that does manufacturing claims that for the proper calculation of RI it is necessary to set vignetting factors (and not to set 'Remove Vignetting Factors' in the relative illumination settings) and to use polarization.


In my opinion if I have defined at all surfaces surface apertures then I need not use vignetting factors for the calculation of relative illumination. Furthermore I should not use these factors as they represent another set of 'virtual' apertures of elliptical shape that would act in addition to the real apertures in my system.


And setting apertures at all surfaces will be more precise than the use of vignetting factors (without surface apertures) as no interpolation has to be carried out in the calculation of RI at intermediate field positions where no vignetting factor has been defined.


Concerning the 'use polarization' issue I think it is not wise to include polarization at a lens that is simulated without AR coating and that in practice will have an AR coating. If I use polarization then Fresnel losses are taken into account but these Fresnel losses are invalid as the coating is missing in the simulation. In case the coating would have been defined layer by layer then of course 'use polarization' would increase precision of the RI simulation especially at high fields. But otherwise I would not use this option.


What is the official recommendation to the questions I have raised? Which is the most precise way to calculate RI with Zemax?


Thanks and best regards,


Kai


 



 

2 replies

Mark.Nicholson
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Hello Kai,

 

Using real apertures is correct. Vignetting factors approximate the off-axis pupil shape as a decentered ellipse, and are useful as they allow you to use Gaussian Quadrature in the merit function, and/or account for vignetting before you have the real apertures in place. But this approximation should be junked once you are out of that stage. VFs are an early-stage design tool, and have no place in the production-ready version.

 

 

Use real apertures, and if you want to be really careful, put the stop on a dummy surface located at the entrance pupil, and turn off ray aiming. You may have to overfill the pupil to account for pupil aberrations and ensure that the stop is correctly illuminated: check that with the footprint diagram.

 

 

Use Polarization will account for surface reflectivities and glass absorption, but it can only be as accurate as the coating information it is given. If you don't have the prescription, use a TABLE coating (coatings manufacturers should be willing to give that data) or worse case an IDEAL coating.

 

 

The only significant assumption that the RI calculation makes is that there are no significant caustics along the beam so that you can map a ray's landing points to a cone. This is the same limitation as all differential ray-tracing methods. You can check the spot diagram and use the 'show direction cosines' options to check this.

 

 

Last, there is a great article here: FAQ about Relative Illumination | Zemax Community

 

 

- Mark

 

 

 

 

K
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Hi Mark,


What a pleasure to meet you here again!


Thanks for your response. That was exactly what I wanted to read ;-)


Best regards,


Kai


 


 

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