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the difference of chief ray definition and convention

  • September 4, 2023
  • 6 replies
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Hi all,

     I tried to figured out the difference between the chief ray definition by the classical books and the convention by ZEMAX. 

     

chief ray definition from Arizona University
chief ray difinition on SPIE website
 
chief ray convention from ZEMAX manual

 the chief ray, by SPIE and Arizona University, is defined to pass through the center of the aperture stop. whereas, is defined to pass through the center of the entrance pupil. the difference causes a lot of troubles using ZEMAX. Can anybody explain this to me ?

Best answer by David.Nguyen

Hi @steven.Ou,

 

I’m just an engineer, but here’s my take.

If the system is paraxial, the entrance/exit pupils are images of the aperture stop. That means, if a ray goes through the center of the entrance pupil. Then, that same ray will also go through the center of the aperture stop, and the exit pupil.

In the first figure that you show from Arizona University, if you loolk at the initial trajectory of the chief ray, it is indeed aiming at the center of the entrance pupil. I’ve tried to highlight this with a blue (dash-dot-dot) arrow in my screenshot below (sorry for colorblind people, its all I could do with the original figure colorcode):

Blue (dash-dot-dot) arrow is pointing at the center of the entrance pupil. The arrow has been offset vertically to conserve the idea of the original screenshot.

Similarly, the chief ray, when looked from the image side, is emerging from the very center of the exit pupil:

Blue (dash-dot-dot) arrow is emerging from the center of the exit pupil. The arrow has been offset vertically to conserve the idea of the original screenshot.

I don’t think the definitions are different between ZEMAX and classical textbooks. I can see why it can be confusing sometimes. For example, in the University of Arizona figure, the entrance pupil is on the image side, and this might not be so intuitive at first glance (perhaps think of the entrance pupil as a virtual image of the aperture stop in this instance).

Does that make sense?

Take care,

 

David

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David.Nguyen
Luminary
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Hi @steven.Ou,

 

I’m just an engineer, but here’s my take.

If the system is paraxial, the entrance/exit pupils are images of the aperture stop. That means, if a ray goes through the center of the entrance pupil. Then, that same ray will also go through the center of the aperture stop, and the exit pupil.

In the first figure that you show from Arizona University, if you loolk at the initial trajectory of the chief ray, it is indeed aiming at the center of the entrance pupil. I’ve tried to highlight this with a blue (dash-dot-dot) arrow in my screenshot below (sorry for colorblind people, its all I could do with the original figure colorcode):

Blue (dash-dot-dot) arrow is pointing at the center of the entrance pupil. The arrow has been offset vertically to conserve the idea of the original screenshot.

Similarly, the chief ray, when looked from the image side, is emerging from the very center of the exit pupil:

Blue (dash-dot-dot) arrow is emerging from the center of the exit pupil. The arrow has been offset vertically to conserve the idea of the original screenshot.

I don’t think the definitions are different between ZEMAX and classical textbooks. I can see why it can be confusing sometimes. For example, in the University of Arizona figure, the entrance pupil is on the image side, and this might not be so intuitive at first glance (perhaps think of the entrance pupil as a virtual image of the aperture stop in this instance).

Does that make sense?

Take care,

 

David


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  • Ultraviolet
  • September 12, 2023

@David.Nguyen 

Thanks David, I’ll give more thoughts to this.


Kevin Scales
En-Lightened
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Hi @steven.Ou ,

Have you had a chance to consider David’s points. I think he gets to the heart of the matter. In normal usage, the definitions of the chief ray as passing through the center of the aperture stop or one of the pupils are absolutely identical. OpticStudio does expand on this a bit, in that if vignetting factors are applied, then the vignetted pupil may not be the aperture stop exactly. You can find this discussion in the help page ‘Chief Ray’ in the section Conventions and Definitions, as well as the link to the vignetting factors help page.

I’m thinking that because OpticStudio is a piece of software rather than a century-old definition from the days of paper-and-pencil design, it made sense to have a more expansive definition. I don’t know this for sure, but it seems a likely explanation. The CR as ray through center of AS is going to be a subset of the possible CRs when vignetting is applied.

If you have any examples of how the differing definitions can cause further difficulties, please don’t hesitate to let us know.


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  • Ultraviolet
  • October 20, 2023

@Kevin Scales 

Thanks for your explanation ,Kevin.


MichaelH
Ansys Staff
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  • Ansys Staff
  • October 20, 2023

To simplify this even more, OpticStudio has Ray Aiming turned off by default.

  • Ray Aiming Off = chief ray through center of Entrance Pupil
  • Ray Aiming On (either Paraxial or Real) = chief ray through center of Stop

For systems with signification pupil aberrations (Analyze > Aberrations > Pupil Aberrations), the ray in the center of the EP will not go through the center of the Stop.

Turn on Ray Aiming and UA, SPIE, & Zemax all agree.


Mark.Nicholson
Luminary
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Ray aiming should be turned on by default in 2023. Back in the 90s when Ken first wrote the code, he defaulted it off because users were running on 286, 386 processors and the like. There’s no reason for that now.

  • Mark

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