OPD /Wave aberration calculation

[dibyendumajumdar]

Hi Mike

I wanted to understand if its possible to calculate OPD purely from the ray tracing data, i.e. compute optical path length of each ray - and then the difference between the length of a ray vs chief ray is the OPD?

I understand that the path length is computed up to the reference sphere.

I have the HH paper and also Welford book on aberrations, but I am struggling to understand the various approaches.

The first confusion is that the exit pupil can be at different places depending on the lens design. The literature often implies exit pupil is after the last surface, but this is not always true. Examples:

• Exit pupil is before entry pupil
• Exit pupil is close to aperture stop

I am wondering if it is possible to visualize the OPD calculation in rayoptics in the context of a layout diagram. Say for a particular field. This would hugely help to understand what is being calculated. I have seen the diagrams in Welford's book and the HH paper, but as I mentioned these assume that exit pupil is located after the last surface.

[dibyendumajumdar]

I found this thread on how Zemax calculates:

https://community.zemax.com/got-a-question-7/opd-reference-sphere-and-exit-pupil-4582

[mjhoptics]

@dibyendumajumdar,
There is no requirement that the exit pupil be located after the final lens surface. The exit pupil can be interior to the lens, after the lens, even after the image surface. The key point is you use the ray positions and directions in image space, i.e. after the final lens surface. The function waveabr.transfer_to_exit_pupil() is used to locate the exit pupil in image space; this location can be virtual.
I agree that the geometry is complicated. There are still papers being published on OPD calculations, often in the context of optical testing. I’ll try to put together something to visualize the reference sphere in the layout.
Mike

[dibyendumajumdar]

Thanks Mike, I am super impressed by your ability to decipher the stuff in HH paper and implement it; I find it extremely difficult to follow. It would be super helpful to have a visual depiction of what is being measured.