Hi Nancy

Yes. In that example, the calculation is performed in the merit function to show how it works. So the OPD plot for the on-axis field is:

Now let’s say I want to recalculate the OPD for the marginal ray so HX=HY=0 and PX=0, PY=1:

So PLEN calculates the optical path length up to surface 4 (Image plane): line 2 is for the chief ray and line 3 is for the marginal ray. So we substract these two values and it gives the optical path difference between the marginal and chief ray up to the image plane.

Then we calculate a correction term. Lines 6 and 7 are the optical paths of the chief and marginal rays from the image plane to the reference sphere (surface 5). Then we compute the difference so it gives the optical path difference between the marginal and chief ray up from the image plane to the reference sphere.

Finally we substract the correction term to the optical path difference between the marginal and chief ray up to the image plane. This is our OPD.

If the system is perfect, all the rays are “like” coming from the reference sphere. When a system is not perfect, the OPD gives us a good idea of the “deformation” from that reference sphere.

I hope it helps but do not hesitate if you have any further questions.

Sandrine