Hi Dirk!

Thanks for your follow-up here. Without being able to see your file, it's hard to say why exactly the distribution of light in your near field detector remains rectangular despite having an absorbing annulus object. One thing you might want to check is that the X-/Y-Width are defined to have the boundaries of the Source Diode itself completely covered by the Annulus object's absorbing area. Depending on the placement of the objects, it might be that the annulus is being ignored, but again, this is hard to say without seeing your file. To demonstrate, though, I've attached a file where the output near field distribution is circular rather than rectangular:

If your file still shows some inconsistency, you could certainly share that in this thread or as a new MyZemax case for us to make more specific comments!

As for other resources that cover DLL creation, I think this article does a nice job summarizing all of the key elements that each DLL requires to properly run. Though it covers all different DLL types (Surface DLL, Source DLL, etc.), the link should directly take you to the Source DLL section. After this article, though, we do recommend looking at some provided sample DLLs and their source code (when provided) to get a more in-depth look at the structure of the code.

Let us know how these thoughts work out for you, and thanks again for the discussion so far!

~ Angel