Hello. Thanks for your question!
Modeling diffraction can be quite tricky, and unfortunately, a ray-based analysis cannot correctly capture the behavior of diffraction through a 5-μm slit (or any diffraction for that matter). Rays are a geometric simulation; you need to consider instead a wave-based simulation that can model diffraction from the edges of your slit aperture.
My suggestion would be to instead consider the Physical Optics Propagation tool in OpticStudio's Sequential mode. You would need to know the LED emission profile to create the correct initial representation of the beam. Modeling this beam correctly could be challenging, but you could start with just a simple Gaussian beam definition to understand how to use the tool. Then you could try propagating that beam through a narrow slit aperture (applied as a rectangular aperture, an example shown below):
I do want to caution you, though; POP can be a very complex tool. You need to be very careful about how you set the simulation up, and due to its complexity, I would first suggest that you study this series of Knowledgebase articles to better understand the pitfalls and its usage: Exploring Physical Optics Propagation (POP) in OpticStudio – Knowledgebase (zemax.com) and Using Physical Optics Propagation (POP), Part 1: Inspecting the beams – Knowledgebase (zemax.com).
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