An Interactive Ray Tracing Simulation of Köhler Illumination
Köhler illumination is one of the foundational concepts in optical microscopy — and also one of the trickiest to truly internalize. Getting the adjustments right is one thing; understanding why they work is another.
There are excellent field guides on the topic worth reading first:
Both also offer interactive simulators that show you the real-world outcome — you see an actual cell image becoming sharper or blurrier as you adjust the controls. These are great for building intuition about what to expect at the eyepiece.
This simulation approaches the same concept from a different angle. Built using ray tracing in OpticStudio, it shows you exactly where every ray travels through the optical system as you interact with the controls. You won't see a cell — but you will see precisely why the image would blur, by following the ray paths themselves. It's a complementary perspective: less about the visual outcome, more about the underlying physics.
I couldn't find a publicly available ray tracing simulation specifically designed to teach Köhler illumination, so I built one.
Three parameters to explore:
- Field stop (field diaphragm): controls the size of the illuminated area at the sample plane
- Aperture stop (condenser diaphragm): controls the angle of the illumination cone
- Condenser axial position: controls the focus of the condenser lens along the optical axis
Each parameter has 20 steps, for a total of 8,000 precomputed ray tracing frames (~335MB) that load into your browser once before you interact.
The OpticStudio archive and macro used to generate the simulation are openly available on GitHub.
The simulation runs entirely in your browser — no installation needed.
Feedback and questions are welcome — this is designed as a teaching tool and I'd love to hear how it works in practice.