Koehler lighting system provides evenly lit images
By incorporating an iris field diaphragm into the path of the microscope illumination beam, the scattered light is minimized and a simple procedure ensures the correct positioning of the condenser for maximum resolution and desired contrast.
Koehler's illumination is based on the presence of an iris field diaphragm inside the microscope.
The correct configuration of the lighting microscope is probably one of the most incomprehensible concepts of optical microscopy and is a critical parameter that must be met for optimal performance.
The intensity spectrum and the wavelength of the light emitted by the light source are of great importance, but even more imperative is that the light emitted from various positions by the light source is collected and focused on the plane of the diaphragm opening of the condenser.

---