Illumination principle used to convert an inhomogene illumination to a homogene Illumination without too much light loss
The main components of a Koehler-Illumination are the “condensor” that maps the illumination (for example some LEDs) to the location of the second component, an Iris, often a manual Iris.
The “Collimator” maps the Iris (and therefore the images of the LEDs) to infinity. So the light will leave the collimation parallel.
Because the LEDs have a certain non-zero diameter, so have it’s image at the Iris. As the result the light from each arbitrary small point leaves the collimatior parallel, but at an Angle.
The sine of this off axis angle is called numerical aperture “NA” (in air) .
The speciality of the Koehler Illumination is, that the light of all these parallel light beams meets at (ideally) one position, markered as “best mix” in the graphics. For best results the real world object to be illuminated shall be at this best mix point.
The main applications of Koehler-Illuminations are back light for specimen in the microscopy.
The NA of the Koehler Illumination can be changed by adjusting the iris diameterand should be adapted to the NA of the microscope lens that focusses at this “best mix” distance.
The higher the NA of the microscope lens (of a given magnification) the higher is the resolution of the lens.
The NA of a microscope lens “in air” is limited, because the sine of the angle is limited to 1.
The NA in a different medium (water, oil) is given by sine of the off axis angle , multiplied by the refraction index n’ of the medium.
Therefore replacing air by water or oil(do not do this if your lens is not marked “oil” or “water”, else this kills your lens!!!) increases the NA , therefore the off axis angle and therefore allows for a brighter and better resolution image of the microsckope lens.