Role and properties
An object can be properly observed only when properly illuminated.
The properties of a good illumination system are the following:
Uniformity (to observe the object and not the illumination inhomogeneities)
An illumination area limited to the microscope field of view {To avoid parasitic light}
A good control of the numerical aperture (or in other words of the partial coherence of the illumination) {see the subsection « Illumination coherence and resolution limit »}
The illumination system must also be able to work in two very different observation modes:
In the first mode, mostly transparent objects are illuminated in transmission; we speak of transmitted (diascopic[1]) light microscopy (from the Greek 'dia-', through).
In the second mode, opaque objects are illuminated by reflection, in general through the observation objective which plays a double role; we speak of reflected (episcopic[2]) light microscopy (from the Greek 'épi-', above).
Those conditions can be fulfilled using various methods, that differ from each other in terms of simplicity and degrees of perfection. The most efficient technique, universally used on professional microscopes, is called Köhler illumination [ [3] [4]] . This method is presented in the next paragraph in the case of transmitted light microscopy. Its implementation for reflected light microscopy is shown on Figure C2, in the supplements at the end of the course sequence.