Koehler illumination

Phase contrast requires Koehler illumination. Early publi-... 

Bertrand lens A lens that, when inserted into the optical train of a microscope, allows the viewer to see an image of the filament used to establish Koehler illumination. A lens that focuses on the rear apeture of the objective lens assembly. 

Koehler illumination In a microscope, the alignment of lamp and lenses that produces optimal and even illumination. 

Figure 10 illustrates schematic drawing of objective-type TIRFM. A high numerical aperture objective lens is mounted on an inverted microscope. A laser beam is passed through a neutral density filter (ND) and a beam expander (BE) to adjust its power and diameter. When the laser polarized linearly is used, the polarization of the laser beam is converted from linear to circular by a quarter-wave plate (A/4). The laser beam is focused by a lens (L) on the back focal plane of the objective, so that specimens are illuminated uniformly with Koehler illumination. By shifting the position of the mirror (M) located between the lens (L) and dichroic mirror (DM), the path of the incident laser light is shifted from the center to the edge of the objective. At the center position, the microscope can be used as a standard epi-fluorescence microscope (Fig. 10b). 

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