48 PART II A Deeper Look into a Light Microscope
Fluorescence
Fluorescence is a low-energy form of radiation (emission) that results from a previous
high energy illumination (excitation). The energy difference between the
excitation and the emission is referred to as the Stokes’ shift. As soon as the excitation
stops, the fluorescence emission stops almost immediately. The particular
advantages of fluorescence microscopy include a strong image contrast and its
specificity; that is, its ability to specifically detect individual structures down to
individual molecules. For the purpose of fluorescence excitation, the specimen is
illuminated through the objective using reflected light.
Gas discharge lamps such as HBO, XBO, HXP or long-life LED light sources are
used as light sources. LEDs make it possible to change the excitation wavelengths
extremely quickly. The downside to LED light sources is that they still exhibit
rather low excitation intensity in certain spectral ranges.
