There are natural limits to resolution: spots that are less than about 300 nanometers apart cannot be displayed separately using a conventional light-optical microscope. Electron-beam, scanning-probe and X-ray microscopes deliver better results, but their high-energy radiation and the vacuum generated in the ray path destroy biological substances and are not suitable for examining three-dimensional structures. Stefan Hell has succeeded in increasing the spatial resolution to within 30 nanometers through a skilled combination of methods using light-optical microscopes. "Even back in 1991, when the inventor was still a student, we recognized the tremendous potential of these ideas", states Dr. Helmut Appel of the Fraunhofer Patent Center for German Research PST. The PST supports inventions that originate at universities and research establishments, for example. It has supported the developments related to these microscopes from the very beginning. It not only guided Hell, who today is a professor and directs the Max Planck Institute for Biophysical Chemistry in Göttingen, through the respective patent application procedures and provided the necessary financial backing, but it also brokered the contact with a partner in industry. Leica Microsystems took out an exclusive license for the basic 4Pi technology in 2001. 4Pi light-optical microscopes have been on the market since 2004 and are selling fast. An important advancement of this technology is based on "Stimulated Emission Depletion", or STED for short: STED samples are tagged with special dyes, as in fluorescence microscopy. The spread of the fluorescent light spots is narrowed down by a technical trick: each spot is irradiated by two precisely coordinated light pulses. The first consists of short-wave green light: this excites the pigment molecules that are located precisely in the focus of the light beam. It is followed by a second pulse of long-wave, red light: the excited molecules emit the energy they have just gained in the form of fluorescence, or "shining". The second beam of light is positioned so that it only excites the pigment molecules at the edge of each spot. As a result, the diameter of the spot of light "shrinks", the number of spots that can be juxtaposed without overlapping is increased, and the resolution is improved. In the fall of 2005, Leica Microsystems pledged to put the first microscope with STED technology on the market within three years.
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