August Köhler – a ZEISS staff member – discovered fluorescence by chance in 1904. He was working on an ultraviolet (UV) microscope when he noticed that a crystal he was examining was emitting light. Köhler initially dismissed it as an irritating side-effect, but he soon recognized its potential. In 1908 he presented the first microscope designed to harness this effect in Vienna. At first, the invention was primarily taken up by botanists and microbiologists. But from the mid-1920s onwards, this type of microscope began to make steady inroads into the medical research arena.
It works on a simple principle: when certain dyes in a sample – known as fluorophores – are excited by high-energy radiation, they begin to emit light themselves. This process is called fluorescence, and the emission from the sample is often a weak green or red light.
The Nobel Prize in Chemistry 2008 marked a further milestone in fluorescence microscopy. Osamu Shimomura, Martin Chalfie and Roger Y. Tsien discovered the green fluorescent protein "GFP" in the luminescent jellyfish Aequorea victoria for the first time. They subsequently attached the GFP gene to other genes which produce proteins. This meant that the fluorescence would indicate when an organism was producing these proteins. As a result, researchers can now use an optical microscope to conduct 'live' observations of protein activity in living cells. To facilitate this process, ZEISS offers laser scanning microscope systems which provide high-resolution and 3D insights without damaging the samples.
Modern methods of fluorescence microscopy such as photoactivated localization microscopy (PALM) can even use fluorescent proteins to circumvent the diffraction limit of optical microscopes first postulated by Ernst Abbe.