| Luminescent and fluorescent molecules both use the same process to emit light: electrons in an excited state emit a photon as they return to their ground state. This light is emitted within defined wavelength ranges depending on the molecular structure and therefore different compounds can be used as markers for different events, processes or molecules. The fundamental difference between luminescence and fluorescence is the way in which the excited state is generated in the first place. Fluorescence occurs when the excited state is caused by external stimulation by light, whereas luminescence is caused by a chemical reaction (either a natural, biological one – bioluminescence, or a purely chemistry based one - chemiluminescence).
Luminescent emissions tend to have varying lifetimes and are often quite faint, but due to the absence of background they have a high signal-to-noise ratio (S/N). Because there is no need for illumination researchers have not to worry about bleaching and phototoxic effects. This makes bioluminescence an ideal tool for long live cell observations, studying photosensitive cells and quantitative analyses. Due to the short half life of most bioluminescence markers (firefly luciferase) the signals are not accumulated in the cell. This enables a high dynamic resolution in gene expression studies.
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