What is fluorescence quantum yield?

The term fluorescence quantum yield (Φ) is a measure of the efficiency of photon emission through fluorescence, which is the loss of energy by a substance that has absorbed light via emission of a photon. It is often defined as the ratio of the number of photons emitted to the number of photons absorbed. In other words the fluorescence quantum yield gives the probability of the excited state being deactivated by fluorescence rather than by another, non-radiative mechanism such as internal conversion or vibrational relaxation (non-radiative loss of energy as heat to the surroundings).

Fluorescence quantum yield is of major importance because it is a physical characteristic of a substance in a specified condition. Fluorescence quantum yield values range between 0 and 1. The higher the fluorescence quantum yield values the brighter the fluorophore and stronger the fluorescence signal intensity will be. For example, Calbryte™ 520 (20653) has a quantum yield three times greater than that of Fluo-3 (21011) or Fluo-4 (20550). This means that per photon of light it absorbs, Calbryte™ 520 fundamentally emits more fluorescence than either Fluo-3 or Fluo-4 could, resulting in stronger signal intensity.

AAT Bioquest offers FluoroQuest™ Fluorescence Quantum Yield Determination Kit (20) for calculating fluorescence quantum yield. This kit provides all the essential components and is optimized for fluorescence quantum yield determination of biological conjugates such as protein conjugates, peptides, nucleotides and nucleic acids. 

For fluorescence quantum yields of common compounds, use our Quantum Yield Lookup database.