In particle physics, the quantum yield (denoted Φ) of a radiation-induced process is the number of times a specific event occurs per photon absorbed by the system.

$${\displaystyle \Phi (\lambda )={\frac {\text{ number of events }}{\text{ number of photons absorbed }}}}$$

The fluorescence quantum yield (FQY) is defined as the ratio of the number of photons emitted to the number of photons absorbed.

$${\displaystyle \Phi ={\frac {\rm {\#\ photons\ emitted}}{\rm {\#\ photons\ absorbed}}}}$$

Fluorescence quantum yield is measured on a scale from 0 to 1.0, but is often represented as a percentage. A quantum yield of 1.0 (100%) describes a process where each photon absorbed results in a photon emitted. Substances with the largest quantum yields, such as rhodamines, display the brightest emissions; however, compounds with quantum yields of 0.10 are still considered quite fluorescent.

Quantum yield is defined by the fraction of excited state fluorophores that decay through fluorescence:

$${\displaystyle \Phi _{f}={\frac {k_{f}}{k_{f}+\sum k_{\mathrm {nr} }}}}$$

where