The quantum yield (Φ) of a process in which molecules give up their excitation energy (or "decay") is the fraction of excited molecules that decay via that pathway (Clayton 1971, 1980). Mathematically, the quantum yield of a process, such as photochemistry, is defined as follows:

The value of Φ for a particular process can range from 0 (if that process is never involved in the decay of the excited state) to 1.0 (if that process always deactivates the excited state). The sum of the quantum yields of all possible processes is 1.0.

In functional chloroplasts kept in dim light, the quantum yield of photochemistry is approximately 0.95, the quantum yield of fluorescence is 0.05 or lower, and the quantum yields of other processes are negligible. The vast majority of excited chlorophyll molecules therefore lead to photochemistry.

The quantum yield of formation of the products of photosynthesis, such as O₂, can be measured quite accurately. In this case the quantum yield is substantially lower than the value for photochemistry, because several photochemical events must take place before any O₂ molecules form. For O₂ production the measured maximum quantum yield is approximately 0.1, meaning that 10 quanta are absorbed for each O₂ molecule released. The reciprocal of the quantum yield is called the quantum requirement. The minimum quantum requirement for O₂ evolution is therefore about 10 (see textbook Figure 7.11). Quantitative measurements of the absorption of light and the fate of the energy contained in the light are essential to an understanding of photosynthesis.