How is quantum yield calculated for a photochemical reaction?

In the context of photochemical reactions, the quantum yield (often denoted by the Greek letters $\gamma$ or $\Phi$) is a quantitative measure of the efficiency of a light-induced process.1

It is calculated as the ratio of the number of moles of a specific product formed to the number of einsteins of light absorbed. Alternatively, it can be expressed as the number of molecules of product formed per photon absorbed.1

Mathematically, this relationship is represented as:

$$\Phi =\frac{\text{moles of product formed}}{\text{einsteins of light absorbed}}$$

In an ideal scenario, such as those described by the photochemical equivalence law, the quantum yield is unity. However, in practice, $\Phi$ can vary significantly:21
* It may approach zero if a back reaction occurs.1
* It can be as high as 1,000,000 if the primary product initiates a chain reaction.1

When a reaction produces multiple primary products, distinct quantum yields exist for each, and these ratios may change depending on the frequency of the absorbed light.1