Emerson et al. (1957) found that if light of shorter wavelengths was provided at the same time as the longer red wavelengths, photosynthesis was even faster than the sum of the two rates with either colour alone. This synergism or enhancement became known as the Emerson enhancement effect.

The two separate groups of pigments or photosystems cooperate in photosynthesis-long red wavelengths are absorbed only by one photosystem, called photosystem I (PS I) and the second photosystem, photosystem II (PS II), absorbs wavelengths shorter than 690 nm, and for maximum photosynthesis wavelengths absorbed by both systems must function together. The two photosystems normally cooperate to cause photosynthesis at all wavelengths shorter than 690 nm, because both photosystems absorb those wavelengths. The importance of Emerson’s work is that it suggested the presence of two distinct photosystems.