Lettuce is a relatively fast-growing vegetable that is divided into four main types: crisphead (or iceberg), romaine, loose-leaf and butterhead . Loose-leaf lettuce has the shortest growth cycle of the varieties, while romaine, butterhead and especially crisphead requires more time until they are ready to harvest . Although there are some variations between the types, lettuce is generally a cool-weather crop which requires specific temperature conditions with moderate daytime temperatures and cool nights to prevent bolting  .
Lettuce thrives under moderate light conditions , with light intensities that varies depending on cultivar and photo period . The spectral composition has a significant impact on the development of lettuce, and a well-adapted spectrum is therefore of great importance for the quality of the final harvest. A full, white spectrum increases harvest compared to narrowband RB spectra, both in terms of leaf area, fresh weight, and dry weight . The different growth stages of the lettuce may benefit from temporal spectrum differentiation, and studies have shown that the initial light treatment influences final fresh weight of the plant significantly . Therefore, a full spectrum with an increased proportion of blue light that promotes compact plants, development of vigor roots and growth after transplantation is beneficial during the propagation phase   . In the following production phase, a spectrum with higher red light content can favourably be introduced since red light in higher degree stimulates biomass accumulation and extension growth .
This kind of spectrum differentiation is only possible with LED lighting and not with fixed spectrum HPS lighting. In addition to customized spectra, replacing HPS with LEDs have other advantages. A study published in 2021 compared HPS and LED lighting for the cultivation of lettuce and concluded that a switch to LEDs can result in reduced water consumption by 15.27% and up to 76% energy savings for lighting, without compromising the yield . Even when total energy consumption was calculated and additional heat for the LED system to compensate for the loss of radiant heat from the HPS lamps was included, the energy savings for the LED system amounted to 10-25% .