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The Hybrid Solar and Artificial Lighting (HYSAL) system used in this study consisted of a mirror-based Optical Waveguide (OW) Solar Lighting System as the solar component and four 60-W xenon-metal halide illuminators as the artificial-light component. A reference (or control) system consisted of a conventional 250-W high pressure sodium (HPS) lamp. Solar irradiance was harnessed whenever available for the HYSAL treatment. During the course of the 30-day growth period for lettuce (Lactuca sativa), the HYSAL's solar PPF varied with the natural fluctuations of terrestrial solar irradiance, which changed dramatically within each day and between days. When averaged over the entire growth period, the average instantaneous solar PPF for the HYSAL treatment turned out to be 322 μmol m−2 s−1 for an average daily photoperiod of only 3.86 hours owing to numerous cloudy days.