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. Over the whole growth period, the xenon-metal halide lamps provided an average instantaneous PPF of 30 μmol m−2 s−1 continuously for 24 hours each day. The resulting total moles of photons received by the HYSAL treatment for 30 days were 199 moles/m2, being 60.6 % solar and 39.4 % artificial. The HPS reference was made to receive equal daily moles of photons as the HYSAL treatment throughout the growth period, resulting in both HPS reference and HYSAL treatment having the same total number of moles (199 moles/m2) at the end of the growth period. Over the entire growth period, the HPS reference had an average instantaneous PPF of 194 μmol m−2 s−1 and an average daily photoperiod of 9.5 hours. The resulting average total dry weight per plant for the HYSAL treatment of 1.37 ± 0.38 g exceeded significantly by 76% (α = 0.05) that for the HPS reference of only 0.78 ± 0.17 g. This significant discrepancy could be explained physiologically by the HPS reference having both significantly longer dark period and higher light compensation point (LCP) than the HYSAL treatment. Further experiment showed that it was indeed the composite lighting profile of the HYSAL treatment, not the light-quality factor, that effected the biomass discrepancy.