Phosphor-converted white LEDs (PCW-LEDs) of numerous types with different relative spectral photon-flux-density distributions (SPDs) are commercially available today. Some are regarded as promising light sources for use in plant factories with artificial lighting. The leaf net photosynthetic rate (P n ) measured under PCW-LED light is an important criterion for evaluating PCW-LEDs in terms of photosynthesis performance. To ascertain P n rapidly under dozens of PCW-LED lights having different SPDs, we have developed a rapid and semi-automated P n -quantification method. The method uses a modified LED-artificial sunlight source system (LASS system) and a P n -measurement system. The modified LASS system can produce light with an SPD, which can accurately approximate that of any PCW-LED light at a photosynthetic photon flux density (PPFD) of 150 μmol m -2 s -1 . First, PCW-LED lights of 30 types at a PPFD of 150 μmol m -2 s -1 were produced using the modified LASS system within 2.5 h. We then measured the P n of cos lettuce, red-leaf lettuce, and green-leaf lettuce (Lactuca sativa L.) plants. In a 16-h P n measurement repetition, the modified LASS system supplied all the produced lights automatically and successively to an identical leaf of a lettuce plant. A P n -measurement system simultaneously measured P n under the produced light. Results show that the mean P n values of the cos lettuce, red-leaf lettuce, and green-leaf lettuce under the 30 produced lights at 20 days after sowing were, respectively, 7.11 -8.02, 5.76 -7.11, and 4.83 -6.17 μmol m -2 s -1 . A rapid and semi-automated method was developed for successive measurement of P n under dozens of combined lights, of which each SPD approximated that of the selected PCW-LED lights, within days, which indicates that the method can determine the P n quickly under numerous PCW-LED lights. Results show that the system contributes to rapid selection of PCW-LED lights performing high P n .