## 3 Results and Discussion
### Scenario  
The nature of light interception on the canopy surface was obviously changed under different light source arrangements and lighting distances (Figure 8). For the low lighting distance, the light interception was focused on the leaves directly under the light source and was near zero on the other leaves. Meanwhile, for lighting distances over 30 cm, the light interceptions were less influenced by the light source arrangement and were overall uniformly distributed on the canopy, but some decrements in uniformity were again observed at lighting distances over 35 cm, which resulted in the lowest CV\({}_{\text{LI}}\) at 30 cm (Figure 8). The effect of reflective material on canopy light interception also varied between different lighting distances and arrangements (Figure 9). On BP, the increment in light interception showed U-shaped patterns according to lighting distance, which was largest at 20 cm, with 3.13 umol m\({}^{-2}\) s\({}^{-1}\), and lowest at 30 cm, with 2.54 umol m\({}^{-2}\) s\({}^{-1}\). Conversely, the increment tended to be larger at long lighting distances on VAP--largest at 35 cm, with 3.06 umol m\({}^{-2}\) s\({}^{-1}\). On average, the light interception was increased by approximately 3.6% by introducing the reflective material. As a result, the total light interception was larger on VAP than BP and was larger at 30 cm between several lighting distances in most cases (Figure 10A). The estimated \(P_{n}\) showed similar patterns with light interception (Figure 10B), but was reversed in some cases (e.g., VAP-NR) due to low LUE\({}_{\text{I}}\) at short lighting distances (Figure 10C). Light uniformity in PFELs is crucial not only for even growth between plants [31] but also for efficient photosynthesis due to the convex shape of the light-response curve [32]. The light concentration and overlapping on leaves derived from the emitting distribution of light sources was found in this scenario (Figure 8), and the lack of uniformity was connected to low LUE\({}_{\text{I}}\), especially for intact lighting (Figure 10C). The use of reflective material on the floor distinctly improved the light interception, but it is shown not to be the prior factor for designing a light environment because the large amount of increased light by reflection was not mostly connected to the increment in total light interception (Figure 9, Figure 10A). The emitting distribution affected the differences in light interception between VAP and BP, which is related to the plant structure. The lettuce is a rosette-type plant with no petiole, so the angle of the leaf surface is positive, that is, the light incidence angle is near perpendicular for  
Figure 7: Simulation results of vertical light interception at different planting densities and canopy positions (**A**) and the averaged light interception of lettuce plants according to position in the canopy (**B**). The canopy was divided into one plant in the center and eight outer plants. HD and LD indicate high planting density (25 plants m\({}^{-2}\)) and low planting density (16 plants m\({}^{-2}\)), respectively. VAP and near horizontal for BP. On the leaf level, light interception is highly differentiated by incidence angle and is larger for vertically emitted light [15; 18], which results in larger light interception for VAP. In this respect, other types of plants were not simulated, but some plants having negative leaf angles can induce the opposite results under different light source arrangements. Finally, for all scenarios, the PPFD\({}_{\text{opt}}\) for achieving \(\text{LUE}_{\text{E,max}}\) was found (Figure 11). The range of PPFD\({}_{\text{opt}}\) was between 300 and 380 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\) and was mainly distributed at approximately 360 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\) for homogeneously intercepted cases.