Figure 6 – Monthly average DLI distribution ( Vancouver , Canada . July 1970 ).
Vertical Farms
Where the U80 and U90 metrics become more interesting is in fully enclosed plant factories , particularly stacked trays in vertical farms . Electrical energy represents between 25 and 35 percent of vertical farm operating costs , and so lighting uniformity for trays is a significant issue . Figure 7 shows three trays measuring 64cm wide by 1.5 metres long , and with a height of 28cm . The plant canopy height is 4.5cm . The four rows of luminaires are spaced at 20cm intervals , which appear to provide good uniformity .
Figure 9 – Vertical farm tray PPFD distribution — canopy height 10.5cm .
Figure 10 confirms this observation , where the U90 metric is only 21 percent .
Canopy Height : 10.5cm | Max PPFD : 1973.1 | U90 : 21.4 | U80 : 55.0
Figure 10 – Vertical farm trays isoPPFD plot — plant height 10.5cm .
To be fair , the tray height of 10.5cm is unusually low . It was chosen specifically to highlight the importance of taking the change in canopy height into consideration when designing tray lighting systems . Greater tray heights will result in improved uniformity , but then reducing the number of luminaire rows for example from four to three will significantly reduce the consistent growing area . Regarding tray lighting , there is another issue that uniformity metrics cannot address : spill light . Most horticultural luminaires designed for tray lighting do not include optics to control the beam spread , and so there will be a significant amount of light that ends up on the aisle floor rather than the plant canopy . Properly designed luminaire optics could address this issue to both save electrical energy ( and money ) and improve lighting uniformity .
Figure 7 – Vertical farm tray PPFD distribution — canopy height 4.5cm .
Three trays are modeled because , as we saw at the borders of the greenhouse models , we can expect there to be light drop-off at the tray ends . The isoPPFD plot shown in Figure 8 , however , shows rather poor uniformity — only 40 percent of the canopy is within the desired ± 5 percent uniformity for consistent crop yield .
Canopy Height : 4.5cm | Max PPFD : 1682.8 | U90 : 39.7 | U80 : 57.6
Figure 8 – Vertical farm trays isoPPFD plot — canopy height 4.5cm .
Equally important is that these results are for plants in the seedling stage . As they mature , the canopy height increases to perhaps four inches or so . As they do , the lighting uniformity will decrease . Figure 9 shows the PPFD distribution for a plant height of 10.5cm , where the uniformity is obviously less .
Summary
Simply viewing an installed lighting system for a greenhouse or vertical farm will not indicate whether the lighting uniformity is acceptable while measuring it by hand is a timeconsuming and laborious process that may be complicated by the existing infrastructure . It is much better if the uniformity can be calculated during the design stage . To this end , the U90 and U80 lighting uniformity metrics presented here are specific to horticultural rather than architectural lighting and , along with isoPPFD plots , clearly indicate whether the uniformity is acceptable for consistent crop yield . This report does not consider plant factories with a single layer of plants , such as cannabis production . In this situation , the walls and ceiling should ideally be painted white to maximise reflected light that will improve lighting uniformity . Again , however , calculating the light uniformity during the design stage will avoid potentially expensive situations once the installation has been completed .
Finally , it should go without saying that accurate predictions of lighting uniformity are only possible with careful measurements of the photosynthetic photon intensity ( PPI ) distribution of the luminaires and careful use of lighting design software to model the application .
34 Maximum Yield