Given this , Runkle ( 2017 ) has recommended that , “ Generally , a 10-20 percent variation in intensity is acceptable .” However , Yelton ( 2017 ) and others have proposed that ± 5 percent is a more appropriate target for PPFD measurements in greenhouses and plant factories . Figure 1 shows a 10 percent difference in plant yield in response to a 10 percent increase in PPFD .
Horticultural Uniformity Metric
This suggests yet another uniformity metric , but one that is designed specifically for horticultural lighting . Suppose we are given a virtual model of a small 20x40-metre greenhouse with PPFD values due to electric lighting calculated at two-foot intervals ( Figure 2 ). There are 2,048 values , with a maximum of 291.3 µ mol / m2-s . The proposed lighting uniformity metric U90 is defined as the percentage of values that are greater than 90 percent of the maximum value , while the proposed lighting uniformity metric U80 is similarly defined as the percentage of values that are greater than 80 percent of the maximum value ( the measurement positions are assumed to be regularly spaced ).
“ Calculating the light uniformity during the design stage will avoid potentially expensive situations once the installation has been completed .”
The first reaction will , of course , be that 44.5 percent usable floor area is terrible in terms of lighting uniformity . This , however , is precisely the point — the lighting uniformity in a small greenhouse with a regular array of identical luminaires will always be terrible . However , the lack of uniformity is due to the area adjacent to the greenhouse walls . Yelton et al . ( 2017 ) noted the uniformity can often be improved by varying the mounting height of the luminaires adjacent to the greenhouse walls , but this may not be an option for practical reasons . Figure 4 shows a rendering of a commercial greenhouse measuring 512x1024 feet , with 7,680 luminaires , while Figure 5 shows its isoPPFD plot . The U90 metric is 90.0 percent , with the non-uniform areas confined to the floor area adjacent to the greenhouse walls .
Figure 4 – 156x312-metre commercial greenhouse .
Figure 2 – PPFD distribution in a 64x128-foot greenhouse .
Unlike the architectural U1 and U2 metrics , these two metrics are particularly useful in that they indicate the relative floor area of the greenhouse that is suitable for growing crops . The U90 metric indicates the percentage of illuminated area that satisfies the ± 5 percent uniformity requirement , while the U80 metric indicates the percentage of illuminated area that satisfies the ± 10 percent uniformity requirement . Even more useful is that these metrics can be shown in an isoPPFD plot ( Figure 3 ).
Canopy Height : 4.5cm | Max PPFD : 301.3 | U90 : 90.0 | U80 : 92.6
Figure 5 – Commercial greenhouse isoPPFD plot .
The U90 uniformity metric is excellent , but it should be noted that the lighting layout for the commercial greenhouse was chosen to ensure uniformity . Without lighting calculations to validate the uniformity , it cannot be assumed that a given design will satisfy the U90 requirements .
Canopy Height : 4.5cm | Max PPFD : 291.3 | U90 : 44.5 | U80 : 60.3
Figure 3 – Small greenhouse isoPPFD plot .
Daylight
We must also remember the Daily Light Integral ( DLI ) inside greenhouses is due mainly to daylight , which is much more uniform . Figure 6 , for example , shows the monthly average DLI for July with the same greenhouse located in Vancouver , Canada , based on Typical Meteorological Year ( TMY3 ) weather records and solar radiation data from the EUMETSAT weather satellite ( Thomas and Ashdown 2022 ).
32 Maximum Yield