Simply viewing an installed lighting system for a greenhouse or vertical farm will not indicate whether the lighting uniformity is acceptable , and measuring it by hand is a time-consuming and labourious process . Calculating uniformity during the design is the best way to go .
Written in 2015 , “ Greenhouse Design and Control ” ( Ponce et al . 2015 ) is extraordinarily comprehensive in its coverage of greenhouse design issues , from site selection through structural load-bearing analysis and ventilation technologies to greenhouse automation using adaptive neural fuzzy inference systems . On the topic of greenhouse lighting , however , it has only this to say : “ The light level in the greenhouse should be adequate and uniform for crop growth .” The authors note that quantum ( PAR ) sensors are commonly used for photosynthetic photon flux density ( PPFD ) measurements , but give no guidance on how to measure the lighting uniformity in a greenhouse . This topic has been occasionally discussed in the literature ( Both et al . 2002 , Ciolkosz et al . 2001 , Eaton 2021 , Ferentinos et al . 2005 , Runkle 2017 ), but only in the context of supplemental lighting using lighting design software intended for architectural applications . The combination of ever-changing daylight and electric lighting has yet to be considered . None of these discussions , however , address the central question : what is “ lighting uniformity ” in horticulture , and how do we measure ( or predict ) it ?
Architectural Lighting
Lighting uniformity in architecture has been a topic of interest for at least 70 years ( e . g ., IES 1947 ), where the goal is to encompass the range of illuminance values measured at an array of positions on a horizontal plane , all in a single metric . These metrics include :
• Coefficient of Variation ( Armstrong 1990 )
• Entropy-based ( Mahdavi and Pal 1999 )
• Maximum to average
• Maximum to minimum
• Minimum to average ( EN 12464-1 )
• Minimum to maximum
• Statistical ( Mathieu 1989 )
• Uniformity Gradient ( Houser et al . 2011 )
In practise , only the minimum-to-average illuminance metric is recognised as an international standard ( EN 12464-1 ) for interior lighting design . This is sometimes designated as “ U1 ,” while the minimum-to-maximum illuminance metric is designated as “ U2 .” To be honest , however , uniformity metrics represent a very simplistic approach to architectural lighting design . They make sense when illuminating large flat surfaces outdoors , such as sports fields , parking lots , and roadway interchanges , but for most interior lighting design applications , they provide little useful information .
Horticultural Lighting
Both the U1 and U2 metrics have been proposed for use in horticultural lighting design by a few luminaire manufacturers , where photosynthetic photon flux density ( PPFD , measured in µ mol / m2-s ) replaces illuminance ( lux ). This again makes sense because with overhead lighting and daylight , the plant canopy can usually be considered as large flat surface .
The problem , however , is that while we perceive visible light reflected from surfaces , plants utilise it for their photosynthetic needs . What we might consider to be almost imperceptible differences in illuminance may be significant in terms of photosynthetic activity and hence plant growth and yield . Commercial growers have for many decades relied on a rule of thumb that a one percent increase in sustained PPFD ( and hence Daily Light Integral ) results in a one percent increase in plant growth and yield . Marcelis et al . ( 2006 ) quantified this assumption by analysing yield data from some one hundred academic papers and nearly ninety commercial growers , followed by opinion surveys with 18 growers . Their research results are summarised in Table 1 .
Crop Group Crop Yield Difference Soil-grown vegetables Lettuce 0.8 %
Radish 1.0 %
Fruit vegetables Cucumber 0.7 – 1.0 % Tomato 0.7 – 1.0 % Sweet pepper 0.8 – 1.0 %
Cut flowers Rose 0.8 – 1.0 % Chrysanthemum 0.6 %
Bulb flowers Freesia 0.25 – 1.25 % Lily 0.25 – 1.25 %
Flowering pot plants Poinsettia 0.5 – 0.7 % Kalanchoe 0.8 – 1.0 %
Non-flowering pot plants Ficus 0.65 % Dracaena 0.65 %
Table 1 – Crop yield difference for one percent difference in sustained PPFD . Source : Marcelis et al . ( 2006 ).
The study results are , of course , more nuanced than this . For example , the yield difference is greater at lower PPFD levels , higher CO 2 concentrations , higher ambient temperatures , and leaf area index . As a result , growers may choose higher temperatures and change their plant density and cultivar choice during times of greater DLI or increased supplemental light levels . In other words , light levels are only one component of farm management . The key point here is that uniformity matters when it comes to horticultural lighting . For example , the two gray squares shown in Figure 1 may appear to be almost the same shade of gray to us , but for many plants , they represent a 10 percent or so difference in photosynthetic activity and hence plant yield .
Figure 1 – What we see versus what plants respond to .
Maximum Yield 31