Lighting Uniformity in Horticulture

Written in 2015, “Greenhouse Design and Management” (Ponce et al. 2015) is awfully complete in its protection of greenhouse design points, from website choice by means of structural load-bearing evaluation and air flow applied sciences to greenhouse automation utilizing adaptive neural fuzzy inference programs. On the subject of greenhouse lighting, nonetheless, it has solely this to say: “The sunshine stage within the greenhouse needs to be satisfactory and uniform for crop progress.”

The authors be aware that quantum (PAR) sensors are generally used for photosynthetic photon flux density (PPFD) measurements, however give no steerage on the way to measure the lighting uniformity in a greenhouse. This subject has been often mentioned within the literature (Each et al. 2002, Ciolkosz et al. 2001, Eaton 2021, Ferentinos et al. 2005, Runkle 2017), however solely within the context of supplemental lighting utilizing lighting design software program meant for architectural purposes. The mix of ever-changing daylight and electrical lighting has but to be thought of.

None of those discussions, nonetheless, deal with the central query: what’s “lighting uniformity” in horticulture, and the way will we measure (or predict) it?

Uniformity in Architectural Lighting

Lighting uniformity in structure has been a subject of curiosity for at the very least 70 years (e.g., IES 1947), the place the objective is to embody the vary of illuminance values measured at an array of positions on a horizontal airplane, all in a single metric. These metrics embody:

  • Coefficient of Variation (Armstrong 1990)
  • Entropy-based (Mahdavi and Pal 1999)
  • Most to common
  • Most to minimal
  • Minimal to common (EN 12464-1)
  • Minimal to most
  • Statistical (Mathieu 1989)
  • Uniformity Gradient (Houser et al. 2011)

In follow, solely the minimum-to-average illuminance metric is acknowledged as a global commonplace (EN 12464-1) for inside lighting design. That is typically designated as “U1,” whereas the minimum-to-maximum illuminance metric is designated as “U2.”

To be trustworthy, nonetheless, uniformity metrics characterize a really simplistic method to architectural lighting design. They make sense when illuminating massive flat surfaces outside, comparable to sports activities fields, parking tons, and roadway interchanges, however for many inside lighting design purposes, they supply little helpful info.

Uniformity in Horticultural Lighting

Each the U1 and U2 metrics have been proposed to be used in horticultural lighting design by a number of luminaire producers, the place photosynthetic photon flux density (PPFD, measured in μmol/m2-s) replaces illuminance (lux). This once more is sensible as a result of with overhead lighting and daylight, the plant cover can normally be thought of as a big flat floor.

The issue, nonetheless, is that whereas we understand seen gentle mirrored from surfaces, vegetation put it to use for his or her photosynthetic wants. What we would think about to be virtually imperceptible variations in illuminance could also be vital by way of photosynthetic exercise and therefore plant progress and yield.

Business growers have for a lot of many years relied on a rule of thumb {that a} one % enhance in sustained PPFD (and therefore Day by day Gentle Integral) leads to a one % enhance in plant progress and yield. Marcelis et al. (2006) quantified this assumption by analyzing yield knowledge from some 100 educational papers and practically ninety business growers, adopted by opinion surveys with 18 growers. Their analysis outcomes are summarized in Desk 1.

Crop Group Crop Yield Distinction
Soil-Grown Greens Lettuce 0.8%
Radish 1.0%
Fruit Greens Cucumber 0.7 – 1.0%
Tomato 0.7 – 1.0%
Candy Pepper 0.8 – 1.0%
Reduce Flowers Rose 0.8 – 1.0%
Chrysanthemum 0.6%
Bulb Flowers Freesia 0.25 – 1.25%
Lily 0.25 – 1.25%
Flowering Pot Vegetation Poinsettia 0.5 – 0.7%
Kalanchoe 0.8 – 1.0%
Non-Flowering Pot Vegetation Ficus 0.65%
Dracaena 0.65%

Desk 1 – Crop yield distinction for one % distinction in sustained PPFD. Supply: Marcelis et al. (2006).

The examine outcomes are, after all, extra nuanced than this. For instance, the yield distinction is larger at decrease PPFD ranges, larger CO2 concentrations, larger ambient temperatures, and leaf space index. Because of this, growers could select larger temperatures and alter their plant density and cultivar alternative throughout instances of higher DLI or elevated supplemental gentle ranges. In different phrases, gentle ranges are just one element of farm administration.

The important thing level right here is that uniformity issues in terms of horticultural lighting. For instance, the 2 grey squares proven in Determine 1 could look like virtually the identical shade of grey to us, however for a lot of vegetation, they characterize a ten % or so distinction in photosynthetic exercise and therefore plant yield.

illustration of how plants see lightingDetermine 1 – What we see versus what vegetation reply to.

Given this, Runkle (2017) has beneficial that, “Typically, a 10-20 % variation in depth is appropriate.” Nonetheless, Yelton (2017) and others have proposed that ±5 % is a extra applicable goal for PPFD measurements in greenhouses and plant factories. Determine 1 exhibits a ten % distinction in plant yield in response to a ten % enhance in PPFD.


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Horticultural Uniformity Metric

This implies yet one more uniformity metric, however one that’s designed particularly for horticultural lighting. Suppose we’re given a digital mannequin of a small 64×128-foot greenhouse with PPFD values because of electrical lighting calculated at two-foot intervals (Determine 2).

There are 2,048 values, with a most of 291.3 μmol/m2-s. The proposed lighting uniformity metric U90 is outlined as the proportion of values which might be higher than 90 % of the utmost worth, whereas the proposed lighting uniformity metric U80 is equally outlined as the proportion of values which might be higher than 80 % of the utmost worth (the measurement positions are assumed to be frequently spaced).

PPFD distribution in a 64x128-foot greenhouse

Determine 2 – PPFD distribution in a 64×128-foot greenhouse.

In contrast to the architectural U1 and U2 metrics, these two metrics are notably helpful in that they point out the relative ground space of the greenhouse that’s appropriate for rising crops. The U90 metric signifies the proportion of illuminated space that satisfies the 5 % uniformity requirement, whereas the U80 metric signifies the proportion of illuminated space that satisfies the ten % uniformity requirement.

Much more helpful is that these metrics could be proven in an isoPPFD plot (Determine 3).

Small greenhouse isoPPFD plot.Determine 3 – Small greenhouse isoPPFD plot.

The primary response will, after all, be that 44.5 % usable ground space is horrible by way of lighting uniformity. This, nonetheless, is exactly the purpose — the lighting uniformity in a small greenhouse with a daily array of equivalent luminaires will at all times be horrible.

Nonetheless, the shortage of uniformity is because of the space adjoining to the greenhouse partitions. Yelton et al. (2017) famous the uniformity can usually be improved by various the mounting top of the luminaires adjoining to the greenhouse partitions, however this might not be an possibility for sensible causes.

Determine 4 exhibits a rendering of a business greenhouse measuring 512×1024 ft, with 7,680 luminaires, whereas Determine 5 exhibits its isoPPFD plot. The U90 metric is 90.0 %, with the non-uniform areas confined to the ground space adjoining to the greenhouse partitions.

512x1024-foot commercial greenhouse.Determine 4 – 512×1024-foot business greenhouse.

Determine 5 – Business greenhouse isoPPFD plot.

The U90 uniformity metric is great, but it surely needs to be famous that the lighting structure for the business greenhouse was chosen to make sure uniformity. With out lighting calculations to validate the uniformity, it can’t be assumed {that a} given design will fulfill the U90 necessities.

Daylight

We should additionally bear in mind the Day by day Gentle Integral (DLI) inside greenhouses is due primarily to sunlight, which is far more uniform. Determine 6, for instance, exhibits the month-to-month common DLI for July with the identical greenhouse situated in Vancouver, Canada, primarily based on Typical Meteorological Yr (TMY3) climate data and photo voltaic radiation knowledge from the EUMETSAT climate satellite tv for pc (Thomas and Ashdown 2022).

Monthly average DLI distribution Determine 6 – Month-to-month common DLI distribution (Vancouver, Canada. July 1970).

Vertical Farms

The place the U80 and U90 metrics change into extra fascinating is in absolutely enclosed plant factories, notably stacked trays in vertical farms. Electrical vitality represents between 25 and 35 % of vertical farm working prices, and so lighting uniformity for trays is a major situation.

Determine 7 exhibits three trays measuring 2.1 ft broad by 4.9 ft lengthy, and with a top of 11 inches. The plant cover top is 1.8 inches. The 4 rows of luminaires are spaced at 8-inch intervals, which seem to supply good uniformity.

Three trays are modeled as a result of, as we noticed on the borders of the greenhouse fashions, we will anticipate there to be gentle drop-off on the tray ends.

Vertical farm tray PPFD distributionDetermine 7 – Vertical farm tray PPFD distribution — cover top 1.8 inches.

The isoPPFD plot proven in Determine 8, nonetheless, exhibits slightly poor uniformity —solely 40 % of the cover is throughout the desired 5 % uniformity for constant crop yield.

Vertical farm trays isoPPFD plot — canopy height 1.8 inches.Determine 8 – Vertical farm trays isoPPFD plot — cover top 1.8 inches.

Equally vital is that these outcomes are for vegetation within the seedling stage. As they mature, the cover top will increase to maybe 4 inches or so. As they do, the lighting uniformity will lower. Determine 9 exhibits the PPFD distribution for a plant top of 4.2 inches, the place the uniformity is clearly much less.

Vertical farm tray PPFD distribution — canopy height 4.2 inchesDetermine 9 – Vertical farm tray PPFD distribution — cover top 4.2 inches.

Determine 10 confirms this statement, the place the U90 metric is just 21 %.

Vertical farm trays isoPPFD plot — plant height 4.2 inchesDetermine 10 – Vertical farm trays isoPPFD plot — plant top 4.2 inches.

To be truthful, the tray top of 11 inches is unusually low. It was chosen particularly to spotlight the significance of taking the change in cover top into consideration when designing tray lighting programs. Larger tray heights will end in improved uniformity, however then lowering the variety of luminaire rows for instance from 4 to 3 will considerably cut back the constant rising space.

Concerning tray lighting, there’s one other situation that uniformity metrics can’t deal with: spill gentle. Most horticultural luminaires designed for tray lighting don’t embody optics to manage the beam unfold, and so there will probably be a major quantity of sunshine that finally ends up on the aisle ground slightly than the plant cover. Correctly designed luminaire optics might deal with this situation to each save electrical vitality (and cash) and enhance lighting uniformity.

Abstract
Merely viewing an put in lighting system for a greenhouse or vertical farm won’t point out whether or not the lighting uniformity is appropriate, and measuring it by hand is a time-consuming and laborious course of that could be sophisticated by the prevailing infrastructure. It’s a lot better if the uniformity could be calculated in the course of the design stage.

To this finish, the U90 and U80 lighting uniformity metrics introduced listed here are particular to horticultural slightly than architectural lighting and, together with isoPPFD plots, clearly point out whether or not the uniformity is appropriate for constant crop yield.

This report doesn’t think about plant factories with a single layer of vegetation, comparable to hashish manufacturing. On this scenario, the partitions and ceiling ought to ideally be painted white to maximise mirrored gentle that can enhance lighting uniformity. Once more, nonetheless, calculating the sunshine uniformity in the course of the design stage will keep away from doubtlessly costly conditions as soon as the set up has been accomplished.

Lastly, it ought to go with out saying that correct predictions of lighting uniformity are solely attainable with cautious measurements of the photosynthetic photon depth (PPI) distribution of the luminaires and cautious use of lighting design software program to mannequin the applying.

All greenhouse and vertical farm lighting calculations introduced on this report have been generated with SunTracker Applied sciences’ Cerise365+GreenhouseDesigner™ horticultural lighting design software program.