Plant lighting with white LEDs

The intensity of photosynthesis under the red light is maximum, but under the red alone the plants die or their development is disturbed. For example, Korean researchers [1] showed that when illuminated with pure red, the weight of the grown lettuce is greater than when illuminated with a combination of red and blue, but the leaves contain significantly less chlorophyll, polyphenols and antioxidants. A biofacter of Moscow State University [2] found that in the leaves of Chinese cabbage under narrow-band red and blue light (compared to the sodium lamp illumination), the synthesis of sugars is reduced, growth is inhibited and no flowering occurs.





Fig. 1 Leanna Garfield, Tech Insider - Aerofarms



What kind of lighting is needed to get a fully developed, large, fragrant and tasty plant with moderate energy consumption?



How to evaluate the energy efficiency of the lamp?



The main metrics for assessing the phytosvet energy efficiency are:

• Photosynthetic Photon Flux ( PPF ), in micromoles per joule, i.e. among the quanta of light in the 400–700 nm range emitted by a lamp that consumed 1 J of electric power.
  
  
• Yield Photon Flux ( YPF ), in effective micromoles per Joule, that is, in the number of quanta per 1 J of electric power, taking into account the multiplier, the McCree curve.
  
  

PPF always turns out to be slightly higher than YPF (the McCree curve is normalized to one and in the greater part of the range is less than one), therefore, it is advantageous to use the first metric for luminaires sellers. It is more profitable to use the second metric to customers, as it more adequately assesses energy efficiency.



Efficacy of HPS



Large agricultural enterprises with vast experience, counting money, still use sodium lamps. Yes, they willingly agree to hang over the ledges provided by them LED lamps, but do not agree to pay for them.



From fig. 2 shows that the efficiency of the sodium lamp is highly dependent on power and reaches a maximum at 600 watts. The characteristic optimistic YPF value for sodium lamp 600–1000 W is 1.5 eff. µmol / j. Sodium lamps 70–150 W have one and a half times less efficiency.









Fig. 2. Typical spectrum of sodium lamp for plants (left) . Efficiency in lumens per watt and in effective micromoles of serial sodium lamps for greenhouses of the Cavita , E-Papillon , Galad and Reflax types (right)



Any LED lamp having an efficiency of 1.5 eff. µmol / W and a reasonable price, can be considered a worthy replacement for the sodium lamp.



Doubtful efficacy of red-and-blue phyto lighting



This article does not give the absorption spectra of chlorophyll because it is incorrect to refer to them in the discussion of the use of light flux by a living plant. The chlorophyll invitro , isolated and purified, really absorbs only red and blue light. In a living cell, pigments absorb light in the entire range of 400–700 nm and transfer its energy to chlorophyll. The energy efficiency of light in the sheet is determined by the “ McCree 1972 ” curve (Fig. 3).









Fig. 3. V (λ) is the curve of visibility for a person; RQE — relative quantum efficiency for a plant ( McCree 1972); σ _r and σ _fr are the absorption curves of red and far red light by phytochrome; B (λ) - phototropic efficiency of blue light [3]



Note: the maximum efficiency in the red range is one and a half times higher than the minimum - in green. And if you average the efficiency over any wide band, the difference will become even less noticeable. In practice, the redistribution of part of the energy from the red range to the green energy function of light sometimes, on the contrary, enhances. Green light passes through the thickness of the leaves to the lower tiers, the effective leaf area of ​​the plant increases dramatically, and the yield, for example, of lettuce rises [2].



Plant lighting with white LEDs



The energy feasibility of plant illumination with common white-light LED lamps was investigated in [3].



The characteristic shape of the white LED spectrum is determined by:

• the balance of short and long waves, correlated with color temperature (Fig. 4, left);
• the degree of fullness of the spectrum that correlates with color reproduction (Fig. 4, right).

Fig. 4. Spectra of white LED light with one color rendition, but different color temperature CCT (left) and with one color temperature and different color rendition R _a (right)



Differences in the spectrum of white diodes with one color and one color temperature are barely perceptible. Therefore, we can estimate the spectro-dependent parameters only by color temperature, color rendition and luminous efficiency - the parameters that are written in the ordinary white light on the label.



The results of the analysis of the spectra of serial white LEDs are as follows:



1. In the spectrum of all white LEDs, even with a low color temperature and with maximum color rendition, as with sodium lamps, there is very little far red (Fig. 5).









Fig. 5. Spectrum of white LED ( LED 4000 K R _a = 90) and sodium light ( HPS ) in comparison with the spectral functions of plant susceptibility to blue ( B ), red ( A_r ) and high red light ( A_fr )



Under natural conditions, the plant shaded by the canopy of alien foliage receives more far red than the neighbor, which in light-loving plants triggers the “shadow avoidance syndrome” - the plant stretches upwards. Tomatoes, for example, at the stage of growth (not seedlings!) Far red is necessary to stretch out, increase growth and the total occupied area, and hence the harvest in the future.



Accordingly, under white LEDs and under sodium light, the plant feels like under the open sun and does not stretch upwards.



2. Blue light is needed for the “sun tracking” reaction (Fig. 6).





Fig. 6. Phototropism - turning the leaves and flowers, pulling the stems to the blue component of white light (illustration from Wikipedia)



In one watt the flow of white LED light to the 2700 K phytoactive blue component is twice as large as in one watt of sodium light. Moreover, the proportion of phytoactive blue in white light increases in proportion to the color temperature. If it is necessary, for example, to turn ornamental flowers in the direction of people, they should be illuminated from this side with an intense cold light, and the plants will unfold.



3. The energy value of light is determined by the color temperature and color rendition and with an accuracy of 5% can be determined by the formula:

$$

Where $$ - light return in lm / W, $$ - the general index of a color rendition, $$ - correlated color temperature in degrees Kelvin.



Examples of using this formula:



A. Let us estimate for the basic values ​​of the parameters of white light what the illumination should be so that, for a given color rendition and color temperature, to provide, for example, 300 eff. μmol / s / m2:









It is seen that the use of warm white light of high color rendering allows the use of slightly lower light levels. But if we consider that the light output of warm-light LEDs with high color rendering is slightly lower, it becomes clear that the choice of color temperature and color rendering cannot be energetically meaningful to win or lose. You can only adjust the proportion of phytoactive blue or red light.



B. Let us evaluate the applicability of a typical general-purpose LED luminaire for growing microgreen.



Let a lamp with a size of 0.6 × 0.6 m consume 35 W, a color temperature of 4000 K , a color rendering of Ra = 80 and a light return of 120 lm / W. Then its efficiency will be YPF = (120/100) (1.15 + (35⋅80 - 2360) / 4000) eff. µmol / j = 1.5 eff. µmol / j. That, when multiplied by the consumed 35 watts will be 52.5 eff. µmol / s



If such a lamp is lowered low enough above the bed of a microgreen with an area of ​​0.6 × 0.6 m = 0.36 m ² and thereby avoiding the loss of light to the sides, the illumination density will be 52.5 eff. μmol / s / 0.36m ² = 145 eff. µmol / s / m ² . This is about half the usual recommended values. Therefore, the power of the lamp must also be doubled.



Direct comparison of phytoparameters of lamps of different types



Let us compare the phytoparameters of the usual office ceiling LED luminaire, produced in 2016, with specialized phyto-lamps (Fig. 7).









Fig. 7. Comparative parameters of a typical 600W sodium lamp for greenhouses, a specialized LED plant lighting and a lamp for general indoor lighting



It can be seen that an ordinary general-lighting luminaire with a diffuser removed when illuminating plants is not inferior in energy efficiency to a specialized sodium lamp. It is also seen that the phyto-illuminator of red-blue light (the manufacturer is not intentionally named) is made at a lower technological level, since its total efficiency (the ratio of the luminous flux in watts to the power consumed from the network) is inferior to the efficiency of an office lamp. But if the efficiency of red-blue and white lamps were the same, then the phytoparameters would also be about the same!



Also from the spectra can be seen that the red-blue phyto-lamp is not narrow-band, its red hump is wide and contains much more far red than the white LED and sodium light. In cases where far red is necessary, the use of such a luminaire as a sole or in combination with other options may be appropriate.



Evaluation of the energy efficiency of the lighting system as a whole:



The author uses the UPRtek 350N hand-held spectrometer (Fig. 8) provided by Intech Engineering.





Fig. 8. Audit of phyto-lighting system



The following model UPRtek - spectrometer PG100N, according to the manufacturer, measures micromoles per square meter, and, more importantly, the luminous flux in watts per square meter.



Measuring the luminous flux in watts is an excellent feature! If you multiply the illuminated area by the density of the light flux in watts and compare with the consumption of the lamp, the energy efficiency of the lighting system will become clear. And this is currently the only indisputable criterion of efficiency, in practice, for different lighting systems, it differs by an order of magnitude (and not by several times, or even by percents, as the energy effect changes when the shape of the spectrum changes).



White Light Examples



Examples of illumination of hydroponic farms with red-blue and white light are described (Fig. 9).









Fig. 9. From left to right and top to bottom farms: Fujitsu , Sharp , Toshiba , a farm for growing medicinal plants in Southern California



The system of farms Aerofarms (Fig. 1, 10), the largest of which is built near New York, is quite well known. Under the white LED lamps in Aerofarms grow more than 250 types of greenery, shooting over twenty harvests per year.









Fig. 10. Farm Aerofarms in New Jersey ("State of the Gardens") on the border with New York



Direct experiments comparing white and red-blue LED lighting

There are very few published results of direct experiments comparing plants grown under white and red-blue LEDs. For example, a glimpse of such a result showed the Moscow Agricultural Academy. Timiryazev (Fig. 11).









Fig. 11. In each pair, the plant on the left is grown under white LEDs, on the right - under red and blue (from the presentation by I. G. Tarakanova, Department of Plant Physiology, Moscow Agricultural Academy named after Timiryazev)



Beijing University of Aviation and Cosmonautics in 2014 published the results of a large study of wheat grown under LEDs of various types [4]. Chinese researchers have concluded that it is advisable to use a mixture of white and red light. But if you look at the digital data from the article (Fig. 12), you notice that the difference of parameters with different types of lighting is not at all radical.









Figure 12. The values ​​of the studied factors in the two phases of wheat growth under red, red-blue, red-white and white LEDs



However, the main focus of research today is to correct the shortcomings of narrow-band red-blue lighting by adding white light. For example, Japanese researchers [5, 6] found an increase in the mass and nutritional value of lettuce and tomatoes when white is added to red light. In practice, this means that if the aesthetic appeal of the plant during growth is unimportant, it is not necessary to discard already purchased narrow-band red-blue lamps, white light lamps can be used additionally.



The effect of light quality on the result



The fundamental law of ecology “Liebig's barrel” (Fig. 13) states: development limits the factor that deviates from the norm more than others. For example, if water, minerals and CO _{2 are} fully provided, but the illumination intensity is 30% of the optimal value, the plant will give no more than 30% of the maximum possible yield.





Fig. 13. Illustration of the limiting factor principle on YouTube



The reaction of the plant to light: the intensity of gas exchange, the consumption of nutrients from the solution and synthesis processes - is determined by laboratory. The responses characterize not only photosynthesis, but also the processes of growth, flowering, synthesis of substances necessary for taste and aroma.



In fig. 14 shows the response of a plant to a change in the wavelength of light. Measured the intensity of consumption of sodium and phosphorus from the nutrient solution of mint, strawberries and lettuce. Peaks on such graphs are signs of stimulation of a specific chemical reaction. The graphs show that to exclude any ranges from the full spectrum for the sake of saving - it's like removing a part of the piano keys and playing the melody on the remaining ones.









Fig. 14. The stimulating role of light for the consumption of nitrogen and phosphorus by mint, strawberries and lettuce (data provided by Fitex)



The principle of limiting factor can be extended to individual spectral components - for a full result, in any case, a full spectrum is needed. Taking a certain range out of the full spectrum does not lead to a significant increase in energy efficiency, but a “Liebig barrel” may work - and the result will be negative.

The examples demonstrate that ordinary white LED light and specialized “red-blue phytosvet”, when illuminated by plants, have approximately the same energy efficiency. But broadband white complexly satisfies the needs of the plant, which are expressed not only in the stimulation of photosynthesis.



Removing green from a continuous spectrum so that the light from white turns into purple is a marketing move for buyers who want a “special solution” but do not act as qualified customers.



White light adjustment



The most common white general-purpose LEDs have a low color rendering of Ra = 80, which is caused primarily by the lack of red color (Fig. 4).



The lack of red in the spectrum can be replenished by adding red LEDs to the lamp. Such a solution promotes, for example , CREE . The logic of "Liebig's barrel" suggests that such an additive would not hurt if it is really an additive, and not a redistribution of energy from other ranges in favor of red.



An interesting and important work was done by the IMBP RAS in 2013–2016 [7, 8, 9]: they investigated how the light of white 4K LEDs 660 nm to the light of white LEDs 4000 K / Ra = 70 affects the development of Chinese cabbage.



And found out the following:

• Under LED light cabbage grows about the same as under sodium, but it has more chlorophyll (the leaves are greener).
• The dry weight of the crop is almost proportional to the total amount of light in moles produced by the plant. More light - more cabbage.
• The concentration of vitamin C in cabbage slightly increases with increasing illumination, but significantly increases with the addition of red light to the white light.
• A significant increase in the proportion of the red component in the spectrum significantly increased the concentration of nitrates in biomass. It was necessary to optimize the nutrient solution and introduce some of the nitrogen in the ammonium form, so as not to go beyond the MPC for nitrates. But in pure white light it was possible to work only with the nitrate form.
• At the same time, an increase in the share of red in the total light flux has almost no effect on the yield mass. That is, the completion of the missing spectral component does not affect the quantity of the crop, but its quality.
• Higher efficiency in moles per watt of a red LED leads to the fact that adding red to white is also more energy efficient.

Thus, adding red to white is advisable in the particular case of Chinese cabbage and is quite possible in the general case. Of course, with biochemical control and proper selection of fertilizers for a particular crop.



Options for enriching the spectrum with red light



The plant does not know where the quantum came from it from the spectrum of white light, and where it came from - the "red" quantum. There is no need to make a special spectrum in one LED. And there is no need to shine with red and white light from one of a special phytolamp. It is enough to use white light for general use and an additional red light to illuminate the plant additionally. And when there is a person next to the plant, the red lamp can be turned off by the motion sensor to make the plant look green and pretty.



But the reverse decision is also justified - by picking up the phosphor composition, expand the white LED's emission spectrum towards the long waves, balancing it so that the light remains white. And get the white light extravagant color, suitable for both plants and humans.



Open questions



You can identify the role of the ratio of far and near red light and the feasibility of using the "shadow avoidance syndrome" for different cultures. It is possible to argue on which areas during the analysis it is advisable to break the wavelength scale.



One can discuss whether a plant is needed for stimulation or a regulatory function of wavelengths shorter than 400 nm or longer than 700 nm. For example, there is a private message that ultraviolet significantly affects the consumer qualities of plants. Among other things, red-leaved varieties of lettuce are grown without ultraviolet, and they grow green, but they are irradiated with ultraviolet before selling, they turn red and go to the counter. And is the new metric PBAR ( plant biologically active radiation ), described in ANSI / ASABE S640 , Quantities and Units of Electromagnetic Radiation for Plants (Photosynthetic Organisms , correctly prescribes to take into account the range of 280-800nm)?



Conclusion



Chain stores choose more old-fashioned varieties, and then the buyer votes with a ruble for brighter fruits. And almost no one chooses the taste and aroma. But as soon as we become richer and begin to demand more, science will instantly give the necessary varieties and recipes of the nutrient solution.



And in order for the plant to synthesize everything that is needed for taste and aroma, lighting with a spectrum containing all the wavelengths to which the plant will react, that is, in general, a continuous spectrum, will be required. Perhaps the basic solution will be white light with high color rendering.



Thanks

The author expresses sincere gratitude for the help in the preparation of the article to the employee of the State Research Center of the Russian Federation-IMBP RAS n Irina Konovalova; Fitex project manager Tatiana Trishina; CREE specialist Mikhail Chervinsky

Note1: This post is a translation of the White LED Lighting for Plants article.



Note2 : The following cycle article: Estimate PPFD when the plant is illuminated with white LEDs simply: 1000 lux = 15 µmol / s / m2