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Designing a Grow Light: To Full-Spectrum and Beyond

In this post, Shelley Barak, CTO at iShence Agtech, explains what goes into designing a grow light and where things are heading in the industry.

Light is one of the most important environmental parameters that impacts plant growth and development. It has a wide range of effects on photosynthetic activity and photomorphogenic responses throughout the plant’s life (Pocock, 2015; Naznin et al., 2016; Ouzounis et al., 2016). Light quality, light intensity, and photoperiod - all play a significant role in a successful growth protocol.

Grow lights are high-energy consumers. A staggering 1%-3% of the total energy consumption in the USA is used for plant cultivation. Thus the importance of high efficiency grow lights.

High-Pressure Sodium (HPS) is the mainstay of horticulture lighting today. Although some of these lights (mainly fixtures with double-ended bulbs) have a relatively high efficiency of 1.4-1.7µmol/J, they still suffer from known limitations: their spectrum is not ideal for photosynthesis, and their heat output is extremely high.

In the last decade, with the improvement of LED, grow lights were built with an emphasis on blue and red LED diodes, as the common conception was these were the only wavelength important for plant growth. They offered lower power consumption, but its impact on flowering stage and crop quality were subpar.

Is Red And Blue Light Enough?

While in the recent past, the answer was yes (Diagram 1 from Plant Physiology & Development 2003), recent studies changed the conception (Diagram 2 Plant Physiology & Development 2015)

Graph of red and blue light on plant growth

The reason for this is that beyond the direct effect of photosynthesis, light spectrum and irradiance level also impact plant metabolism with the help of photoreceptors (Diagram 3). The plant receives signals from the light environment through photoreceptors that are responsible for various processes. That includes induction of seed germination, seedling de-etiolation, flowering time (Franklin and Quail, 2010; Strasser et al., 2010), fruit quality (González et al., 2015), root elongation (Salisbury et al., 2007; Costigan et al., 2011) and tolerance to biotic and abiotic stressors (Ballaré et al., 2012).

Different types of photoreceptors
Diagram 3 - Different types of photoreceptors (Möglich 2010)

Recent studies (Massa, Gioia D, et al. 2008; Kim, Hyeon-Hye, et al. 2004) showed that plant growth is better under full-spectrum light in terms of higher quality in biomass and biochemical ingredients. In addition, recent studies on cannabis showed a significant effect of full light spectrum on both cannabinoid content (Gianmaria Magagninia et al. 2018) - with up to 50-200% more CBD/THC/CBG, as well as impacting morphology and yields.

Light Penetration - Color Matters

Photosynthetically active radiation, often abbreviated PAR, is the spectral range of light radiation from 400 to 700 nanometers that photosynthetic organisms can use in the process of photosynthesis. (see diagram 4 below).

PAR wavelength graph
Diagram 4 - PAR wavelength

Light penetration is a key factor. Different colors (red, green, blue) have different leaf penetration capabilities. A study done in 2010 (Brodersen and Vogelmann, 2010) has shown a significant difference in that aspect (see diagram 5 below). The chlorophyll completely absorbs red and blue light at the top layers of the leaf. Green light alone penetrates all through the leaf. Despite being low in the range of 500-600 nm, chlorophyll absorption, is still enough to penetrate the leaves and perform photosynthesis. However, when mixed with red and blue into white light, the green will reach the chlorophyll deeper in the leaf and maximize overall photosynthesis. Far-red penetrates the leaf and is photosynthetically active, but it can lead to cell expansion and stretching in flowering plants that can have an adverse effect.

Light penetration into leaf graph
Diagram 5 - Light penetration into leaf (Brodersen and Vogelmann, 2010) (Bugbee 2019)

Conclusion - What's Next for Grow Lights?

The AgTech industry is slowly catching up with the recently published academic research on grow light. There are several key points which gain faster adoption by manufacturers and growers, mainly:

  • Light spectrum and intensity have proven to be critical factors in plant development.

  • Blue and red lights are not enough for growing plants, as recent studies showed that a combination of green with red and blue would dramatically improve results, even compared to natural sunlight.

  • White LED grow lights provide a full spectrum of light designed to mimic natural light - a well-balanced spectrum of red, blue, and green.

So, is that it for grow lights advancements? Far from it. Shifting to the full spectrum and abandoning the legacy of red/blue is just the first step in the long journey. Once we are ready to utilize the great benefits of the full spectrum, we need to tweak it and use each color/wavelength at the right intensity. The challenge ahead is making subtle changes in the spectrum will help us “hack” the internal mechanisms of the plant.

Full-spectrum is great as a baseline, but different crop types require a different mixture of light depending on their growth stage. On top of that, environmental parameters play a significant role in the plant’s light absorption capabilities.

This complexity pushes the agricultural domain towards the realms of big data, IoT, and machine learning, to reach optimal performance and ensure farms’ profitability in the years to come.

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