Utilizing LEDs in Horticultural Lighting

In the approximate 12,000 years of human agriculture history, we can be sure that it wasn’t long before humans learned that the amount of sunlight plants received had a direct effect on their characteristics and growth.

Humans began applying artificial light to help promote plant growth almost as soon electric light bulbs were invented in the 1860s.1

When there is a shortage of natural sunlight, electrical lighting can fill the gap, for instance, on overcast days in a greenhouse or during the winter. Indoor growing on a large scale can also be enabled by the right light fixtures.

Artificial illumination has opened the opportunity to grow crops under many conditions which were not previously possible by supplying light energy.

Plants and Light

Photosynthesis is the key process which makes earth habitable. Plants (including some bacteria and algae) take in nutrients and water (H2O) from the soil, absorb carbon dioxide (CO2) from the air and then use energy (photons) from sunlight to convert these components into glucose, which is the essential sugar that sustains human and animal life.

Plants release oxygen (O2) back into the air in the process, making it breathable.

Photosynthesis is the process by which plants use sunlight to produce glucose, the essential foundation of Earth’s food chain.

Photosynthesis is the process by which plants use sunlight to produce glucose, the essential foundation of Earth’s food chain. Image Credit: National Geographic)

However, plants respond differently to light energy at various wavelengths, and the visible spectrum of sunlight extends from deep violet at one end to red at the other (wavelengths of approximately 380 nm – 750 nm).

Scientists have known that photosynthesis is optimized by red wavelengths for decades, especially around 660 nm. (Plants appear green to our eyes because they are best at absorbing red light to utilize for photosynthesis, while reflecting mostly green light.)

For root strength during germination, among other things, plants also need light waves in the blue portion of the spectrum. Far red, a barely visible range beyond red, promotes larger branching, leaves and flowering.2

Development of Horticultural Lighting

Alongside the development of different lighting technologies, research into agricultural light applications and plant response has continued.

From carbon-arc and incandescent light fixtures in the early 1900s, through to the development of mercury and fluorescent lamps in the 1930s, to today’s newest color-tunable LEDs, growers, researchers and farmers alike have sought artificial sources which provided “sunlight-like” light, while also being energy efficient and affordable.

A handful of technologies have become commonplace in horticultural lighting applications over the last few decades. Known as High Intensity Discharge (HID) lights, these fixtures provide higher lumen-per-watt intensity than earlier technologies.

Yet, they also generate significant radiant heat, which does not contribute to photosynthesis, so they lose some efficiency.

In addition, they are also not spectrally optimized for plant growth but are good enough for numerous general applications. HID lights include metal halide, mercury vapor, ceramic metal halide, conversion bulbs and high pressure sodium.

Types of Horticultural Lighting Applications

The specifics of each horticultural lighting application present a large range of needs, just as the water, nutrient and light requirements of various types of plants can differ. In broad terms, they can be categorized into supplemental lighting, photoperiodic lighting and sole-source lighting situations.3

Supplemental Lighting

The delivery of moderate-intensity lighting on cloudy days and at night to enhance plant growth and quality is known as supplemental lighting. For instance, this method is utilized for crops which need a lot of light, like tomatoes, grown in greenhouses during the dark days of winter.

Currently, the most common source in these settings are high-pressure sodium lights.

Photoperiodic Lighting

A number of ornamental crops (like those utilized in landscaping), regulate their flowering in response to cycles of daylight and nighttime.

Photoperiodic lighting provides low-intensity light to mimic light and dark cycles to encourage these plants to produce blooms when desired. It can encourage plant height, but does not promote photosynthesis, and so, does not increase plant growth.

Sole-Source Lighting

It is becoming more common to grow some crops in entirely indoor settings instead of greenhouses.

For instance, vertical farms are becoming more popular for high-density agricultural operations. Indoor growers are increasingly turning to LEDs, as artificial light is the only energy source that now provides a number of advantages for horticultural applications.

Major lighting applications and characteristics used in the production of horticultural crops: photoperiodic lighting, supplemental lighting, and sole-source lighting.

Major lighting applications and characteristics used in the production of horticultural crops: photoperiodic lighting, supplemental lighting, and sole-source lighting. Image Credit: Source

LED Advancements Provide New Horticulture Lighting Capabilities

The earliest LEDs, first invented in 1962, provided relatively low brightness and only came in red. Developers have been able to create LEDs in a rainbow of colors since then, including high-intensity white LEDs, by utilizing a range of methods and materials.

These newer LED products provide better efficiency and performance and can have a large variety of spectral properties. They allow lighting schemes which are precisely tuned to different color spectra and that generate specific lighting attributes to enhance plant development.

LEDs have started to become the horticultural lighting solution of choice for a growing number of applications due to their additional benefits of low thermal radiation, low power consumption and durability.

LEDs have created a new technology platform for researchers and designers, leading to an expanding range of choices for growers.

LED technology is driving vital conversations around spectrum, energy efficiency and advanced control systems; features which can improve crop performance and business.4 In fact, shipments of LED lights to growers around the globe are expected to increase at an average annual rate of 32% until 2027.5

The emerging practice of vertical farming stacks layers of plant trays indoors with LED lighting optimized to promote growth cycles. The technique maximizes crop yield for the available footprint in locations where land acreage is scarce or expensive, such as urban areas.

The emerging practice of vertical farming stacks layers of plant trays indoors with LED lighting optimized to promote growth cycles. The technique maximizes crop yield for the available footprint in locations where land acreage is scarce or expensive, such as urban areas. Image Credit: Radiant Vision Systems

The ability to precisely control the hue and output is one of the exciting aspects of LEDs to horticulturalists. This is the correlated color temperature (CCT) and spectral power distribution (SPD) of LEDs.

A new agricultural discipline of light “recipes” has been discovered. With advances in LED technology, light recipes, establishing the number of hours illuminated, the intensity of photons directed at plants and the mix of colors can be finely tuned to each crop and even to each stage in a crop’s life.6

Horticulture Light Recipes

Two key elements are optimized by plant lighting, photosynthesis rates and growth morphology (which is the growth of the structures of the plant like roots, stems, leaves and fruit). Both of these elements are dependent on the amount of incident photons and the wavelengths of light which are absorbed by plant structures.

Human-oriented lighting sources are usually characterized by metrics like illuminance, lumens and luminous flux as these terms help quantify how the human eye perceives light emitted by a source.

A different set of metrics is employed to characterize horticultural lighting - how plants “perceive” and respond to light:

  • Photosynthetic Photon Flux (PPF) – Expressed in micromoles per second (μmol/s), a measurement of the photosynthetically active photons emitted by a lighting system in the PAR wavelength region per second. PPF is the plant equivalent of lumens (unit of luminous flux). 

The photosynthetically active region (PAR) of light is 400 nm – 700 nm. The PAR encompasses light waves that plants can use for photosynthesis.

The photosynthetically active region (PAR) of light is 400 nm – 700 nm. The PAR encompasses light waves that plants can use for photosynthesis. Image Credit: BIOS Lighting 

  • Photon Efficacy – The efficiency of a horticultural lighting system is its photon efficacy, also known as PPF per Watt (PPF/W). Electrical lighting systems work by converting energy (electricity) into photons (light). It is expressed in micromoles per Joule (μmol/J). 
  • Photosynthetically Active Radiation (PAR) – The wavelength range from 400 nm – 700 nm is known as the PAR region. This is where photosynthesis happens due to photon absorption. This is roughly equivalent to the range of light visible to the human eye, from around 380 nm to 750 nm. 
  • Day Light Integral (DLI) – DLI is expressed in mol/m2/day and is a cumulative measure, counting the total number of photons (in the PAR) that reach a given surface during one day’s photoperiod. Establishing the natural DLI of a greenhouse allows growers to calculate the amount of supplemental lighting required for their desired crop yield. 
  • Photosynthetic Photon Flux Density (PPFD) – The amount of photosynthetically active photons which fall from a light source onto a given surface each second is the PPFD of that light. PPFD is the plant equivalent of lux (unit of illuminance) and is expressed in micromoles per square meter per second (μmol/m2/s). 

During the four stages of plant growth: germination, vegetation, flowering and fruiting, lighting recipes can be employed to optimize activity. Different light wavelengths are absorbed by photopigments which promote and control growth. 

Different light recipes (wavelength and intensity combinations) can be employed to:

  • Achieve more biomass
  • Achieve optimal plant size
  • Enhance the taste or color of produce
  • Shorten the growth cycle 

Examples of how different light recipes can be used to alter plant growth and characteristics.

Examples of how different light recipes can be used to alter plant growth and characteristics. Image Credit: Source

Measuring and Characterizing LEDs

Different plants respond differently to various combinations of light, and each plant reacts differently to light at various stages of its growth. 

Custom calculations can be used to establish the correct lighting recipe for various crops to achieve specific outcomes in each unique setting, considering spectral factors, PPFD, beam angles, DLI and more.

Each LED bulb or fixture utilized must be carefully considered for how it fits into the complete agricultural lighting plan. 

Radiant provides a variety of integrated solutions for both R&D and production-line testing of intensity, luminance, illuminance, dominant wavelength and CCT of various lighting sources to help both LED lighting design engineers and manufacturers measure and verify the output of lighting components. 

Compact goniometer systems, which measure light as a factor of angle, pair with application software and imaging technology in R&D environments to provide fast, cost-effective and comprehensive data for 3D light modeling. 

Radiant’s systems allow lighting engineers to optimize their product designs and produce standard output files easily, including IES, EULUMDAT, or Radiant Source Model™ (RSM) formats. 

Their ProMetric® Imaging Colorimeter and Photometer solutions ensure product quality and consistency in real-time on the production line, increasing manufacturing yields and optimizing the efficiency of light measurement processes. 

With the growing adoption of LED lighting, trends for retrofitting agricultural light systems with LEDs, and the global horticultural lighting market predicting a 21.4% CAGR to 2025,7 it’s safe to say that use of LEDs in agriculture applications will continue to “take root.” 

References

  1. Wheeler, R., “A Historical Background of Plant Lighting: An Introduction to the Workshop.” HortScience, Vol 23:7(1942-1943), December 2008. DOI: https://doi.org/10.21273/HORTSCI.43.7.1942
  2. Higgins, A., “Growing the future.” Washington Post, November 6, 2018.
  3. Runkle, E., “Horticultural Lighting Applications.” Greenhouse Product News, June, 2018.
  4. Alex Bodell, as quoted in Konjoian, P. and Bodell, A., “Horticulture’s History with Disruptive Technology; LED Lighting Takes Its Turn.” Greenhouse Product News, June 2019.
  5. Marshall, C. and Maxwell, K., LED Lighting for Horticultural Applications. Research Report by Navigant, Q1 2018
  6. Higgins, A., “Growing the future.” Washington Post, November 6, 2018.
  7. Horticultural Lighting Market…Global Forecast to 2025. Markets and Markets, 2020 

Acknowledgments

Produced from materials originally authored by Anne Corning from Radiant Vision Systems.

This information has been sourced, reviewed and adapted from materials provided by Radiant Vision Systems.

For more information on this source, please visit Radiant Vision Systems.

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