Spectrum, Micromole, and everything in between

LED lighting for crops brings benefits similar to those of the commercial industrial market - saving power and operating costs, controlling light colors and photometry. But plants have different lighting needs than humans.

Human-centered parameters such as luminous efficacy (lm/w) or color rendering index (CRI) may not always be good indicators of the effectiveness of lighting for flowers or vegetables.

LED lighting technology is changing indoor farming and supplementary lighting for greenhouses.

There is still much to learn about changing lighting requirements for different plants, type and location of the facility, environmental conditions, growth stages and the desired results.

There is no doubt that as the technology advances, the solutions go to light fixtures with spectrum control. In the RA and PFL series that we offer, for example, you can get light fixtures with separate control in three groups of wavelengths of blue, red and far red to match the type of plant, the stage of growth and the desired results.

Until a few years ago, most studies connected the compatibility of the chlorophyll absorption graph to the peak of the spectrum at the blue (450 nm) and red (600-660 nm) wavelengths. The studies showed no association with chlorophyll absorption in the green wavelengths. The result was a variety of lighting solutions that produced only those wavelengths, with a fragmented spectrum. The theory behind lighting product design was focused on energy efficiency. The common opinion was that if there was no effect along the green wavelength or wavelengths other than blue or red, why should we waste energy producing those wavelengths. In lighting fixtures, monochromatic LEDs were installed that produced only the two desired wavelengths and reached an energy efficient product that is supposed to be the most effective for plants.

Or so they thought.

Over time, further studies have found that the whole spectrum in the PAR range and beyond has an effect on the plant.

A different SPD can affect the quality of the produce, cycle time and morphology of the plant. So today we'll see more and more light fixtures with different colored diodes and additional white LEDs to complete the spectrum, and a more complete sequence of produced wavelengths.

While green leafy vegetables such as basil, cilantro and lettuce still focus on the blue and red wavelengths, in plants where the biomass of flowers is the main thing, like strawberries or cannabis, some add a far red component to the spectrum and focus on the high level of light the plant needs.

Each grower is looking for a proven recipe for growing his target plant, and the ASABE (American Society of Agricultural and Biological Engineers) began work on standardization in 2015. The work focuses mainly on the requirements of PAR (Photosynthetic Active Radiation) - the portion of the spectrum that refers to wavelengths of 400-700 nm.

There are three main issues that need to be addressed when choosing a lighting solution:

• Composition of the spectrum - what wavelengths the lighting fixture is able to provide.
• The spectrum power distribution (SPD) - the percentages of the different wavelengths in the light produced.
• Illumination levels - how many µmols per square meter per second (µmol/m²/sec) each plant needs at various stages of growth.

The hours of lighting required for a plant are based on its circadian cycles, but they are different from those of humans and also vary among plant types.

It seems that the distance to a reliable recipe book that will describe spectral compositions, illumination hours, combinations and precise intensities of illumination for different plants is still great.

These days reaching a winning formula still requires a trial and error process. You can get recommendations from various sources, but the specific adjustments to your facility can only be made at your facility.