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.
Until a few years ago, most agronomic lighting products developed were 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 little to no association with chlorophyll absorption in the green wavelengths. The result was a variety of lighting solutions that produced only those blue and red 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 a broader spectrum, and a more complete sequence of produced wavelengths.
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 an agronomic lighting solution:
- The spectrum power distribution (SPD) – the percentages of energy distribution between the different wavelengths in the light produced.
- Lighting levels – how many µmols per square meter per second (µmol/m2/sec) each plant needs at various stages of growth.
- Photoperiod – Will be decided according to the plant and the effect we are trying to achieve.
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, photoperiod, combinations and precise intensities of illumination for different plants is still great.
