Limited light range in LEDs fails to maximise crop growth

LEDs offer great potential for protected cropping but many of the units that are currently available are limited in light range so fail to maximise crop growth and quality, University of Lincoln professor of agri-food technology Simon Pearson told the Organic Open Day.

Pearson: more white-light LEDs coming on the market - image: HW
Pearson: more white-light LEDs coming on the market - image: HW

He said AHDB-funded tests by his department "reviewed a huge number of models at the Lighting Industry Association laboratory, though worryingly a lot of companies wouldn't submit their equipment for testing". Those that were assessed tended to peak in the red and blue parts of the light spectrum, the tests found.

"Plants need light of 400-700nm wavelength, but like a broad diet of light within that and they don't yet get that from LEDs," said Pearson. "Reds are cheaper and most systems look pink. You get pink glasshouses in the Netherlands, but they are difficult to work in because you can't see the plants properly - strawberries and tomatoes look brown. But we are getting more white-light LEDs coming on the market."

He explained: "Light colour has a dramatic effect on quality of the plant, but balance is important. The flavour profiles of herbs come from the plants' protection from ultraviolet light - basil with no UV doesn't taste of basil, though too much UV and you can't eat it. Insect pollinators also rely on UV. You can get UV-A and UV-B chips, though there are human health issues with UV, particularly UV-C."

In general, he added: "LEDs are around 30 per cent more efficient than high-pressure sodium and can run for 25,000 hours. The technology is coming on quickly, bringing a rapid increase in efficiency." He pointed to Haitz's Law, which states that the cost per lumen of LED lighting falls by a factor of 10 every decade. "But buy from a reputable seller. There is a lot of cheap, dodgy equipment out there," he warned, noting that a promising alternative is plasma lighting, which "has incredibly white light" including some ultraviolet.

Pearson said his own university "is investing heavily in agri-food", which also includes development of polytunnel films as well as robotic weeding and harvesting projects.

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