At the cost of more than one sad-looking hanging basket, I have come to understand that plants essentially require three core ingredients to flourish: water; good soil; and sunlight. It would seem that the least important of these would be sunlight, requiring minimal intervention from the would-be gardener. However matters change substantially when, for commercial or environmental reasons, the decision has been made to grow plants indoors or underground.
For example, Growing Underground, a London based urban farming tech company, took the decision in 2012 to convert disused tunnels below the Northern Line into a state-of-the-art underground growing centre.
That said, sunlight is in short supply 60 metres below the surface, and so indoor growers have turned to artificial lights to provide the necessary illumination of their plants. This is not as straight forward, however, as screwing in a few bulbs and flicking the switch. Sunlight, essential for photosynthesis and a host of other biological processes, is a complex mixture of different frequencies. From ultraviolet all the way down to infrared and beyond, sunlight is formed of 5 main region: Ultraviolet C, Ultraviolet B, Ultraviolet A, Visible light, and Infrared. Each of these regions spans at least a hundred nanometres in wavelength, and are not present in equal measure.
Looking then at the artificial options, for a long time the go-to light source was the incandescent bulb. A familiar fixture in homes and businesses, a metal wire filament (typically tungsten) is heated by passing a current through it. However, these sorts of bulbs are not really suitable for simulating sunlight. Their output is overwhelmingly in the infrared part of spectrum and lower frequencies, with actually very little light in the visible part and even less in the UV part of the spectrum. Some limited success was achieved by using coatings on light bulbs to cause the spectrum to shift more towards one mimicking sunlight. But this introduces a whole host of other issues, a notable one being the increase in heat emitted from the bulbs. This heat can negatively affect the plants, and cause a drop in yields.
A new contender entered the scene in the early 2000s with light-emitting diodes (LEDs). LEDs have had a long development history, being conceptually demonstrated in 1907, but not enjoying any real commercial success until the 1960s when infrared LEDs were developed. Their method of operation is fundamentally different to that of an incandescent lightbulb, exploiting a phenomena called electroluminescence, in which electrons and electron holes are recombined to produce light. This means that the frequency output of an LED is fixed, and extends over only a very small range of frequencies. As has been mentioned before, sunlight is formed of a mix of frequencies, and so for a while LEDs seemed a non-starter when it came to indoor growth.
That changed, however, with the introduction of ‘white’ LEDs. These used a combination of red, green, and blue LEDs to produce a broader spectrum of light than would otherwise be possible. The first attempts at white LEDs used phosphors, a technology already well understood in the context of fluorescent bulbs. The light emitted by the LEDs is converted, using a phosphor coating, into frequencies more suited for indoor growth. The technology was first introduced by the Nichia Company in 1996, and ever since incremental improvements in power, efficiency, and brightness have made LEDs the go-to light source for indoor or underground growing. As discussed in a recent interview, Growing Underground have made extensive use of LED technology in their growing setup. They’ve even gone so far as to record bespoke growing ‘recipes’ involving different combinations of light at differing frequencies.
It seems then that the future of unconventional agriculture is indeed a bright one. With companies like Growing Underground demonstrating that the flexibility offered by LEDs can be powerfully coupled with other technologies to enhance the growth of crops.
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