Thanet Earth has posted a response (pdf) on their website, explaining what causes this "light pollution", and how they are trying to prevent it. Both of the tomato glasshouses at Thanet Earth are equipped with growlights which provide the plants with a light intensity that replicates the light levels of a typical spring day. In total, the two operational lit greenhouses have some 20,000 lights at work. Each is around 1000W.
"These lights are usually switched on by around midnight, and will stay on until the afternoon. The plants still need time in darkness as part of their photosynthesis process, so they ‘sleep’ from sunset through until we turn the lights on again during the night.
"Whilst the manufacture of these blinds is extremely precise and the maximum coverage possible is provided, for ventilation purposes we have to leave small gaps where the blinds meet. The blinds are also not ‘black-out’ blinds. It’s imperative that the blinds allow the humid air out and the dry air in. A blackout blind would not allow this; plant health and fruit quality would suffer. As with a blind on the window of a home, light will be visible from above, and a glow may be seen."
According to Thanet Earth, their coastal setting brings with it some additional challenges: "When a heavy, low mist covers the site then we seem to suffer from a visible night glow from the lit glasshouses." They say they have consulted experts in this field who have confirmed that this is caused by the escaping light being trapped and reflected repeatedly, thereby having a magnifying effect.
The company confirms that light pollution is a serious issue, but emphasizes that the growlights are not used during key production times in Spring and Summer. And by leaving the lights off until 11pm at the earliest, they aim to minimise any impact of the lights on their neighbours.
Many people have asked Thanet Earth about LED light technology. They have so far chosen not to use LEDs in our greenhouses for a number of reasons:
- "Our plants benefit from the additional warmth of our sodium lights."
- "Plant performance under LED lighting simply isn’t as good as we find with our sodium lights."
- "The light glow from LEDs is much more intense and would be purple in tone. This would take our night light emissions to extremes that neither we nor our neighbours would tolerate."
Source: CPRE Kent / Thanet Earth