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PEARL's magic tree...

Image of a tree in unusual light
The cherry tree for the park experiment in PEARL

We are about to undertake some experiments which require a park to be created in PEARL. The park has four large screens around it, which have back-projection movies of a London park so that the mid-to-far distance is represented with a surrounding dynamic image of a park - trees, grass, people and so on - and the 'join' of these screens to the actual environment in the laboratory is made with real mulch and leaves so that they match as closely as possible the images on the screens. We also have a range of artificial plants to generate the three-dimensional sense of the park. We also have a real cherry tree in the park, placed, and lit so that the shadows cast by its branches match the intensity and direction of the shadows cast in the background images. You can see the idea under construction in the image below.

Image of trees and plants in a park, including projections on background screens
The PEARL park under construction, with real tree, artificial plants and back-projected images of a park...

So why do we have a bright pink picture of a tree at the beginning of this article? Well, of course our real cherry tree is being kept indoors for the experiments, so is away from natural light. To help keep the tree healthy in these conditions, apart from ensuring that it has the right nutrients and water it needs, we provide a regular dose of the light that trees like while they are growing their leaves in Spring. Daylight is a mix of wavelengths of light, which, when all are present, gives the impression of the 'white' light we, as humans, are familiar with. The distribution of wavelengths varies during the day, with shorter wavelengths stronger in the early morning and longer wavelengths stronger in the late afternoon, but the overall image is always one of the light being 'white'. However, unlike us, trees use only a part of the light spectrum that is visible to humans, so in PEARL we need to make sure that they receive the light that they need. This is a mixture of very short wavelengths - into the deep blue and indigo - and very long wavelengths - deep red and towards infrared - with a dash of green. If we extract from the light all the other wavelengths, the overall appearance of the light is the sort of bright pink colour that you see in the picture at the top of the article. We provide this for a few hours a day so that the tree can 'sleep', as it would if it were outside.

The net result is that the tree has been developing its new leaves really well and we are looking forward to see how it thrives during the experiment period. After these experiments have finished, we will move the tree outside to enjoy the real daylight...

In the actual experiment of course, the light will be set for human consumption, and will be set according to the time of day - but our tree will still be able to enjoy its doses of its very own special light - as you can see in the image below!

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