We evolved in greener shades

Lighting designers could make better use of natural light patterns for reasons rooted deeply in human evolution, and the new British Standard on lighting of workplaces helps them to do just that

“Look! Look at the light in the trees!” is something you will often hear me say.

All my life I have loved the interaction between natural light and trees; it lifts my spirits, and I am sure that I am not the only one.

When I was studying for my MSc we were given an interesting paper, ‘The Effects of Environmental Illumination on Melatonin, Bodily Rhythms and Mood States: a Review’ [by PT Stone in Lighting Research and Technology, Volume 31 (3): 71-79], about an evolutionary possibility which might help to explain this particular fascination. It discussed the idea that humans developed on the plains of Africa.

Animals that live in that environment to this day do not need eye protection against the sun, so why do we now wear sunglasses?

Stone’s hypothesis, based on how the human eye works now, was that we must have evolved under the tree canopies for protection against the harsh savannah sunlight.

Sensitivity to light

The paper’s first line of evidence, as I have pointed out, is our present need for sunglasses on a bright day. Stone writes that animals on the plain have a limited vertical and very wide horizontal angle of view. This does not take in the high sky-dome and, therefore, the sun.

We, on the other hand, have an increased vertical angle of view. His argument is that this indicates that the human eye must have evolved in a different environment to that of the open plains.

Stone’s second point is that the human eye’s sensitivity to light ranges between the 400nm and 700nm wavelengths, with a peak at 555nm, in the yellow/green part of the visible spectrum. The anoraks among us will already know that lamp manufacturers try to ‘pack’ their lamps with these wavelengths, as this increases the lumen package, which will be lambda-corrected. Now, what is the colour of sunlight as it descends through the tree canopy? Well, you got it – yellow/green light that matches the colour-sensitivity peak of human vision.

Whether the theory is sound or not, people love the essence of this phenomenon, the natural pattern of light through the tree canopy. From the humble ‘GOBO’ (GOes BefOre) stage lighting device, which projects a simulation of a tree canopy onto stage scenery, to the translucent supergraphics in trendy offices, we replicate this visual impression in our concrete jungle.

Opposing forces

What is the antithesis of this uplifting, shared experience? In my view it is an overcast sky. If the bright light filtering through tree canopies uplifts and energises us, then the grey light from an overcast sky depresses the spirit and flattens definition in the world around us. And, at last, this negative effect is being recognised in recommendations for lighting design.

The new BS EN 12464-1 2011 standard now addresses this issue by giving a wider variety of uniformities of illuminance over direct task area and the surround. The new required ratio for general areas is a magnificent 0.4! Why is this so good? Because the old argument for more lighting fittings – and therefore more energy – arising from a ridiculous ratio of 0.8, has now been consigned to history.

The new British Standard is also awash with lovely little design comments among the figures, which helps to cement the relationship between human response and light.

Under the heading ‘Modelling’ it says: ‘The general appearance of an interior is enhanced when its structural features, the people and objects within it, are lit so that form and texture are revealed clearly and pleasingly.’ Fabulous – a design conversation!

We are moving forward again. As we seek to understand more about how we respond to all light, I look forward to hearing someone say: “Look! Look at the light in the office!”

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