Light work: following on from his thoughts in Peter Davey's valedictory issue, in which contributors were invited to speculate on the future, services engineer Max Fordham considers lighting in the workplace and how it is evolving to meet the challenges of environmental responsibility.

The need for natural light will make buildings thinner so that the light
can penetrate from the windows. The use of glass in all buildings has to
become more rational. Horizontal rooflights provide two and a half times
more light than vertical windows and glass close to the floor level
provides very little useful light, so 100 per cent glazing is not
sensible. Of course, modern High-Tech glass construction is so stylish
that it will be a pity to see it go--unless something better turns up
to replace it. That will be the innovation. Max Fordham, AR March 2005

Before the Industrial Revolution, natural light was the only available option to illuminate the world of work. Rooms in which people worked tended to be tall with large openings to admit light. By the era of tall buildings, floor plates had increased to give deep plans, while storey heights had diminished. By then, however, electricity was available for producing artificial light. Just as the lift was essential for the development of tall buildings, so was electric light.

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Today, discussion of buildings and lighting strategies is dominated by the need to conserve energy. For every 1000 lumens provided by the best fluorescent lights, 10W of electricity is consumed, equivalent to 30W of carbon fuel. For comparison, an incandescent light bulb generates only 100 lumens from the same quantity of electricity. 1000 lumens of natural light is equivalent to 10W of solar energy and if infra red light is filtered out, then 1000 lumens is available from 5W of natural light.

Current recommendations for light levels in offices are generally about 500 lux. The standard lighting template for offices is a volume of just under 3m high and 6m deep, typically lit from one side. On a dull day, the sky provides around 5000 lux, so the natural lighting requirement in this kind of environment is around 50 to 100 lux (about what we expect at home). Electric lighting is designed to provide around 500 lux, so it is hardly surprising if lights are turned on all the time. Cursory observation reveals that lights in city centre offices are nearly always on, but this is extremely wasteful. Typically, 20W per sqm over 8000 hours per year generates around 100kg/sqm of carbon dioxide annually. Total carbon dioxide production, including lighting and heating, of some recent energy efficient office buildings is about 40 kg/sqm per year.

The shift to natural light

In the early '70s, energy conservation strategies focused on using energy from lighting to provide winter heating. The aim should now be to design buildings so that they use practically no energy specifically for heating the building. Good thermal insulation coupled with heating incoming ventilation air from outgoing stale air should enable the general metabolism of the building (heat from people, computing equipment and other electrical appliances) to be used to keep the building comfortably warm. Clearly then, natural light should replace artificial light, and windows meet the demand for illumination. It is difficult, however, to see how light from windows can provide 500 lux illumination in a deep plan building. Roof-lit buildings are one answer, such as Edward Cullinan's Ready Mixed Concrete (RMC) headquarters at Thorpe Park (AR September 1990) which memorably employed rooflights at the core of the building.

This strategy came into play when my firm was asked to make a bid as mechanical and electrical consulting engineers for the National Trust building at Swindon designed by Fielden Clegg Bradley. Our proposal suggested a roof-lit building that would provide good natural lighting with a minimum daylight factor of 5 per cent. The bid was accepted and the Helis building won the RIBA Sustainability Award for 2006. As the notion of a roof-lit building is difficult to justify financially, the area of the void in the upper floor was reduced to allow for some courtyards to be added so that the natural lighting on the ground floor was a little restricted.

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The building is now in use and it is evident how the lighting strategy is working. On the upper floor there is plenty of natural light, but electric lights are in use more than anticipated. Though these are controlled automatically, to set lighting levels, there has been a tendency for occupants to request that set levels be increased. Since the lighting strategy was designed to provide light at the working surfaces, this is perhaps why the occupants felt they wanted to increase the general light level.

Time to adapt is also an issue in a working environment. As you look up from a piece of work or computer screen to a window with a bright sky, you are dazzled by the bright sky. When you look down again the work is dim. This has little to do with the total level of illumination, but depends on contrast. It means that if artificial light is available which is brighter than the natural light, it will be turned on to offset the dimness when looking back down. The amount of light also depends on what you need to see. You certainly need bright light if you are threading a needle, but the light level inside a gallery showing delicate watercolours is necessarily very low, around 50 lux. The transition from outside to inside usually takes time, giving eyes a chance to adapt to the low light level.

Brightness and distribution

Lighting design also must address the way the light is distributed in the space and the colours of the surfaces. It is important that light has a directionality and should not be diffuse (i.e. coming from all directions evenly). Moreover, the field of view should be bright and cheerful. Brightness refers to the amount of light being reflected off a surface into the eye. This is the technical meaning, but we could also describe it as 'cheerful', 'bright' or 'jazzy'. An object has colour because it does not reflect all the light it receives. A yellow object only reflects the yellow part of the light it receives, while a white surface reflects almost all the light. Yet a brightly coloured object appears 'bright' by any normal use of language. On a visit to the Royal College of Art I was struck by a large picture, around 2m in diameter, coloured bright red with large white flecks. Such a powerful image cheered and illuminated the field of view. Perhaps a similar picture on the gable wall at Helis would reduce the demand for electric lighting.

Sustainable design requires careful consideration of the relationship between natural and artificial lighting, and how this impinges on both external form and internal space. As the impetus to be more environmentally aware grows, architects and services engineers should take such responsibilities seriously.