Materials that are light-permeable are usually called ‘trans­parent’ or ‘translucent’, although it might be more accurate to call them ‘diaphanous’ – of such fine texture as to allow light to pass through; translucent or transparent (from dia-, ‘through’ + phainen, ‘to show, to appear’). It is related to ‘phantom’, something apparently sensed but having no physical reality.

Diaphanous: it’s a beautiful word and yet seldom used by architects and designers for materials that allow light to pass through them, either transparent (crystal-clear), or translucent (‘misty’, diffuse and ‘vague’). They can be dense materials like glass or have an open structure, like wire mesh or a woven fabric. At www.materialexplorer.com visitors can search for materials on a variety of characteristics, such as gloss, texture, hardness, temperature, acoustics, smell and transparency. It turns out that architects and designers most often select ‘transparency’ when searching for a material. And the number and variety of materials that can be used to manipulate light penetration is growing by the day, as are the ways in which they achieve this – through structure, texture, volume, colour, relief, prisms, openness, an illusion of depth and reflection. Where does that fascination come from, what is it with architects and transparency? Is it a longing for the ephemeral and immaterial, or is it simply that transparency satisfies a basic human need for light?

Nature
Translucent structures and materials also occur in nature and when they do, there is always a functional reason. For example, the window plant (Fenestraria) is a succulent that lives in the desert where, to avoid drying out, it spends ninety per cent of its time hidden in the sand. But in order to admit the daylight it needs for photosynthesis, the top of each ‘leaf’ consists of a thin, transparent material that ensures that the plant is able to absorb enough light. Just as this plant needs its ‘windows’ in order to live, human beings need the windows in buildings for we, too, require daylight for a healthy life.
Some organisms use transparency as a form of protection, like the jellyfish which makes itself almost invisible in order to escape the notice of predators. In order to survive, the jellyfish needs a high degree of invisibility or non-materialization.

But predators, too, can turn transparency to their advantage. The spider’s web is made from transparent spider’s silk which is woven in a very open structure so as to be as invisible as possible to the unsuspecting fly that the spider has set its sights on. Architecture to snare a quarry? Architecture as a trap, or as a seductive image?

Unintentional examples of the first are not unknown. The reflective south elevation of the administration building of the University of Saskatchewan, Canada, reflects the surroundings and the sky so perfectly that the facade itself ‘dissolves’. Every day dozens of birds crash into this glass wall – chickadees and sparrows, emerald hummingbirds, warblers and cedar waxwings. The least elegant solution would be to plaster the windows with stickers of birds of prey. Yet a film that would render the facade opaque from outside, would immediately obliterate the three-dimensional effect of the transparency.

Quality
In architecture it is not so much the amount of light that enters a building that matters, as the way it enters, the quality of the light. It’s not about bringing as much light as possible inside, but about how it is done, the contrast between light and dark, the effect of light on materials, the reflection on a surface, the interplay between direct and diffuse light.

Diffuse light, such as occurs on cloudy days, illuminates the environment more evenly and penetrates more thoroughly than direct sunlight in a clear sky. Special glasshouse coverings ensure that more diffuse light enters a greenhouse, thereby increasing productivity. Diffuse light is also essential for lighting works of art in museums to the best effect; museums consequently make great use of translucent material. The manipulation of light is as old as building itself. We find it in Japanese paper screens (shoji) and in traditional Islamic facades decorated with lacy screens (mushrabiyah), and it continues into modern architecture, from the thick walls with tapering windows that cast light deeply and subtly into an interior space, as in Le Corbusier’s church at Ronchamp, to today’s layered facades that are made up of reactive, manipulable screens, slats and light-diffusing materials.

In theory there are four routes whereby daylight can have a positive affect on the health and well-being of a human being: through the visual, the biological, the physico­chemical and the psychological systems. Lengthy exposure to dim or flickering light or too sharp a contrast between light and dark can lead to eye problems, resulting in headaches and tiredness.

But although we need light first and foremost to see, its most profound impact on our well-being is via our hormonal system. Light entering the eye affects our biological clock, which in turn regulates all the cyclical processes in our body, such as sleeping and waking. When there is insufficient or inappropriate light, the clock doesn’t run as it should. A chronic disturbance soon leads to sleep disorders and health and performance problems. Shorter, but no less disruptive disturbances are caused by jet lag or shift work.

Biological effect
Light also stimulates the brain. When a person is exposed to too little light, as in winter for example, they may experience some or all of the following symptoms: tiredness, melancholy, a desire to sleep all the time, poor concentration, irritability and weight gain. This typically occurs during the shorter days of the year, from September to March in the northern hemisphere, and is caused by a shortage of bright light of a sufficiently high intensity.
Many years ago, the US space organization NASA looked into the biological effects of light and dark. In space the light-dark cycle is much shorter than on earth. The hormonal system has difficulty keeping up with that short cycle of four hours of light, four hours of dark, leaving astronauts active and sleepy at the wrong times. Jet lag has a similar effect: during the day you can’t keep your eyes open but at night, when you go to bed, you can’t fall asleep.

The rhythm of the biological clock is driven by the production of two hormones: cortisol, called the ‘stress hormone’ because it activates, and melatonin, which causes sleepiness. The production of cortisol is highest in the early morning, after which it gradually tapers off. With melatonin it is the reverse: it increases from around ten in the evening, reaching its peak in the early hours of the morning and diminishing sharply during the day. When functioning properly, these hormonal cycles ensure that we get up fit and alert in the morning and are able to fall asleep at night.

The effect of light on health has spawned a new discipline. Relatively speaking, it is quite a short time since human beings started shutting themselves up in buildings where it is darker than outside and the human body is still programmed to the much higher levels of outdoor light. Even on very grey days light levels can reach 2000 lux and on a sunny day that can rise to as much as 100,000 lux. Inside buildings the light intensity seldom gets above 500 lux. (By way of comparison: street lighting starts at about 5 lux in a small residential street rising to 20 lux on a motorway. In the home the average lighting level is 150 lux, in factories and offices the norm is 300 and 500 lux respectively; out of doors, an overcast still produces 10,000 lux). So it is scarcely surprising that people who spend too much time indoors receive too little light stimulation. Although the effects of deprivation are not visible from one day to the next, the brain does react rapidly to the reverse situation: anyone who gets a large dose of light will perform well for the next two to three hours.

Winter blues
Many people are especially sensitive to an enforced absence of high intensity light, in other words, to a shortage of cortisol. Three to five per cent of the population is so badly affected that they become clinic­ally depressed and need to seek the help of a doctor and psychiatrist. Ten per cent of the population are not sufficiently depressed to need treatment but suffer from what is variously referred to as winter blues, autumn, winter or spring fatigue, or a mild form of Seasonal Affective Disorder (SAD). Race, skin colour and social status play no role, but gender does appear to matter. As far as is known, the number of women who suffer from winter blues is four times greater than the number of men.

But too little darkness can also pose a threat to health. There are indications that a shortage of darkness might be one of the causes of breast cancer. American scientists recently discovered that in women who experience too little darkness the production of melatonin may be disrupted and melatonin is known to play a role in suppressing tumours.

The natural rhythm of the human body diverges slightly from the 24-hour rhythm of the earth’s orbit around the sun. In several places around the world students were placed in a windowless room where the lights burned constantly at the same level. They were forced to develop their own rhythm for sleeping, eating and working. All the experiments led to the same conclusion: the natural rhythm is on average not 24 hours but 24 hours plus 20 to 30 minutes. In people who spend part of every day outdoors, their biological clock automatically adjusts to the natural cycle of light and dark. But in people who spend every day working in spaces where no daylight penetrates – such as department stores and warehouses – no such adjustment takes place. After 24 days they are twelve hours (24 x 30 minutes) out of phase meaning that they produce a maximum of melatonin just when they should be producing a maximum of cortisol and vice versa. Result: during the day they have trouble staying awake and at night they have trouble sleeping. After a further 24 days they are once again in phase.

Philips has been researching the interaction between light and health for ten years now and has acquired a considerable body of knowledge in that field which they have channelled into the development of artificial light that can prevent health problems and increase productivity in the workplace. Research has shown that if the light level in a factory is increased from 300 to 2000 lux, the productivity of the people who work there increases by twenty per cent.

Artificial light is indispensable in all kinds of underground structures, and in other places  where it is not possible to use daylight. But in many cases daylight is permissible and then diaphanous materials are essential, precisely because they not only admit the changing levels of daylight, but also because they make contact with the outside world possible.