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Diffuse light in focus

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CI/SfB (N3) Technologies which focus diffuse light promise to open up a range of possibilities for light fittings, windows and signage

Luminaires, louvres, glazing, partitions - all may use diffusing glass or plastic to mask the image or light source behind.

Where the surface could be described as fully diffusing, there is no visible relationship between the incoming light and the exiting light apart from its intensity.

For privacy, this may be just what is needed. For glazing, there can still be glare from intense sunlight. For lighting, the relatively even spread of illumination can wastefully spread light where it is not needed. Controlling diffuse light into a beam and directing it where it is needed could open up a variety of possibilities.

One way of controlling diffuse light is to profile the surface of glass very finely.

The National Physical Laboratory (NPL) has been doing this in creating diffraction gratings for optical measurement.

Generically, what is created is a controlled optical diffuser. The consultancy Scientific Generics has been commissioned by the DTI to explore commercialisation of such spin-offs from NPL.

Antony Hurden of Scientific Generics explains controlled diffusers on a spectrum of possibilities from the lens to the fully diffusing surface (bulk diffusers).

Starting with the lens on the left and working to the right (see diagram), methods have been found of focusing light using finer and finer profiling of the glass surface. The scale of profiling of the Fresnel lens and lenticular array is also the scale of prismatic glazing - sheets of prisms used to redirect using reflection or refraction. A few examples of this exist, for example the roof of the refurbished Billingsgate fish market 1.As we move to a finer scale of glass (or plastic) profiling, light is instead bent by diffraction at the profiles; there remains some ability to focus this diffuse light. (More technically, a controlled diffuser randomises transmitted light whilst controlling the solid angle and direction into which the light is scattered. ) While the commercialisation exercise is focused on NPL's diFfractive optic capability, NPL is also involved in the finer scale of holographic optics, a technology that can also bend light by diffraction. Other groups are currently developing building applications (see box).

Options for buildings Scientific Generics' brief is to look at all possible applications of NPL's diffractive optics technology, which includes ideas for instrumentation, photographic enlargers, lasers and inspection. Possibilities related to construction include:

focusing of diffuse beams from luminaires which could save energy, as could applications for illuminated emergency notices and other signage output from uplighters could be more focused glare could be reduced from point sources, for general and display lighting new effects could be produced in theatre lighting with 'diffuse beams' directional diffusing glass could redirect sunlight up to the ceiling (like an internal light shelf ), so brightening the ceiling and reducing glare close to the window. Diffusers (being lenses) are sensitive to the angle of incidence, so without sun-tracking the sunlight patch on the ceiling would change position diurnally and annually. (Of course clear glass for view out with its own shading would also be needed in a facade. ) These are early days. NPL is looking for partners to commercialise its capability. Available cutting and coating technologies should enable the technology to be scaled up. (Holographic films are already produced. ) In the course of his six-month project, Hurden has been approached by glass and lighting companies who see immediate potential for controlled diffusers.

1 Designing with Innovative Daylight. Paul Littlefair. 1996. BRE Report BR 305. From CRC, tel: 0171 505 6622. 66pp. £35.

HOLOGRAPHICS Littlefair has summarised recent progress in holographics, primarily in the US and Germany.

Though still in its infancy, a few experimental projects have been Built.

Some volume manufacturing techniques exist.

In the US, relatively cheap holographic plastic film is produced by embossing a structure of sloping saw teeth 0.5m-6 apart.

Used for side windows, some sunlight is redirected, so there remains some view out but some risk of glare.

In Germany one holographic treatment is a set of microscopic stripes on glass (called zone plates). These were used for the moveable louvres of atrium glazing for a house at the last International Garden Festival in Stuttgart. Some 94 per cent of direct sunlight was redirected. Around 45 per cent of diffuse skylight was admitted.

Littlefair recently suggested that suntracking would not necessarily be needed - adjustment of such louvres a few times a year might be enough.

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