Distant Lights
Distant Lights
Hollow light guides, or "light pipes", can be used in many different applications. For instance, they are used in explosion-risk areas and where maintenance is difficult and expensive, such as high-bay industrial lighting, road tunnels, and street lighting. Leading daylight into buildings is another promising application. Light guides are still rare in most countries, but in the former Soviet Union alone-with a long tradition of light guide research-there are ~45000 installations.
A hollow light guide could, in principle, be constructed of any material with a highly reflective surface. However, a smooth inner surface makes it hard to control the light. If microprismatic films are used, higher reflectance efficiencies are achieved, and the different optical qualities and reflectance angles of various films can be combined, so the light can be both transported along the tube and redirected. Many of the acrylic films have additional qualities, such as allowing light that hits them at certain angles to pass through, while light from other angles is reflected. In the Forestall Building (see A Systems Approach to Remote Light Sources, IAEEL Newsletter 4/94), 3M optical films have been used, and this article is based on the optical qualities of these films.
The basic optical film is called OLF (Fig.1). Up close, one surface of the film looks like an infinite series of perfectly parallel mountain ranges and valleys. The height of the "peaks" is about 0.5 mm. The other surface is smooth. If the light hits the film within at angle of 27.6° or less, almost all light is reflected. Light from other angles will pass through the film.
Fig. 1
A simple light pipe can be made by clothing the interior of a clear acrylic tube with this optical film. Here, the light bounces between the inner walls of the tube, causing it to move onwards (Fig. 2). Very little light exits the tube through the walls. This type of tube has been used in light poles where a lamp is placed at the bottom of a vertical tube. At the top, there is a reflector that reflects the light down to the road.
Fig. 2
Since some light is lost in each reflection, a more narrow beam would reduce the number of reflections, and thus reduce losses. If the purpose is to simply move the light from the light source to another point (as in the light pole above), a very good reflector with a narrow beam could eliminate the need for the light pipe.
However, the fact that the tube loses some of the light can also be an advantage. If the amount of light entering the tube is not too high, the tube will appear as a softly glowing line. This technique has been used to make drivers aware of a long curve at the top of a steep cliff.
The Forestall Building offers a more advanced design, combining two optical films with different qualities. The basic tube is still the one described above. However, at the bottom of the tube many small holes have been punched. A light ray that hits such a hole will go straight through the tube wall instead of being reflected back into the tube. Here, a so-called Transmissive Right Angular Film (TRAF) has been used (Fig.3).
Fig. 3
When the light hits this film within the acceptance angle, it changes direction and is directed vertically down to the ground. (Fig. 4)
| Fig.4 Combination of two films with different optical qualities. |
By varying the width of the TRAF, the light distribution on the ground can be varied. In the Forestall Building's 80-m tube the light distribution over the road differs from that over the pedestrian area.
Nils Borg
See also: CIE hollow-light guide committee (IAEEL 1/95)