A Systems Approach To Remote Light Sources
A Systems Approach To Remote Light Sources
However promising the new, efficient light sources, such as the sulfur lamp, may be, they will be of little use if not coupled with systems that can distribute the light efficiently over large areas.
The new electrodeless microwave sulfur lamp described in the IAEEL newsletter 3/94 (see "Sun on Earth") is a demonstration lamp with an extremely high luminous flow. Each 5.9-kW unit produces ~450 000 lumens, the equivalent of some four-hundred 100-W incandescent bulbs. Even though lamps expected to reach the market in 1995 are to be less powerful, their light output will still be so high that unconventional distribution systems will have to be applied. Otherwise, their theoretical market and energy-savings potential may never be reached. Three demonstration projects exemplify how their sunlight-like light can be distributed over large areas.
HOLLOW LIGHT GUIDES ("Light pipes")
In one of the large halls at the National Air and Space Museum, full of rockets and satellites, changing lamps and maintaining the luminaires in the ceiling always were very expensive and time-consuming tasks, and thus much neglected.
Here, the 94 high-intensity discharge (HID) lamps have been replaced by three sulfur-lamp units used in combination with three 30-m hollow light guides ("ligth pipes"). These light pipes are internally clad with a highly reflective optical film that transports and distributes the light evenly towards the floor from over their entire length. Each sulfur lamp unit feeds light into a pipe from an easy-to-access catwalk at the side of the room. The rail-mounted pipes are divided into three sections and can be rolled back to the cat walk for cleaning and repair. The system offers improved color and reduced shadowing. Moreover, unwanted UV radiation was cut by half, an important feature for a museum.
The efficiency of hollow light guides is a much-debated issue: The above installation appears to be very efficient, with lighting levels increased threefold and energy use cut by 1/3. Although a modern HID-lamp system also would have increased efficiency, the maintenance problems would have been the same, with the luminaires getting more and more dirty. Here, it appears that it is the combination of efficient light distribution (the light pipes in this case both transport the light over a long distance and distribute it evenly) and the easy-to-maintain feature that accounts for the savings. A similar explanation also must be sought at the US Department of Energy's Headquarters (the Forestall Building) where an 80-meter-long outdoor light pipe installation is fed by one sulfur-lamp unit at each end. These two lamps replaced ~240 old luminaires with 175-W, conventional, HID lamps. Light levels were increased fourfold, while power requirements decreased by 2/3. (Also see Distant Lights, IAEEL Newsletter 4/94)
REFLECTOR-BASED DISTRIBUTION
If the purpose is to move light without providing illumination at the same time, a reflector-based system may be cheaper and more efficient. Since the sulfur lamp provides extremely stable light from a point source, an efficient reflector in the lamp unit can "collect" the light into a narrow beam that moves from the lamp, via reflectors placed outside the lamp unit. If the air is clean, very little light is lost between the lamp unit and the reflectors. (See figure below)
At the University Hospital in Lund, Sweden, the sulfur lamp is being used in combination with reflectors made of microprismatic optical materials. This hospital, typical of those constructed during the building boom in the Nordic Countries in the 60's and 70's, has large areas where daylight has little or no access. Moreover, the present lighting systems are inefficient, and it is believed that the large, dark areas adversely influence the well being of humans. Although this is still a somewhat controversial issue, the hospital authorities believe that more sunlight-like light could have positive biological effects. The ability to achieve higher lighting levels while reducing energy costs and cooling loads is another advantage.
The building housing the first prototype installation was erected in 1967. The original intentions of the architects were to create a soft and welcoming atmosphere in the entrance area by allowing natural light to enter through the glass facade. However, the facade didn't let in as much sunlight as had been hoped, and the inner parts of the entrance area were found to be very dark. Here, two parallel sulfur lamps with custom-built reflectors have been arranged in a way that directs the light upwards.
A system of reflectors (not light pipes) in the ceiling distributes the light over the entrance area. Thus, the light is transmitted from the lamp unit via a primary reflector in the ceiling, as beams through the air, to a number of secondary reflectors that direct the light down towards the floor. The reflectors are clad with highly reflective films, but shaped so as to avoid any glare. Moreover, since these films have a microprismatic surface structure that "splits up" the beams, the risk of glare problems is further reduced. The fact that the reflectors "move" the light source far away from the eye of anyone that would happen to look into them helps to further eliminate glare problems.
Similar installations are planned for three more hospitals in southern Sweden. For instance, for the first time an underground radiotherapy clinic will be bathed in sunlight, albeit artificial.
See also 1000-watt sulfur lamp now ready in IAEEL 1/96
Nils Borg