Guiding Light Efficient At Times
Guiding Light Efficient At Times
Hollow light guides remain an interesting niche application for distributing light from artificial light sources. Although their optical efficiency appears to be low, their use in certain applications (including daylighting) may offer energy benefits. If light guides are ever to become cost effective their use will have to be motivated primarily by various non-energy-related benefits-even in niche applications.
One relatively unusual category of lighting technology is the hollow light guide (HLG), through which light from a bright source is transmitted and distributed rather evenly over a large area. Light is reflected down the length of the guide, either from mirrored surfaces or via total-internal-reflection prismatic surfaces. Light guides can be oriented horizontally or vertically, or they can be manufactured in non-linear configurations such as arches.
Although optical efficiencies are not high, light guide technology may bring with it some opportunities for energy savings. For instance, light guides offer one valuable means of utilizing highly efficacious and high-lumen-output light sources (e.g., in low-ceiling premises and premises with low illumination levels, hazardous areas, very cool or hot environments, clean-rooms). In such situations other types of luminaire technology cannot be applied or may be impractical. Moreover, the relatively low dirt-depreciation characteristics of HLGs translate into a reduced maintenance factor, and hence a reduction in initial wattage or lumen output.
The potential for using HLG systems to distribute daylight throughout the inner core of buildings could represent the most important application of this technology from an energy standpoint.
It is necessary to emphasize that light guides are highly specialized technologies with a restricted range of uses. For instance, reflector-based lighting or various indirect lighting concepts may offer cheaper and more practical solutions to many of the problems associated with distributing light from high-and ultra-high-output sources.
A Century of Development
Light guides have a long history. Although invented in 1880, it took about 100 years before light guides were first produced in substantial numbers. About 50 000 units have been made in Russia since 1980, and since 1986 ca 15 000 units have been manufactured in North America.
Between 1980 and 1994, the VATRA (Ternpol, Ukraine) manufactured over 47 000 systems measuring 6 m x 275 mm and 18 m x 600 mm, and coated with a reflective 20-50 micron, polyethylene terephtalate film. These were powered with 700W reflector metal-halide lamps (3-4 lamps in the larger-diameter systems).
Most of these systems were installed in hazardous areas (high risk for explosions or fires) and in situations where the maintenance costs of changing lamps were high. An additional 5 000 extruded aluminum systems have been produced in Russia. By late 1986, over 15 000 systems utilizing prismatic films had been produced in North America (mostly 6 m x 150 mm, utilizing 250W metal-halide lamps).
Today, 15 companies in 12 countries manufacture the systems. Research on technology and design approaches continues in Canada, Switzerland and Russia as well as in the Ukraine and the USA. Thanks to these efforts, light guides (once limited to explosion-hazard environments) are now being widely used for specialized applications, such as roadway lighting, architectural lighting, industrial lighting, and museum lighting, as well as for ordinary indoor lighting tasks.
Beyond Energy Savings
Hollow light guides epitomize the interesting subject of non-energy benefits. Indeed, energy savings are not the main motivation for using light guide systems. The following factors are generally more important, although it should be stressed that HLG technology is not the only option in many of the situations below.
HLGs offer:
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Possibilities to utilize ultra-high-output light sources which cannot be used with ordinary luminaires.
- Uniform illumination and low glare-the luminous flux from the point-source or linear high-intensity light sources can be distributed relatively evenly over a large area.
- Reduced maintenance-fewer lamp changes, due to a reduced number of light sources, and easier access to the lamp compartment which can, for example, be located at ground level even though the light is delivered far overhead.
- Thermal management-relatively easy to capture and remove unwanted waste heat from light sources.
- Reduced dirt depreciation-sealed light guide systems accumulate less dirt than conventional luminaires which, even if sealed, have to be opened to change lamps. In addition, HLG systems may have aerodynamic properties that direct particles away from the light-emitting surface of the luminaire.
- Considerably improved safety features reducing the risks for electrical shock, explosion and fire, due to the low temperatures and absence of electric potential in the channel of the guides, and reduced electromagnetic fields (EMF). These are all benefits in specialized environments, such as chemical factories or clean-rooms, since the electronics of the lamp unit can be situated far from the sensitive environment.
- UV filtration facilitated by the isolation of the light source from the final point of light distribution and the relatively small size of the filtration material required.
- An easy way to vary light color since color filters or lamps can be inserted at the end of the guide.
- New opportunities for lighting architecture when using extended lighting devices with different shapes and colors.
Three Technology Approaches
Although the details may vary, all HLG systems reflect and redirect light from one or more point sources, achieving an even distribution over a large area. One distinguishing characteristic of HLG systems is that light undergoes numerous internal reflections, whereas in normal luminaires the number of reflections is normally quite low. The large number of reflections, in combination with the fact that light typically must be transmitted through the light guide material, is the main explanation for the low optical efficiency of HLGs compared with that of good reflector- or mirror-based systems.
There are three main types of light-guide technology that differ in terms of the optical properties of the materials used and the geometry of the guides. One commonly used figure of merit is the maximum length/maximum diameter ratio, an indicator of the quality of the reflectivity or light conductivity of the inner surface and the collimation of the light source itself.
Cylindrical, metallic mirror reflection light guides(also known as Slit Light Guides, SLG).Selected interior light-guide surfaces are coated with reflective material, and light escapes via an opening (or slit). One advantage of SLG systems is that the distribution of light can be easily controlled by rotating the cylinder and/or varying the width of the light-emitting aperture. This is the least expensive of all light-guide technologies. Ratio of length- to-diameter 40-50 to 1.
Cylindrical Prism Light Guides (PLG). Geometrically similar to SLG systems but lined with total-internal-reflectance films rather than mirrored optical materials. Excellent luminance uniformity. Ratio of length-to-diameter: 80-100 to 1.
Wedge-shaped (flat), metallic mirror reflection Light Guides (WLG). Rather than being cylindrical in cross-section, these systems are wedge-shaped and can be relatively wide. The top and sides are mirrored, and the bottom surface is light-emitting. The advantages are that they can utilize linear light sources and have a large light-emitting area. Ratio of length-to-height: 30-40 to 1.
The SLG is the simplest system to design and build and is thus the least expensive. This comes with a trade-off in efficiency and optical performance. In all cases, the coupling of the light-producing section with the light guide section is a critical factor in the overall efficiency.
Elusive Efficiencies
Quantifying the efficacy of HLG systems is far from easy. The optical efficiency of HLG systems is typically in the range of 45-55% (amount of light distributed to a given area divided by the total flux from the light sources). As a general rule, wedge-shaped light guide systems achieve 5-10% greater efficiency than cylindrical geometries. This performance does not compare well with good conventional luminaires; thus the non-energy benefits discussed above must be considered.
Given the complexity of light-guide systems (and numerous component efficiencies to consider) it is difficult to generalize about performance. The International Commission on Illumination's committee TC 3.30 is developing various information resources on light guides. One example of how the efficacy of a light guide installation can be calculated is given in the adjacent article.
Economics
Light guides made in small production runs can be expected to be relatively expensive. Costs may decline by a factor of 2-2.5 with a 10-fold increase in production volume. But capital costs are only part of the equation. Light guide systems must be compared with conventional lighting systems on the basis of energy, maintenance, labor costs for installation and, in many cases, the differential capital cost savings for heating, ventilating, and air conditioning systems. Viewed as a daylighting technology, cost comparisons must be made against the alternative requirements for windows, light shelves, prismatic panels, controls, and the like. Sun-tracking systems sometimes used with light guides are a considerable cost factor.
The true economics of light guide systems hinge more on the non-energy benefits than on the potential energy savings achieved by using highly efficacious sources or daylight.
Julian Aizenberg
Evan Mills
Julian Aizenberg has conducted research on light-guides for more than 30 years and has overseen many large-scale applications. He founded the CIE Technical Committee 3.30 on Hollow Light Guides which he also chairs.
Evan Mills is Associate Editor of the IAEEL Newsletter and researcher at Lawrence Berkeley National Laboratory.