Manufacturers of hollow light systems have generally followed the same design protocols as for standard lighting systems. Information is available that uses the typical coefficient of utilization (CU) procedure, which allows a designer to determine illuminance within a space. To use these systems, an engineer needs to know the illuminance required for a specific space in order to determine the size and number of separate lighting systems needed. The basic zone cavity or lumen method design procedures, described by the Illuminating Engineering Society of North America (IESNA), uses published manufacturer data for light guide materials and systems. The basic procedure involves:
1.determining required illuminance level using IES recommendations;
2.determining the area of the space to be lighted;
3.defining room characteristics, including surface reflectance;
4.determining the number of systems for supplying illuminance, using published CU data; and
5.producing a uniform layout using multiple fixture systems, based on spacing/mounting height criteria data.
Designers who use fiber optic light delivery systems are mainly interested in supplying a particular quantity of light to a task. A primary objective in a lighting design is providing illuminance. Illuminance is flux density on a surface, expressed as lumens (light flux) per square foot, typically known as a foot-candle. Consensus recommendations for appropriate foot-candle levels for particular tasks can be found in a variety of industry guidelines and lighting handbooks. Illuminance can be estimated or calculated using some basic principles in conjunction with published photometric performance data for a particular lighting system. The first step is to determine the amount of light flux, measured in lumens, that is produced by a source. If the source is incandescent, the rated lumens are typically what are available to the system. If the system is using an HID (high-intensity discharge) lamp, such as a metal halide, the designer needs to determine system lumens. System lumens are a product of the rated lumens of the lamp and the relative ballast factor of the ballast designed to operate the lamp. Ballast factor accounts for the difference between the lumens obtained with a reference ballast in the laboratory and the lumens available with a commercial ballast. Typical ballast factors can range from 0.7 to 1.2. (See Eley et al. 1993 for a discussion of determining ballast factor.) The designer must also consider a source's lumen maintenance, the light output characteristics over the source's life. This information is typically available from the lamp manufacturer.
Not all of the lumens from a particular source are available. All fiber optic systems use optic coupling systems that direct flux into a fiber optic material using complex reflectors, lenses, or other optical techniques. Total lumens of a fiber optic system can be determined from the coupling efficiency of the system. The coupling efficiency is simply the quotient of total lumens produced by the lamp and the total lumens accepted by the fiber. Coupling efficiency is measured under laboratory conditions and is usually published. This efficiency factor accounts for losses that occur through the reflector or lens system as well as reflection losses that occur at the face of the fiber optic bundle itself. Coupling efficiencies typically range from 50 to 80 percent, depending upon the structure of the coupling system. Light that enters the fiber optic system within a given critical angle with respect to the face of the fiber is efficiently transmitted to the terminal end via total internal reflectance. Typically, a fiber optic system absorbs a small amount of the light flux along its length. This loss is called attenuation and is a function of the fiber optic material and the distance that the flux travels.
In order to determine the total available flux provided by an end emitter, the engineer needs to know the length of the fiber and the attenuation per unit length. Attenuation refers to the percent of light that is absorbed per unit length of fiber optic. If attenuation is 2 percent per meter, then each meter length transmits 98 percent (100 percent–2 percent) of the flux entering that length. This number in known as transmittance.
