- Lumen method
Also known as the zonal cavity method, the lumen method is a series of calculations that uses horizontal
illuminancecriteria to establish a uniform luminairelayout in a space. In its simplest form, the lumen method is merely the total number of lumens available in a room divided by the area of the room [cite web|url=http://www.lightcalc.com/glossary.html|title= Light Calc|work= Glossary|accessdate=2008-03-20] . In order to perform this calculation, many factors, coefficients, lamp lumen data and other quantities must be gathered. Despite the scientific impression of the lumen method equations, there are inaccuracies and assumptions built into the method. Therefore, the lumen method should not typically be used as a standalone, final solution; it should be used as a tool in particularly uniform settings of lighting design if a simple, rough technique of illuminance quantification is desired.Steffy, LC, IES, FIALD, Gary 1963. "Architectural Lighting Design, 2nd edition" John Wiley & Sons, Inc., New York]
For a step-by-step guide, reference the IESNA Lighting Handbook [Rea, PH. D., FIES, Mark S., Illuminating Engineering Society of North America (2000), "The IESNA Lighting Handbook", 9th edition, 8-11 to 8-14] .
The lumen method is outlined in the table:Figure 1: Table 10.2 Lumen method Variables and Formulations (Steffy 2002).
As seen from Figure 1, the lumen method can be manipulated to output a particular variable. This is valuable because certain numbers are needed at different times in the design process. “Number of luminaires” is important because this number can be used to estimate costs and layout the spacing of luminaires in a computer lighting calculation program (Steffy 2002).
The CU value should be obtained by the manufacturer of the luminaire which is to be evaluated. In order to determine the CU on the manufacturer’s table, a room cavity ratio (RCR) must be used. Also, the
reflectanceof the ceiling, walls and floor must be known.
RCR = 5 x (room height) x (room width + room length) / [(room width) x (room length)] [Hughes, S. David (1988), "Electrical Systems in Buildings", 150, Delmar Publishers Inc]
Lumens per lamp should be obtained from the lamp manufacturer.
Light Loss Factorscan be calculated using methods in the IESNA handbook. Sometimes, individual companies have their own rule of thumb for Light Loss Factors. The ballast factor can be obtained from the ballast manufacturer.
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