Today’s focus on saving energy is accelerating demand for luminaires designed to achieve a high level of energy efficiency. Luminaires can be evaluated based on luminaire center beam candlepower (candela), total input watts (W), efficiency (fraction of lamp lumens that exit the luminaire), efficacy rating (lumens/W), coefficient of utilization (CU) and comparative yearly energy cost of light ($/1000 lumens).
While these metrics provide valuable tools for comparing the efficiency of luminaires, it is important to remember that efficiency is only part of the story of a lighting product and should be considered along with how the luminaire distributes the light and at what intensity. This will result in selection of luminaires that are both efficient and likely to achieve good visual comfort.
Luminaire efficiency is the ratio of light output emitted by the luminaire to the light output emitted by its lamps. Another way of looking at it: Luminaire efficiency is the percentage of light output produced by the lamps that are in turn emitted by the luminaire.
Not all light produced by the lamps will exit the luminaire; some will remain trapped inside and dissipate as heat. The luminaire’s physical characteristics will affect how much light will exit and how much will be directed at the workplane.
Luminaire efficiency is important because while you can have a very efficient lamp-ballast system, if the luminaire itself is not efficient at delivering lumens, then the lighting system overall is not either. Factors that affect the efficiency of the luminaire include its shape, the reflectance of its materials, how many lamps are inside the luminaire (and how close they are to each other), and whether shielding material such as a lens or louver is used to soften or scatter the light.
While a high level of luminaire efficiency should be valued, overemphasizing it can lead to poor lighting quality and angry users. After all, a bare lamp offers 100 percent efficiency, but is hardly a good choice. In reality, the most “efficient” luminaires are often candidates for direct glare, particularly unshielded luminaires with direct distribution at lower mounting heights typically found in offices, classrooms and similar applications. In such cases, light may exit the luminaire very efficiently, but the luminaire itself is a “glare bomb,” and users are likely to resort to wearing baseball caps.
Luminaire efficacy describes the efficacy of the entire luminaire, including the light source, ballast and luminaire losses. The Luminaire Efficacy Rating (LER) provides a metric for comparing the relative energy efficiency of fluorescent luminaires. Initiated in response to the Energy Policy Act of 1992, LER offers a voluntary rating standard for several categories of commercial and industrial fluorescent luminaires such as 2×4 recessed lensed and louvered luminaires, plastic wraparounds and striplights (see NEMA LE 5-2001 for more information).
LER is expressed:
LER = [Luminaire Efficiency (EFF) x Total Rated Lamp Lumens (TTL) x Ballast Factor (BF)] ÷ [Luminaire Watts Input]
Some manufacturers publish LER in their products’ photometric reports and specification sheets. Even without it, designers can easily calculate LER themselves as the information required by the above formula should be generally available for the product.
Efficacy is not the entire story of a luminaire. In the above drawing, the left luminaire operates at an efficacy of 28 lumens/W, while the right luminaire generates about 50 percent more light output for about 33 percent more wattage, resulting in a 14 percent higher efficacy. The right luminaire accomplishes this gain, however, through a lack of control of glare. Drawing by Kevin Willmorth.
The coefficient of utilization (CU) metric allows us to look at luminaire efficiency in the context of the actual application. Since all room surfaces are potential reflectors of light, the room itself acts an extension of the lighting system. A given luminaire may emit some of its light directly at the workplane and some at a nearby wall. The wall absorbs some of the light and reflects the rest, some of which in turn reaches the workplane.
CU therefore allows us to compare luminaire efficiencies in a given environment. It shows the percentage of light output produced by the lamps that reaches the workplane after light is lost due to the luminaire’s efficiency at transmitting light, the room proportions, and the ability of room surfaces to reflect light.
Luminaire manufacturers provide CU tables for their products in the photometry report and associated IES files downloadable for design calculations and analysis using software. As Average Maintained Light Levels (fc) = (Lumens x CU x Light Loss factors) ÷ Area (sq.ft.), CU can have a big impact on the capacity needs for a given lighting project and hence both its capital and operating costs. CU shows how changing room finishes can affect light levels.
The Comparative Yearly Energy Cost of Light is another luminaire comparison metric created in NEMA LE 5-2001 in response to the Energy Policy Act of 1992. It is expressed as a $/1000 lumens value based on the below formula:
Energy Cost = (K/LER) x 1000 Lumens
Where K = $0.24/W [(3,000 average operating hours per year x $ 0.08/kWh average energy cost) ÷ 1000]
Specifiers should be prepared to make adjustments as needed to tailor the formula to their project. The operating time averages to about 8 hours per day and be adapted easily. The $0.08 per kWh cost is outdated as a national average and can also be adapted. As of October 2009, according to the Department of Energy, the national average cost per kWh of electric energy was $0.1022 for commercial buildings, increasing K to $0.31/W, and $0.0668 for industrial buildings, reducing K to $0.20/W. For the latest national averages and specific regional and even more specific state averages, click here.
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