It's Not Easy Being Green

Jul 1, 2008 12:00 PM, By Carla Gallina, Lighting Designer

What it takes to achieve a truly green lighting design

There are many ways of evaluating “green” design. Green architecture includes specification of renewable, local, and nontoxic materials, enhanced envelope design, and daylight strategies. Green electrical and mechanical systems are evaluated on energy performance and rewarded for energy reduction. When it comes to lighting, there is much more to going green than simply reducing energy. In an effort to shave watts and evaluate efficacy, designers may overlook an obvious — or perhaps not so obvious — incandescent lamp choice; namely, the 37W/ MR16/ IR. This powerful and efficient halogen source has commercial and retail applications that may make it the lamp of choice for green designs, including those seeking Leadership in Energy and Environmental Design (LEED) Platinum certification.

This historic pedestrian bridge in Hanover, Minn., is lit by 37W MR16 60° flood lamps.This historic pedestrian bridge in Hanover, Minn., is lit by 37W MR16 60° flood lamps.

At minimum, energy code compliance and LEED certification require evaluating lighting system performance based on watts per square foot, a metric that is oblivious to light quality, system performance, embodied energy, and environmental waste. If a lighting system meets energy requirements as measured by a watt-hour-meter but is of poor light quality, introducing glare and providing inadequate light levels, is it truly green? What if it is difficult and expensive to operate and maintain, doesn't account for the actual power delivered to the lighting system, or contains toxic waste and throw-away electronics? Is it sustainable?

To help answer these questions, we'll make two comparisons. First, let's look at the system operation of 37W/MR16/IR/FL35 lamps and PL-T 42W compact fluorescent lamps (CFLs) installed in commercial downlights. Then, let's weigh the advantages of the 35MRC/IRC/NFL24 halogen relative to the PAR38/CDMI/FL25 integrated ceramic metal-halide (M-H) lamp installed in an open lamp holder or mono-point. Conclusions are based on a macro analysis of sustainability, which includes trading energy performance for operational performance and re-examining the way we gauge energy efficiency relative to delivered light.

Performance metrics

Efficacy, as defined by the Illuminating Engineering Society of North America (IESNA), New York, equals lumen per watt and is a preliminary measure used to compare the energy value of various light sources. However, lamp efficacy is not a constant. When a lamp is installed in a luminaire, the efficacy is reduced by the luminaire efficiency. As the lighting system ages, its efficacy degrades further. Discharge lamps — metal halides, in particular — suffer significant losses in lumen output as they age, yet the energy input remains constant. At 40% of rated lamp life, M-H lamps nominally provide 65% of initial lumen output, and fluorescent lamps nominally provide 85% of initial lumen output. The efficacy of a halogen lamp remains relatively constant, losing only about 5% of initial lumen output by end-of-life.

Table 1. Candlepower distribution of a lamp provides precise measures of light intensity and distribution.

The candlepower distribution of a lamp or luminaire provides precise measures of light intensity and distribution, whereas efficacy provides little usable information regarding actual performance. To illustrate this simply, consider two 60W PAR38 halogen lamps from a single manufacturer: one 25° narrow flood (NFL25) and one 30° flood (FL30). The manufacturer's data rate both lamps at 850 lumens, and both lamps have the same efficacy. When calculated at 5° intervals, however, the NFL25 delivers 11,658 total candlepower with a center beam candlepower (CBCP) of 3,909, and the FL30 delivers 9,598 total candlepower with a CBCP of 2,643. If you consider candlepower per watt in place of lumen per watt, the NFL25 significantly outperforms the FL30 at a distribution range up to 40° vertical (Table 1). Light intensity measured in candela is the standard metric used in point-by-point lighting analyses and is used for evaluating lamp and luminaire performance in this article.