Exterior Lighting: Setting LED Upgrade Goalstest

By Thomas A. Westerkamp

June 2017

Setting goals

When planning upgrades of exterior lighting systems, it will become increasingly important for managers to pay close attention to a host of project goals and considerations.

Metrics. They will need to use additional metrics as the watt measure declines in usefulness. The foot-candle measures the amount of light a source produces. The lumen is the degree to which a light source illuminates an object some distance away. The lux — the English measure of lumens — expresses lumens per meter squared, or illuminance over a larger area, one square meter.

The higher a light source’s incandescent wattage, the greater its illumination. But now that other sources, including compact fluorescent lamps (CFL), light emitting diode (LED), and induction lamps, provide superior energy efficiency, the lumen will become the unit of measure. Watts become an important measure when comparing energy cost of different options with the same lumens.

One of the most energy-efficient types of lighting is the LED. As the incandescent and fluorescent lamp phaseouts continue, these other options will replace them at much lower operating cost.

Efficacy. Luminous efficacy is a measure of lighting energy efficiency. To calculate efficacy, managers can divide lumens by rated watts. The LED bulb has the highest efficacy rating and is the most energy efficient, providing the same lighting while consuming the fewest watts.

Color. The color of light can have a distinct effect on its quality. Cool, white fluorescent lights reduce the natural color perceived by the eyes as compared to incandescent lights. Actors do not use cool white fluorescents in their makeup table lighting because of this tendency to distort true color.

Safety. Managers need to understand the impact of a retrofit on the safety of occupants and visitors. The safest outdoor lighting minimizes shadows and glare around buildings and parking lots. It directs most of the lighting downward, and it minimizes wasted upward lighting.

Life. Several factors can cause electric light sources to fail, including faulty electrical components, excessive heat, corrosion inside the lamp, and lumen depreciation. Lumen depreciation is the gradual decrease in lumen output that occurs over the lamp’s life cycle. Incandescent lamps typically last 1,000-2,000 hours and average 1,200 hours, and they lose 10-15 percent of their initial light output before burning out completely.

A CFL lamp lasts 8,000-2,000 hours and loses 10-15 percent of its output before burning out completely. Linear fluorescent lamps typically last 5,000-40,000 hours and lose 5-10 percent of their original lumen output before they fail and develop flickering as lamps and ballasts begin fail.

LEDs do not burn out suddenly in the same way as incandescent or fluorescent sources. Their lumen output continues to decrease gradually over time.

Many LED A19 replacement lamps are rated to last 25,000 hours or more before they lose 30 percent of their initial light output. Also, testing indicates the diodes in these products might maintain useful light output longer than these estimates — up to 50,000 hours. It is possible to replace the diode and LED lamp, but replacement of the fixture and lamp assembly also might be required. Managers should follow the best practices recommended by the lamp manufacturer or lighting engineer in order to maximize the life of LED lighting.