Upcoming Roadway Projects? Forget Watts and Think Lumen Output

May 17, 2017

For Best Results When Specifying LED Roadway Projects, Forget Watts and Look to Lumen Output

Lighting professionals who specify LED roadway replacement projects often make the mistake of referencing old high-intensity discharge (HID) wattage when establishing lighting values. The fact is that wattage is not the way to specify LED requirements. LED systems should be installed in a manner that provides optimum light levels at the lowest cost per kilowatt, and this requires using a new metric. Today, specifying the LED lighting system based on lumen output helps ensure that the specification is written around the best LED luminaire that meets project requirements and delivers the right amount of light.

Here’s an example of how relying on wattage instead of lumen output can lead us astray.

A DOT project calls for replacing outdated 400-watt high-pressure sodium fixtures with an energy-efficient, low-maintenance LED system. The DOT requests through its specifier the “LED equivalent” for the 400-watt HPS fixture.

As a side note: A 400-watt HPS luminaire initially supplies about 50,000 lumens. The rule of thumb is to reduce the HPS lumen output by half to determine the LED lumen output, which would be 25,000 lumens in this case.

If wattage is used as the basis for selecting the LED system, then it’s likely a 209-watt LED, cobrahead-style luminaire with IES Type II distribution would be specified.

Now here’s where a problem can arise: Let’s say two manufacturers offer luminaires that attempt to meet the specifications of 209 watts. One luminaire delivers 209 watts and 25,053 lumens, and the second supplies 210 watts and 23,780 lumens. If the latter luminaire is installed, it could under-light the area.  

On the other hand, by selecting the luminaire by the lumen output (25,000 in this example), then the right amount of lighting is delivered and the wattage then falls into place. 

This lumen output approach is preferable because few LED luminaires produce the same lumen output at the same wattage. Different chips, LED boards, optics and thermal materials impact how much light an LED luminaire delivers.

Leveling the playing field by specifying luminaires based on lumen output rather than wattage allows the end user to select the luminaire that best meets application requirements.

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