Alliance for Lighting Information


Naomi Miller's Comments on the Efficiency of Full Cutoff Lighting

In response to a statement that it seems "counter-intuitive" that Full Cutoff lighting could be less efficient (i.e. MUST be more efficient), Naomi Miller wrote:

It seems intuitive that if you were to redirect the wasted uplight downward, that you would improve the efficiency of the system. Unfortunately, it seldom works that way. As Peter Ngai, an optical engineer for Peerless Lighting says, "if you work the light harder, you lose in efficiency." That is, when you use reflectors and lenses and baffles and optical devices to control the light so that it is directed into more useful zones, those optical devices absorb light, or block light, or cause the light to be bounced so many times that the intensity of the original light ray is reduced. The result is good: you get the light where it is needed, so the "desired" lumens-per-Watt are higher, but the "raw" lumens-per-Watt are lower. Energy efficiency, in classical terms, is measured in raw lumens-per-Watt. Even if we measure in "Downward Lumens-Per-Watt", you most often see that full cutoff fixtures are no higher in "downward efficiency" than their optically poor cousins. Does that mean that we don't want to use full cutoff fixtures for most outdoor applications? No, because there are so many other things to be gained: reduced glare, less light trespass, and less light pollution.

I understand your frustration. You want to be able to explain in simple terms that more responsible outdoor lighting equates to energy savings, so that you have an economic argument for municipalities. In my opinion, energy savings is the weakest link in the argument unless, as Jim Benya pointed out, you can reduce lighting in areas that are overlighted, you can reduce wattage by shifting from HPS to metal halide or compact fluorescent lamps since nighttime visibility and perceived uniformity seem to be improved for drivers and pedestrians, or you can eliminate lighting in some areas altogether.

I don't want to be an impediment in your arguments. I'm trying to support you with accurate information so that your arguments stand up better under scrutiny. Believe me, I also wish physics of light wasn't so stubborn.

used with permission


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