People usually consider walking on water or in thin air a miracle. But I think the real miracle is not to walk either on water or in thin air, but to walk on earth. Every day we are engaged in a miracle which we don’t even recognize: a blue sky, white clouds, green leaves, the black, curious eyes of a child — our own two eyes. All is a miracle.” — Thich Nhat Hanh
Suppose you owned a construction company and, along with it, some big, shiny dump trucks for hauling materials. Now suppose you’re constructing a building, starting from the foundation up. So you hire some drivers to haul sand from the sandufacturer (you know, the place where they manufacture sand) to the construction site so it can be mixed with some cement to pour the foundation.
Back at the sandufacturer, the dump truck is filled with 100 tons of sand (I told you it was a big dump truck) and the driver happily heads over to the construction site. But when he gets there, there’s only 3.5 tons on sand in the back of the truck. And behind him is a very long, thin line of sand stretching for miles all the way back to the sandufacturer. It turns out that those big, shiny dump trucks have a leak and only about 3.5 percent of the sand makes it to the construction site. It’s enough to drive you to the sanditorium.
This scenario may sound ridiculous to you, but you witness it every day. Except it’s not sand we’re talking about, but electricity.
Every time you turn on an incandescent lamp you’re seeing about 3.5 percent of the energy you’re paying for to power it up. The other 96.5 percent is the long, thin line going back to the power plant, except you can’t see it. You can, however, feel it. It can be felt in the heat emanating from the back of the fixture, as well as the heat in the lamp socket, connectors, wires, dimmer, feeder transformer, transmission lines and the power plant.
Most of the energy is lost at the point where the current passing through the filament is converted to light and heat. It seems that Edison’s lamp (at least some of which can be attributed to Joseph Swan) makes a better heater than it does a light. But it’s hard to beat the beauty of the warm glow of an incandescent source, so we continue to use the dump truck with the hole in the bed.
Still, we have a duty to be responsible with our resources, whether that means being economically responsible and/or ecologically responsible. And while we have a desire to create the best possible look, we owe it to our customers to make judicious choices based the needs of the client fueled by our knowledge of light, electricity and energy.
How, you might ask, can we make better choices when it comes to light sources? Glad you asked.
Today, a lighting designer has the widest range of options than ever before and those options are growing. When we have an area to illuminate, we can now choose between incandescent, discharge, fluorescent, electroluminescence and LED sources. Each has their own unique characteristics, applications, assets and drawbacks. One of those characteristics is the efficacy or the lumens per watt. It’s basically a measure of how much visible light it produces for a given amount of electricity. Here is a comparison chart that shows the lumens per watt for several types of light sources.
Light Source Efficacy
HPL 750/115 29.2 lumens per watt
HPL 575/115/LL 21.5 lumens per watt
HPL 575/115/SL 28.7 lumens per watt
MSD 575 78.3 lumens per watt
MSR 575/2 85.2 lumens per watt
MSR 1200 Short Arc 91.7 lumens per watt
CDM150SA 86 lumens per watt
Compact Fluorescent Lamps (CFL) 45-60 lumens per watt
Red LEDs Up to 46 lumens per watt
Red-Orange LEDs Up to 53 lumens per watt
Orange LEDs 18-22 lumens per watt
Yellow and amber LEDs 34-35 lumens per watt
Green LEDs 58-79 lumens per watt
Blue-Green LEDs 50-75 lumens per watt
Blue LEDs 17-26 lumens per watt
White LEDs As high as 98 lumens per watt
Low Pressure Sodium Up to 200 lumens per watt
Before we all run out and start specifying low pressure sodium lamps for the stage, we might want to take into consideration that their color rendering index (CRI) is zero! That’s not a typo and it’s not a joke. The CRI is 0. These babies are typically used where energy efficiency is important but color rendering is not, like for street lighting.
But the lamp efficacy only tells part of the story. The luminaire efficacy should also be taken into account. The luminaire efficacy is the percentage of visible light coming out of the fixture compared to the visible light produced by the source. In the most basic terms, it’s a measure of how much light from the source can be gathered and redirected to the output of the luminaire. Luminaires can vary wildly in terms of efficacy, from as little as about 10 percent in a Fresnel in a spot focus to about 65 percent in an ellipsoidal reflector spotlight with a wide lens.
Of course, we might want to consider all the other characteristics, those little things like cost, size, weight, color temperature and CRI. In many cases, these other factors outweigh the efficiency considerations simply because they are immediately observable while the efficiency is hidden and the impact is delayed. By the time the first electric bill arrives the lighting team is usually long gone.
Look closely and you’ll find that the line of sand leading back to the electric plant also trails back to the designer. We can’t completely plug the energy leaks, but in many instances we can make them smaller.
