An incandescent light bulb is very efficient, 80% efficient in fact. Really.This is because the amount of light output per unit of energy is commonly referred to as ‘efficiency,’ but that is a misnomer. The amount of light put out per unit of energy is actually a measure of efficacy. An incandescent bulb is not very efficacious.The difference between efficiency and efficacy is representative of the many issues that get ignored when discussing lighting and energy. But failing to understand this difference or the other nuances of lighting and energy, can turn what looks like great energy savings in theory into a poor investment.Returning to our incandescent lightbulb, it turns 80% of the input electricity into radiant energy. But while efficient, an incandescent blub is not efficacious. Of the energy it radiates, most of it is invisible infrared energy. Per unit of energy it sucks in, it produces very little useful, visible light.You can think of the difference between efficiency and efficacy as the difference between being efficient –getting a lot of work done in a time-constrained situation— and being effective, i.e. getting a lot of useful work done.A great simple LED lightbulb today puts out 80 lumens per Watt. Compact fluorescents are also very good at 70 lumens per Watt and often cheaper. Both are much better than the 20 lumens per Watt found in an incandescent light bulb.But it is the ever higher levels of efficacy for lighting in the future that is most exciting. Some LEDs for office buildings are now achieving 130 lumens per Watt. Several companies are rolling out smaller light bulbs that are hitting 200 lumens per Watt.Twenty to 80 to 130 to 200…. how high can it go?683.The reason why stems from the difference between how much energy a lightbulb puts out and how human beings perceive light, a measurement called photometric intensity.Unlike a Watt, which is a unit of energy independent of human beings, a lumen, is a human-eye-centered measurement of light. A lumen is defined as the amount of light put out by one standard, old-fashioned candle (and if you want to get extremely technical a lumen is “the amount of light put into one steradian by one candela”). But it is about the experience of light in our eyes and brains. It does not correlate directly with energy.And our eyes have different levels of sensitivity to different colors of light. We are more sensitive to green light than to red light, for example, so one lumen of green light is more useful to us than one lumen of red light.Scientists have discovered that in well-lit conditions we are most sensitive to light that is 555 nanometers in wavelength. This happens to be an intense neon green light—like those newer pedestrian crossing street signs.The amount of energy it takes to produce one lumen of light at the 555 nanometer neon green color to which we are most sensitive is 1/683^rd of a Watt. So a light source that is 555 nanometers in wavelength and that uses one Watt of power and is 100% efficient will produce 683 lumens of light. Hence, our 683 lumens per Watt maximum.What about a redder light? One Watt of 700 nanometer red light, for example, would generate only 3 lumens compared to 683 for the green. It is far less effective.Of course, most of us wouldn’t want to buy a light that only produces an intense neon green no matter how much energy is saved. There are many factors to consider when selecting lighting: how long will the lights last, what is the color shift over time, what is the color rendering index (CRI) of each light, etc. Most commercially available lights combine many different wavelengths to produce a more pleasant light color.

In the quest for most effective light, in finding the most energy sipping light, the ultimate milestone is 683 lumens per Watt.