“Going Beyond the Lumen?” by Jim Brodrick

Republication of Postings from the U.S. Department of Energy (DOE) Solid-State Lighting Program by Jim Brodrick, SSL Program Manager, U.S. Department of Energy

Light sources are typically evaluated in terms of how much light they produce (in lumens) relative to how much power they draw (in watts). From lighting designers to building owners to consumers, we’re encouraged to choose the most efficacious light sources appropriate for the task at hand — those with the highest lumens per watt (lm/W). The numerator of that expression is a measure of light output, based on human visual sensitivity under standard viewing conditions. That makes sense: we want to know how much light the source is producing that will help humans to see and function in the world.

But what we’re also starting to understand is that light has other impacts — and potentially great benefits — that are unrelated to human visual sensitivity. In fact, these other benefits, such as improving productivity, have been receiving an increasing amount of attention. But many potential benefits, especially those that depend on the ability to tune the spectrum of SSL products, can’t be accurately conveyed solely in terms of lumens — which means that we may end up needing additional metrics in order to do them justice.

That’s because the lumen is a measure of light output, where emitted light of various wavelengths is weighted in accordance with humans’ visual sensitivity to those wavelengths. But as we’re still learning, the effect of light on humans (and other living creatures) extends well beyond enabling us to see. Researchers have discovered that the human eye contains at least five different types of nonvisual photoreceptor cells — that is, cells that detect light but that play no role whatsoever in visual perception. We’re still learning about the various functions of such cells, but we know that one thing they do is help regulate the production of melatonin, a hormone that plays a key role in regulating the sleep-wake cycle and affects our health in many other ways as well.

We also know that the peak sensitivity for melatonin suppression is for light at around 464 nm, which is in the blue range. And because LEDs — far more than other light sources — are highly tunable, they can be carefully tailored to adjust their emission around 464 nm, or in any other portions of the spectrum the manufacturer desires. They can also be made to be tunable by the user, so that the spectral power distribution can change with the turn of a dial or the touch of a keypad, in accordance with whatever effect is desired.

While much more research is needed to understand how this spectral flexibility can best be used, the potential benefits range from normalizing our circadian rhythm, to promoting relaxation, to improving mood and concentration, to speeding convalescence, to promoting the optimal growth of plants and animals. Further, spectrally adjusted light can help protect sensitive wildlife that live in areas where outdoor lighting is used. And all of this is to say nothing about visual value that’s not captured by the photopic lumen — such as tuning the spectrum to emphasize certain colors or provide contrast for retail, inspection, and high-value fine-manual work applications such as surgery.

Given that we now have a type of light source that can be spectrally tailored to suit our needs, wants, and whims — visual and otherwise — and given that our knowledge of the nonvisual functions is growing by leaps and bounds, it stands to reason that we may need to more carefully consider additional metrics to capture the full range of gifts SSL has to offer, and develop new metrics as science advances.

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