August 18, 2011 | 12
At the SciFoo conference last weekend, brain scientist and illusionmeister Steve Macknik elevated a basic principle of energy conservation—turn off the lights when you don’t need them—to a whole new level. He showed how you can turn off the lights in a way that no one will even notice.
Right now, an AC light bulb turns on and off 50 to 60 times a second, or even faster for a modern ballasted bulb. If you could stretch out the off intervals, you’d save electricity. And if you could do it cleverly, taking the response of the human eye into account, the flickering would not only be imperceptible, but the quality of light might actually improve. “We’re taking advantage of the dynamics of the visual system,” Macknik says.
So far, most efforts to create greener bulbs have focused on the spectrum of light. Incandescent bulbs have long spectral tails, so even at their best, only about a tenth of their energy goes into useful illumination; most gets squandered as infrared radiation. (You can use this handy online calculator to see how much energy a glowing filament emits over different wavelength ranges.) The three main alternatives—fluorescents, LEDs, and a new technology called electron-stimulated luminescence—all emit light by causing a phosphor to glow at certain wavelengths, lopping off the tail. The downside is that spectral engineering takes a toll on color quality. It took me a long process of trial and error to find a good mix of bulbs for our house; I have a big box in my basement with all the CFLs and LEDs I’ve screwed in and back out again. New York Times columnist Bob Tedeschi recently offered his tips on how to match bulb to room.
Macknik and his colleague Susana Martinez-Conde, who work at the Barrow Neurological Institute in Phoenix and write a column for Scientific American Mind magazine, propose to explore not the spectral but the temporal dimension. The two of them have made a name for themselves coming up with visual illusions, and they think bulbs could exploit these tricks to shed more light with less power.
They start with the famous illusion that two identical gray dots or squares look different when you put them on different backgrounds. They find they can accentuate the difference by flickering the illumination at a certain rate. “You can make something brighter or dimmer if you change the temporal dynamics appropriately,” Macknik says. This result seems simple enough, but he says visual studies are prone to a lot of subtle biases; volunteers’ perception of light and dark can be skewed by the order in which the experimenters present their test scenes. It took years to come up with a controlled experiment that could tease out the optimal flicker rate.
This illusion can work even if you don’t realize the light is flickering. When a light turns off and back on within 20 milliseconds, it looks as if it never went off; retinal cells detect the change, but the stimuli get blurred as the brain processes them—an effect known as flicker fusion. Yet the gray patches still look brighter or dimmer.
To achieve the highest contrast, Macknik says a bulb would turn on for 70 milliseconds, off for 10, and repeat—an 87.5-percent duty cycle. Straight away, this would use 12.5 percent less electricity. You can wring out another 10 percent or so because the enhanced contrast would let you get away with a lower-wattage bulb.
The 12.5-hertz power cycle that Macknik proposes flouts conventional wisdom, which holds that such a low frequency would cause an unbearable amount of flickering. No one wants their bedside lamp to become a dance-club strobe light, which is one reason why power companies chose an AC frequency of 50 or 60 hertz to begin with. (If you still have a CRT computer monitor, try changing the refresh rate to see how low a rate you can stomach.)
Ah, but the AC current is a sine wave. Macknik’s claim is that a different waveform eliminates visible flickering even at very low frequencies. Such a waveform could be readily programmed into LED bulbs, which convert AC power to DC and could modulate the DC output as desired.
A 20-percent savings isn’t all that much, compared to the factor of four you get when changing from an incandescent to a CFL or LED. For me, the bigger lesson is that engineers can draw on the findings of neuroscience and behavior science to fine-tune technology to our needs.
Optical illusion courtesy of Edward H. Adelson
Demonstration of how flickering makes a gray circle look brighter or lighter. (c) Stephen L. Macknik & Susana Martinez-Conde, All Rights Reserved 1998, 2011