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Flickering Light Illusion Could Save 20% of Electricity Consumed

by Charles Q. Choi

Future lights and displays designed to exploit how the brain works could lead to dramatic energy savings, a new study says.

Neuroscientists have discovered a sensory phenomenon that allows a light to essentially flicker off for 13 percent of the time yet appears as bright to the human brain as if it was on the entire time. Their findings were released on Nov. 12 in the journal Proceedings of the National Academy of Sciences.

"This has the potential to lead to lights that save energy and that last longer because they don’t have to be on all the time for them to look as bright to people as they did before," says Stephen Macknik, a cognitive neuroscientist at the Barrow Neurological Institute in Phoenix.

Roughly a quarter of all electric power is used for lighting, according to the U.S. Energy Information Administration. If every light-emitting device adopted the flickering strategies the researchers are working on, Macknik estimates they could save 5 to 20 percent on energy, amounting to $4.7 billion to $19 billion or more saved each year in the United States.

"These results are interesting and will surely stimulate new discussions on how to find the best compromise between illumination quality and cost," says vision scientist Jose-Manuel Alonso at the State University of New York College of Optometry, who did not take part in the study.

Researchers have investigated the flickering of lightbulbs for as long as they have existed. “Thomas Edison studied flicker informally as part of his investigation of movie technology,” Macknik says.

Movies flash at least 24 still images per second at viewers. This flickering is fast enough to create the illusion of moving images. Edison found that lightbulbs that flicker 60 times a second appear to be constant light sources, Macknik says.

Macknik and his colleagues were investigating a mystery in visual perception surrounding what is known as Bloch’s Law. Decades of research has found that humans perceive that the brightness of a flash of light increases with how long it lasts, up to a plateau of about 100 milliseconds. For instance, a 10-millisecond flash will appear twice as bright as a 5-millisecond flash of the same actual brightness, but a 1,000-millisecond flash will appear the same brightness as one lasting 500 milliseconds.

Strangely, other studies discovered a phenomenon dubbed the Broca-Sulzer effect that contradicts Bloch’s Law. This effect finds that if you take a flashing lamp and speed up the rate at which it pulses, people will think it appears brighter as it gets faster and then dimmer again.

After years of research, Macknik and his colleagues found that perceptions of brightness were not only linked with how long a flash of light was, but also how long the period of darkness between flashes were. This factor was something that studies of Bloch’s Law and the Broca-Sulzer effect had not rigorously taken into account.

Their findings suggest that if pulses of light 67 milliseconds long were alternated with 10-millisecond periods of darkness, people would nevertheless perceive these light pulses as fused together and not as flickering. Previous research on visual perception of flickering typically kept the periods of light and dark equal in duration.

This effect is an illusion. It’s not an optical effect caused by the physical nature of light, as is the case with a pencil that looks like it’s bent when placed in a glass filled halfway up with water, which has to do with the different extent to which water and air bend light. Instead, “we’re talking about a sensory or cognitive illusion that depends on how the brain works,” Macknik says.

The brain regularly experiences illusions having to do with brightness. For instance, in the checker shadow illusion, one square on a checkerboard appears to be a darker color than another square even though both are exactly the same color. “This is something the brain does to make an object look the same even under very different lighting conditions,” Macknik says. “We discovered another way our visual system adjusts how bright something looks based on temporal dynamics—maybe something useful if in the flickering light under a jungle canopy.”

It remains uncertain what is responsible for the illusion that Macknik and his colleagues discovered, known as a constancy effect. “It’s got something to do with neurons in the brain and how they transmit information to each other,” Macknik says.

The researchers are now experimenting with lighting that uses their flickering strategies. “It’s always interesting to have basic science that you think is really cool but that no one else cares about that might turn out to have applications that can help people,” Macknik says. "We can save a significant amount of power by tuning lighting devices to work more efficiently with the human brain."

Top Image: Courtesy Flickr user Alca Impenne.


Charles Q. Choi
 has written for Scientific American, The New York Times, Wired, Science and Nature, among others. In his spare time, he has traveled to all seven continents, including scaling the side of an iceberg in Antarctica, investigating mummies from Siberia, snorkeling in the Galapagos, climbing Mt. Kilimanjaro, camping in the Outback, avoiding thieves near Shaolin Temple and hunting for mammoth DNA in Yukon.

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