There are only so many colors that the typical human eye can see; estimates put the number just below 10 million. But now, for the first time, scientists say they’ve broken out of that familiar spectrum and into a new world of color.
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In a paper published in Science Advances, researchers detail how they used a precise laser setup to stimulate the retinas of five participants, making them the first humans to see a color beyond our visual range: an impossibly saturated bluish green.
Human retinas contain three types of cone cells, photoreceptors that detect the wavelengths of light. S cones pick up relatively short wavelengths, which we see as blue. M cones react to medium wavelengths, which we see as green. And L cones are triggered by long wavelengths, which we see as red. These red, green and blue signals travel to the brain, where they’re combined into the full-color vision we experience.
But these three cone types handle overlapping ranges of light: the light that activates M cones will also activate either S cones or L cones. "There’s no light in the world that can activate only the M cone cells because, if they are being activated, for sure one or both other types get activated as well," says Ren Ng, a professor of electrical engineering and computer science at the University of California, Berkeley.
Ng and his research team wanted to try getting around that fundamental limitation, so they developed a technicolor technique they call "Oz."
"The name comes from the Wizard of Oz, where there’s a journey to the Emerald City, where things look the most dazzling green you’ve ever seen," Ng explains. On their own expedition, the researchers used lasers to precisely deliver tiny doses of light to select cone cells in the human eye. First, they mapped a portion of the retina to identify each cone cell as either an S, M or L cone. Then, using the laser, they delivered light only to M cone cells.
It wasn’t exactly a comfortable setup. "This is not a consumer-oriented device, right? This was a basic visual science and neuroscience project," Ng says. In fact, the researchers experimented on themselves: three of the five participants were co-authors of the study. The two others were colleagues from the University of Washington, who were unaware of the purpose of the research.
Ng himself was one of the participants. He entered a darkened lab and sat at a table. "There were lasers, mirrors, deformable mirrors, modulators, light detectors," Ng says. There, he had to bite down hard on a bar to keep his head and eyes still. As the laser shone into his retina, he perceived a tiny square of light, roughly the size of a thumbnail viewed at arm’s distance. In that square, he glimpsed the Emerald City: a color the researchers have named "olo."
What, exactly, did olo look like? Ng describes it as "blue-green with unprecedented saturation" — a perception the human brain conjured up in response to a signal it had never before received from the eye. The closest thing to olo that can be displayed on a computer screen is teal, or the color represented by the hexadecimal code #00ffcc, Ng says.
If you want to try envisioning olo, take that teal as the starting point: Imagine that you are adjusting the latter on a computer. You keep the hue itself steady but gradually increase the saturation. At some point, you reach a limit of what your screen can show you.
You keep increasing the saturation past what you can find in the natural world until you reach the limit of saturation perceptible by humans—resulting in what you’d see from a laser pointer that emitted almost entirely teal light. Olo lies even further than that.
Ng’s team dreams of one day building screens that can scan your retina to display perfect images and videos by delivering light to individual cones—enabling crisp, nonpixelated visuals in impossible colors. "That’s going to be extremely hard to do, but I don’t think it’s out of the realm of possibility," Ng says.
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