Invisible Teal
n the mythical Emerald City from the children’s classic “The Wizard of Oz”, everything seems to shine in radiant green. But appearances are deceiving: every visitor must wear tinted glasses – supposedly to protect against the city's dazzling brilliance, but in truth to maintain a magical illusion. Now, without any glasses but with a new laser-based technique, researchers at UC Berkeley led by Professor Ren Ng have managed to make people see a blue-green color that lies outside their normal color spectrum. In reference to the literary inspiration, they’ve named the technique “Oz.”
“The most saturated natural light looked pale by comparison”: That’s how Austin Roorda, Professor of Optometry and Vision Science at UC Berkeley, describes his experience with the new color called “olo.” He is one of the few people who have seen this blue-green hue with their own eyes and played a key role in developing the underlying technology. The results were published in April in the journal Science Advances.
The “Oz” technique allows researchers to precisely stimulate thousands of light-sensitive cells in the human retina using tiny pulses of laser light.
The “Oz” technique allows researchers to precisely stimulate thousands of light-sensitive cells in the human retina using tiny pulses of laser light. The essential ingredients: precise eye-tracking, adaptive optics to project the light, and software that synchronizes and controls both in real time. In this way, “Oz” can trigger neural signals that don’t occur in our natural color perception.
Our ability to see colors is thanks to special photoreceptors in our retinas: the cones. There are three types – each specialized for a different part of the visible spectrum. S-cones (for “short”) respond to short wavelengths, M-cones (for “medium”) to mid-range, and L-cones (for “long”) to long wavelengths. Blue light falls on the short-wavelength end of the visible spectrum; red light on the long-wavelength end.
Stimulating M-cones also tends to activate S- or L-cones because the sensitivity ranges of the different cone types overlap. (Source: Wikipedia / Public Domain)
However, stimulating M-cones also tends to activate S- or L-cones because the sensitivity ranges of the different cone types overlap. This overlap allows the brain to compare signals and perceive more nuanced colors.
But this also makes it almost impossible to stimulate M-cones in isolation – until now. For the first time, the “Oz” technique makes it possible to almost exclusively activate M-cones. The result: a completely new color that doesn’t fit into the familiar color spectrum.
0 parts L, 1 part M, 0 parts S – that’s how the melodic name for the color “olo” was born.
“If you write the coordinates (0, 1, 0) on paper and squint your eyes a bit, the 0 and 1 look like an ‘o’ and ‘l’.” explains James Fong, the lead author of the Science Advances paper. “I originally came up with this name as a fun exercise, but everyone in the lab ended up really enjoying it.”
Although the researchers haven’t yet managed to stimulate the M-cones entirely in isolation, the new technique did succeed in producing a previously unknown color perception in test subjects. “This is exciting in its own right,” says Fong.
Test subject Austin Roorda sitting in the “Oz” system. (Photo: Austin Roorda)
To classify and measure this perception, the three test subjects compared “olo” to a single-color laser light that came as close as possible to the perceived hue: a teal that already appears intense on screens – but is even more vivid as laser light. Yet even this seemed surprisingly dull compared to “olo.” “The difference was stunning,” says Austin Roorda, one of the test subjects. “To make a match of ‘olo’ to the natural light, we had to dilute it with a white light.”
While the new color itself has no direct application yet – and it’s not entirely certain that no one has ever perceived it before – the real goal of the experiment was to demonstrate that the “Oz” technique works. And that, they’ve achieved – despite some remaining technical hurdles.
The biggest challenge: some light still reaches adjacent cells and activates them unintentionally. The team aims to minimize this in the future. In addition, cones located along the line of sight can’t currently be targeted. These cones are especially small and densely packed, which makes their spatial mapping difficult – a prerequisite for targeting them with the laser. As a result, the “Oz display” currently only appears off to the side of where you are looking, in an area about four times the size of the full moon in the night sky.
The researchers are now working to further develop the “Oz” technique. As they refine the technology, their long-term goal is to explore broader applications in vision science. “We're planning on using this Oz platform to study other aspects of human vision, such as the possibility for treating color blindness or studying retinal degeneration diseases,” says James Fong.
Original publication:
Fong, James et al.
Novel color via stimulation of individual photoreceptors at population scale
Science Advances, Vol 11, Issue 16
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