“There’s no light in nature that can only stimulate the M-cones,” said Austin Roorda, a professor of optometry and vision science at the School of Optometry at Berkeley. 

Roorda explained that the human eye has long, middle and short wavelength-sensitive cones called L, M and S cones. The M cones are sandwiched between the L and the S cones, so when the M cone is tickled, so are the L and the S. 

The study involved a team of researchers, including Ren Ng, a professor of electrical engineering and computer sciences at Berkeley. Their collaboration began years ago when Ng asked Roorda, “What would happen if we delivered light to thousands of M cones only? Would it be the greenest green you’ve ever seen?”

So it’s essentially isolated green or “pure” green.