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Researchers in Israel have made a new type of contact lens that can correct a form of red-green colour blindness known as deuteranomaly. By incorporating plasmonic metasurfaces into standard contact lenses, the researchers were able to restore lost colour contrast and improve colour perception by up to a factor of 10.
Humans can typically distinguish more than a million colours, but for some, colour perception is limited in certain ranges of the electromagnetic spectrum. In these individuals, the response of the light-sensitive cone photoreceptor cells at the back of the eye is attenuated when excited with a specific wavelength of light.
In deuteranomaly, for example, signals from the cells that are sensitive to green-yellow light (known as medium-type cone photoreceptors) are dulled. This means that the brain receives too many signals from longer wavelengths associated with yellow-red light. The result is that people with this form of colour blindness struggle to tell red and green wavelengths apart. Although special glasses that reduce perception of yellow-red light are available, they are bulky and uncomfortable to wear.
Artificially-engineered thin metallic films
Researchers Sharon Karepov and Tal Ellenbogen from Tel Aviv University have now transferred metasurfaces – artificially-engineered thin metallic films that can be fine-tuned to interact with light in very specific ways – onto the surface of commercially-available contact lens to achieve the same filtering capability.
The metasurfaces work by exploiting the physics of plasmons, which are quasiparticles that arise when light interacts with the electrons in a metal and makes them oscillate. The shape, size and arrangement of the nanoscale structures – in this case, a 40-nm-thin film of nanosized gold ellipses – within plasmonic materials makes it possible to support plasmons at specific frequencies. By thus adjusting these structural parameters, the researchers can control which frequencies of light the material will absorb and scatter.
From flat to curved surfaces
Since metasurfaces are usually fabricated on flat surfaces, Karepov and Ellenbogen needed to develop a technique to transfer them onto the curved surface of a contact lens. Their new fabrication process opens the door for embedding these materials into other non-flat substrates as well, they say.
By testing the optical response of the metasurface at every stage of the new fabrication technique and imaging its structure, the researchers confirmed that its light manipulation properties did not change after transfer to the curved surface.
Lab-grown human retinas shed light on colour vision
Factor of 10 improvement in colour perception
They then simulated how a wearer of their new nanostructured contact lens would perceive colour using standard tests based on Commission International de l’Eclairage (CIE) colour spaces and conventional models of human colour-sensitive photoreceptors. They found that the device could shift incorrectly-recognized colours closer to the original hues and that lost visual contrast in red-green colour blindness could essentially be restored (see image). Indeed, they measured an improvement of up to factor of 10 in colour perception. An Ishihara-based colour blindness test (the most well-known colour perception test for red-green colour deficiencies) also confirmed contrast restoration.
While the new lens still needs to pass clinical-stage tests, the researchers say that manufacturers could potentially embed the metasurfaces during the moulding stage of contact lens fabrication or thermally fuse them to a rigid lens. They plan to continue improving their metasurface transfer process and test it for other applications too.
The present work is detailed in Optics Letters.