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15/09/2008
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Embedding an optical system in a lens can now help people suffering from reduced vision. Tom Shelley reports
Many people suffer from reduced clarity of vision that does not make them anywhere blind – but makes it hard to read road signs, for example.
In 36 states of the USA (but not in the UK), people are allowed to drive cars using ‘bioptics’: mini telescopes that hang just in front of one of the spectacle lens, which can be glanced into to read something detailed. Yet they are so ugly that many people will not wear them. More worryingly, when used to look at a magnified image, the rest of the field of view in its vicinity is obscured.
However, Dr Eli Peli and Dr Fernando Vargas-Martin, working together in the Schepens Eye Research Institute in the Harvard Medical School, have come up with a solution, published in the Journal of Biomedical Optics.
This is a combination of ideas, based on astronomical telescopes and periscopes, that embeds an optical path on its side within the thickness of a spectacle lens, so that light coming through that lens in the normal manner is not interfered with.
After several design iterations – involving software simulation and ray tracing, based on Zemax software, along with hardware experimentation – a final prototyped design has emerged, embracing spherical mirrors, polarising beam splitters and quarter wave plates.
The first beam splitter reflects the light entering the carrier lens towards a concave mirror. After reflection and convergence, the light passes through the beam splitter, through the carrier lens, forming an intermediate image plane, and proceeds through the second beam splitter. It then reflects of the second spherical concave mirror – and is then reflected by the second beam splitter into the wearer’s eye. This arrangement provides a Keplerian, astronomical, reversing telescope with significant light loss, due to the four passes through the beam splitters. However, this light loss can, say the authors, “be substantially recovered” using polarising beam splitters and quarter wave plates.
This would reduce light loss to 80%. This may sound bad, but this can easily be accommodated by human vision. Vision multiplexing – allowing the user to see the whole scene, as well as the magnified image – is achieved by tilting the beam splitter cube. Field of view is about 50% wider than conventional bioptic devices available.
The final prototype accommodates all the components in an 8mm thick carrier lens. While the whole concept is still being developed, it has been possible to embed a telescope with a magnification of about three times within a spectacle lens. It also shows a way of building very compact optical systems within each other. The authors believe their in-lens telescope has the potential to be mass-produced as a commodity ophthalmic lens blank that can be surfaced to include the wearer’s spectacle prescription.
Other obvious applications for the technology would be for those who need to have optical sighting systems in front of their eyes, without interfering with the rest of their vision – such as fighter pilots. It might also be attractive to the games and entertainment industries.
Pointers
* The entire telescope optical system is turned on its side and embedded within the thickness of a carrier lens 8mm thick
* The solution gives a magnification of about three times when the telescope section is glanced through, without interfering with the ability to see the rest of the scene
* A technology has been developed to produce the lenses that the inventors believe have the potential for mass production
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Author
Tom Shelley
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