The eye is a simple and robust optical instrument that is fully adapted to serve our visual system. Although the optical quality is not as good in the eye as in the best artificial optical systems, it matches what is required by most of the visual capabilities. There are also a number of compensating mechanisms in the visual system that renders some of the potential optical limitations as invisible. For instance, the large potential deleterious effect of chromatic defocus is limited by proper colorfilters and the band-pass spectral sensitivity.
An interesting discussion in the last decades has been the possibility to correct for the aberrations of the eye using adaptive optics [12]. This is now technically possible in the laboratory and also, although partially, with correcting devices such
. Fig. 12.11 Retinal images of a letter“E”in a normal eye for different angles at the retina, fovea, 10, 20, 30, and 40. At larger eccentricities, the off-axis aberrations further degrade the quality of the retinal images
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as intraocular lenses. The correction of the eye’s aberrations may improve vision in some subjects but there are fundamental limitations that cannot be surpassed. The first is the sampling of the retinal images by the photoreceptors. Even if sharp images are projected into the retina, the smallest letter to be perceived will require several photoreceptors across to be properly interpreted..Figure12.12shows this schematically at a correct scale. Images of letters smaller than those corresponding visual acuity (decimal) two will not be discriminated even if the letter is resolved by the eye’s optics.
However, as was pointed out, the main cases for optical degradation are not higher order aberrations, but defocus and astigmatism. In that context, the manip- ulation of the eye’s optics by different devices has been a successful technological development since the correction of defocus in the thirteenth century to the use of cylindrical lenses to correct astigmatism in the nineteenth century. Today, it is also possible to correct and induce also higher order aberrations in contact lenses, intraocular lenses, or laser refractive surgery procedures.
The future of correcting the eye’s optics is both exciting and promising. And photonics and light technology will surely play a key role. The use of advanced optoelectronics would allow new prostheses to restore accommodation in the presbyopic eye. Two-photon interaction in the cornea by using femtosecond lasers may offer the possibility of changing the optical properties without the need to remove tissue, as is the case in the current ablation-based procedures. Optical technology is also fundamental in new diagnosis instrument. New swept-source optical coherence tomography allows full three-dimensional imaging in real time . Fig. 12.12 The sampling of the images the retina by the photoreceptors imposes the second limit to vision after the optics. Images of letters corresponding to decimal visual acuity 1, 2, and larger than 2.5 are represented. At the limit, a very small letter even if well projected into the retina would cover one single receptor. Then, the details of the letter will not be detectable. This suggests that even if the optics were perfect, visual acuity would not be improved
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of the eye in an unprecedented manner. And ophthalmoscopes equipped with adaptive optics obtain high-resolution images of the retinal structures in vivo.
Optics and photonics are now, more than ever, at service to help our eyes to see well.
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