You are here: vision-research.eu » Vision Research » Visionary of the Quarter » Pablo Artal (Q04-2015)
Adaptive optics (AO) is a mature technology that allows obtaining improved images by correcting the optical aberrations of the instrument. Perhaps the most well known AO application is in astronomical telescopes to compensate for the image degradation induced by the atmosphere. Although with different spatial and temporal scales, the optical problems involved in the human eye are similar as those in Astronomy.
During nearly 20 years, different laboratories in the world, including my own lab1 devoted significant efforts to develop adaptive optics instruments for use in Vision and Ophthalmology.
The human eye is a simple, but extremely robust, optical instrument2. It is composed of only two positive lenses, the cornea and the crystalline lens, which produce real images of the world on the retina, initiating the visual process. When compared with artificial optical systems, often formed by many more lenses, the eye is simple, but well adapted to the requirements of the visual system. The optical surfaces of the eye’s lenses are not exactly spherical in shape and they are not perfectly aligned, producing ocular aberrations and blurring the images on the retina. The impact of the aberrations in a normal eye is approximately equivalent to that of 0.25 D of pure defocus, a small error typically not corrected, although this can be quite significant when imaging the retina. In the normal young eye, the optical properties of the two ocular components are tuned to produce an improved overall image quality. In particular, two of the most important aberrations, coma and spherical aberration are partially corrected in the eye, in a similar way as in an aplanatic optical system3.
When the eye’s optical aberrations are known, it is possible to correct them using a wavefront correcting device that compensates for the eye’s aberrations. In the ideal case, the system of “corrector+eye” becomes aberration-free, producing perfect (diffraction-limited) retinal images. AO in the eye can be implemented using deformable mirrors or liquid crystal spatial light modulators as corrector devices4,5. The main application in the early days of AO for the eye was to improve the resolution and quality of retinal fundus images recorded through the corrected eye optics in ophthalmoscopes.
However, once AO instruments were operative for the eye, we realized that not only ocular aberrations could be corrected, but also any desired aberration pattern could be added to the eye in a controlled manner. By using an additional optical path, visual stimuli were projected to the eye’s subject to perform visual testing for a variety of optical conditions. This is the basis of the concept of the adaptive optics vision simulator. This instrument consists of a wavefront sensor to measure the eye’s aberrations and a correcting device, in our more recent choice a liquid crystal on silicon (LCOS) spatial light modulator, to modify the eye’s optics. A Hartmann-Shack (H-S) wavefront sensor6 operating in infrared light measures the eye's aberrations and residual defocus in real time (25 Hz). In the second pass, after the light is reflected in the retina and passes through the complete system, an array of lenslets, optically conjugated with the subject's pupil plane, produces an image of spots on a camera. The locations of the spots provide the local slopes of the ocular wavefront aberration. The correcting/manipulating device is placed in the system conjugated both with the subject's pupil plane and the wavefront sensor, by using appropriate sets of lenses in a telescope configuration. Subjects view a stimulus (letters or any visual scene) produced either by a pico-projector or an OLED micro-display. Figures 1 and 2 show a schematic diagram of the instrument and a picture of a commercial version developed by Voptica SL (www.voptica.com), a start-up company of the University of Murcia.
The potential applications of this type of AO instruments in Ophthalmology can be enormous. Perhaps the most obvious is the progressive substitutions of phoropters, those old-fashion systems that you find in any ophthalmic clinic, containing wheels with different lenses used during visual testing to determine the required optical prescription. With AO not only defocus and astigmatism, but all optical aberrations could be induced. This would allow optimizing the optical correction for different visual tasks and conditions. For example, in some cases, some residual customized amount of spherical aberration can provide depth of focus in presbyopic eyes. Another powerful application will be the pre-testing of different optical solutions. In invasive procedures, such as laser refractive surgery, before a definitive ablation of the cornea is performed, the optical profile to be induced could be optimized for each patient. This will open the door to an era of true customized eye treatments. In vision research, a variety of experiments can be performed with this instrument. Just as an example, we explained the underlying reasons of the phenomenon known as night myopia7, the myopic shift occurring at low luminance levels that is mainly due to an error in the eye’s accommodation.
The future of this technology looks really promising and I personally expect a rapid transition from the laboratories to the clinics. However, there are still challenges that need substantial work to be carried out. Although we have already proposed binocular AO systems8, they are still limited to the laboratory. In addition, open view systems combining AO and eye–tracker technology should be developed for more practical clinical uses. We have recently developed a multi-signal sensor operating in open view (Figure 3), although the compensations part is not yet implemented9.
I am convinced that these tools will revolutionize the way vision is tested and corrected today. In addition, this will offer patients the possibility to evaluate vision before a surgery and to surely optimize the clinical outcomes. As a final comment, I would like to emphasize that this is an example of a technology originated in academic laboratories that will eventually emerge as an indispensable clinical tool to improve the quality of vision of patients.
Pablo Artal developed new optical approaches to better understand the optical properties of the human eye. His first contribution was a new double-pass instrument, using a laser source to illuminate the eye and a camera for recording the retinal image. The analysis of the images provided with new insights into the eye’s optics and more importantly opened the door for the use of new technology in other apparatus, notably the Hartmann-Shack wavefront sensor. This early research led to the aberrometers, of common use today in Ophthalmology.
Artal and his co-workers used innovative laboratory prototypes to discover that the cornea and the lens present a coupling of the aberrations that improve the retinal image quality. They showed that the eye behaves optically as an aplanatic system, with a partial correction of spherical aberration and coma. They also found that the eye’s aberrations increased with age and the reason was actually a decoupling between the cornea and the lens with age. These findings were used as the theoretical bases for the new generation of aspheric intraocular lenses now widely used in the clinic.
The first adaptive optics system to correct for the eye’s aberration in real-time was also developed in Artal’s laboratory. These concepts were applied later for the first time for visual testing as adaptive optics vision simulators. A plethora of new applications has from them transformed vision correction, providing new ways to image the retina and to evaluate vision.
These are some of his most significant scientific achievements:
The real impact of Artal’s work can be found in significant improvements affecting people quality of life. In more practical terms, these are some of the social implications:
Full Professor of Optics
LOUM – Laboratorio de Óptica de la Universidad de Murcia
Centro de Investigación Óptica y Nanofísica
Universidad de Murcia (Spain)
Laboratorio de Óptica
Universidad de Murcia
Centro de investigación en Óptica y Nanofísica (CiOyN) Campus Espinardo – 30100 Murcia,Spain
Phone: +34 868888555
Fax: +34 868883528
Email: loum[at]um.es