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Optical Instruments for Viewing Applications
Published in Abdul Al-Azzawi, Light and Optics, 2018
Three common defects of vision are myopia (nearsightedness), hyperopia (farsightedness), and astigmatism. In myopia, the eyeball is too long, or the cornea is too curved. Light from a very distant object comes to a focus in front of the retina, as shown in Figure 14.5(b). This occurs even though the ciliary muscle is completely tensed. Accommodation permits nearby objects to be seen clearly but not more distant ones. A diverging lens of the proper focal length can correct this condition, as shown in Figure 14.5(b). In hyperopia, the eyeball is too short or the cornea has insufficient curvature. Light from a very distant object does not come to a focus within the eyeball, even when the ciliary muscle is relaxed. Its power of accommodation permits a hyperopic eye to focus on distance objects, but the range of accommodation is not enough for nearby objects to be seen clearly. In this condition, light from a distant object is focused behind the retina. The correction for hyperopia is a converging lens, as shown in Figure 14.5(c).
Optical Instruments for Viewing Applications
Published in Abdul Al-Azzawi, Photonics, 2017
Three common defects of vision are myopia (nearsightedness), hyperopia (farsightedness), and astigmatism. In myopia, the eyeball is too long, or the cornea is too curved. Light from a very distant object comes to a focus in front of the retina, as shown in Figure 14.5(b). This occurs even though the ciliary muscle is completely tensed. Accommodation permits nearby objects to be seen clearly but not more distant ones. A diverging lens of the proper focal length can correct this condition, as shown in Figure 14.5(b). In hyperopia, the eyeball is too short or the cornea has insufficient curvature. Light from a very distant object does not come to a focus within the eyeball, even when the ciliary muscle is relaxed. Its power of accommodation permits a hyperopic eye to focus on distance objects, but the range of accommodation is not enough for nearby objects to be seen clearly. In this condition, light from a distant object is focused behind the retina. The correction for hyperopia is a converging lens, as shown in Figure 14.5(c).
Corneal onlays and inlays
Published in Pablo Artal, Handbook of Visual Optics, 2017
Hyperopia is a condition where the optics of the eye are not strong enough or the axial length of the eye is too short for images to come into focus on the retina without additional power. This additional power is either provided by positive (plus) powered lenses or contact lenses or may be achieved by strong accommodative effort. If a patient has had their natural crystalline lens removed and not had an IOL implanted, a condition known as “aphakia,” the eye is left with a significant amount of hyperopia, or undercorrection. Positive spectacle lenses are also used to correct aphakia; however, they are extremely thick. Positive contact lenses are an option for aphakia; however, most patients who are aphakic are older and are less able to handle contact lenses or tolerate contact lens wear. For both hyperopia and aphakia, a change in the power of the eye can be achieved by either increasing the curvature of the cornea or introducing a positive powered implant.
A perspective of contemporary cataract surgery: the most common surgical procedure in the world
Published in Journal of the Royal Society of New Zealand, 2020
Charles N. J. McGhee, Jie Zhang, Dipika V. Patel
The vast majority (>90%) of the eye’s refractive error (myopia, hyperopia, astigmatism) can be corrected by simple sphero-cylindrical lenses i.e. spectacles, however, higher order aberrations (HOA) of the light wavefront are components that are not correctable in this manner and may degrade visual acuity before and after cataract surgery. Interestingly, Sachdev et al. (2004) highlighted that even early cataract development increases HOA, including positive spherical aberration, notably conventional IOLs also add positive spherical aberration post-cataract extraction (Sachdev et al. 2004). Using the same wavefront aberrometer, McKelvie et al. (2009) developed a physical model eye to objectively compare HOA introduced by each of 3 aspheric IOL models, independent of patient or surgeon variables. IOLs of different design and power were noted to have significantly different HOA profiles, with the greatest variability observed in higher power IOLs (McKelvie et al. 2009). Significant differences in HOAs were also observed in response to changes in pupil size (minimal HOA with small pupil diameter), decentration (increased HOA with a displacement of the IOL away from the optical axis), and tilt angle (the least significant factor affecting HOA) (McKelvie et al. 2011).
Liquid crystal technology for vergence-accommodation conflicts in augmented reality and virtual reality systems: a review
Published in Liquid Crystals Reviews, 2021
The second major challenge, vision correction (the so-called prescription problem), originates from the need for eyes to see objects and virtual images clearly when the eyes have refractive errors. Currently, a pair of personal prescription eyeglasses is one of the solutions to correct refractive errors of eyes, but this solution is incompatible with the AR and VR systems. The AR and VR optical systems are usually designed for people with 20/20 vision without considering the refractive errors in the eye [3]. Therefore, vision correction function should be designed into AR and VR optical systems with tunable optical elements. For a normal eye, light from infinity can converge at the surface of the retina, which means that the eye can clearly see objects that are far away, as shown in Figure 4(a). When the eyes have refractive errors (e.g. the cornea is curved more than required), light from infinity does not converge at the surface of the retina, as shown in Figure 4(b). When the light converges between the crystalline lens and retina, the eye can only see an object close to the eye. This condition is called near-sightedness or myopia. Similarly, when the convergent light is located outside the eyeball, the eye sees a blurry image when an object is near the eye; this condition is called far-sightedness or hyperopia. Myopia requires prescription lenses with negative focal lengths (or negative lenses, f < 0), while hyperopic condition require positive lenses (f > 0) for eye corrections. Both myopic and hyperopic eyes may have normal accommodation abilities. Another eye condition, presbyopia, mainly results from the weak accommodation ability of crystalline lenses of the eyes. From statistical results [13], the accommodation ability of crystalline lenses decreases with age, and people start to have presbyopia after 40 years of age and may eventually lose accommodation ability. People with myopia and presbyopia usually need two pairs of eyeglasses: one with positive lenses and the other with negative lenses. It is estimated that half of the world population will suffer from myopia by 2050 [14]. Based on other statistical results [15–17], the range of lens powers of prescription lenses should be from −2 diopters (D) to +2 D (i.e. 4 D in total) in order to cover the prescription lens requirements of ∼80% of the population. To cover a larger population, the range of the lens power must be larger. Thus, implementing tunable lenses in AR and VR systems to provide an appropriate lens power is one of the solutions to the prescription problem.