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Visual Perception
Published in Robert W. Proctor, Van Zandt Trisha, Human Factors in Simple and Complex Systems, 2018
Robert W. Proctor, Van Zandt Trisha
For nearsightedness, or myopia, the eye is too long, resulting in a focal point that is in front of the receptors when the lens is relaxed. For farsightedness, or hyperopia, the eye is too short, resulting in a focal point that is behind the receptors when the lens is fully flexed. As people become older, the speed and extent of their accommodation decrease continually. With age, the lens becomes harder and less responsive to the pulls of the muscles in the eye, so accommodative ability decreases and essentially all people become hyperopic. This condition is called presbyopia, or old-sightedness. The near point can increase from as close as 10 cm for 20-year-olds to as far as 100 cm by age 60. Presbyopia can be corrected with reading glasses or bifoc als, which typically are not prescribed until age 45 years or older. A person can have perfect vision in all other respects but still need reading glasses to compensate for the decreased accommodative ability of the lens.
Lasers in Medicine: Healing with Light
Published in Suzanne Amador Kane, Boris A. Gelman, Introduction to Physics in Modern Medicine, 2020
Suzanne Amador Kane, Boris A. Gelman
Excimer lasers produce ultraviolet wavelengths that are absorbed well in general by water and proteins. This means their power can be absorbed by the transparent structures of the eye, allowing surgery on the lens and cornea. One of the newer uses of lasers involves sculpting the cornea to correct problems in vision. The most common example is myopia (or nearsightedness), in which a steeply curved cornea results in images being focused in front of, rather than on, the retina itself. In one form of this operation, called photorefractive keratectomy (PRK), an excimer laser is used to directly remove material from the surface of the cornea to flatten it out. A different technique, called LASIK (for laser-assisted in-situ keratomileusis), uses a thin knife called a microkeratome (or sometimes another laser) to slice a flap off the top of the cornea first, after which an excimer laser is used to reshape and flatten the cornea. The flap is replaced, and a flatter cornea overall results after healing. While these procedures are typically used to correct nearsightedness, in some cases they can also be used to correct for farsightedness due to an overly flat cornea. In wavefront-guided LASIK, the excimer laser is used to sculpt the cornea to attempt to correct subtler visual errors. While many have benefited greatly from PRK and LASIK, the utility of laser surgery for refractive corrections remains limited. Not everyone is a good candidate for LASIK, and ophthalmologists now assess potential recipients for various risk factors, such as a rapidly changing refractive error, overly thin corneas, and a tendency toward dry eyes. In a small percentage of cases, patients report serious side effects from LASIK, such as light sensitivity, glare, dry eyes, and problems with night vision.
The Visual Environment: Measurement and Design
Published in R. S. Bridger, Introduction to Human Factors and Ergonomics, 2017
In hypermetropia, light rays entering parallel to the optic axis are brought into focus behind the retina. This can be caused by the eye being too short anteroposteriorly or by insufficient curvature of the refractive surfaces of the eye. Hypermetropia is sometimes referred to as farsightedness because the near point is further away from the eye (for an equal amount of accommodation) than it is in a healthy eye. Hypermetropic individuals can be said to lack refractive power and may tire quickly when carrying out work in which the viewing distance is short (such as using a VDT).
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.