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Optics and optical instruments
Published in Andrew Norton, Dynamic Fields and Waves, 2019
It often comes as a surprise to people to learn that the major part of the eye’s focusing power is contributed not by the crystalline lens, but by the cornea + aqueous humour combination, acting as a fixed focal-length converging lens. Yet what makes the eye an exceptional optical system is the ability of the crystalline lens to change its focal length (on instructions from the brain) by changing the curvature of its surfaces. The ciliary muscle can squash or stretch the thousands of layers that make up the lens, thereby changing its shape. So although the crystalline lens may make only a minority contribution to the total focusing power of the eye-lens system, it is this contribution that lends the system its distinctive capability — that of being able to vary its focal length.
Corneal onlays and inlays
Published in Pablo Artal, Handbook of Visual Optics, 2017
Presbyopia is a condition of the eye associated with aging where the accommodative mechanism of the eye, consisting of the crystalline lens and ciliary muscle, is no longer able to increase the power of the eye to view near objects. Patients who do not require glasses to see distance (emmetropes) usually just use reading glasses to correct presbyopia. Patients who are myopic, hyperopic, or astigmatic (ametropes) generally use glasses to correct their distance vision with an addition (bifocals) or multifocal contact lenses in order to see clearly at near. Monovision is an approach where one eye is corrected for distance and the other eye is corrected for near. Monovision can be achieved with contact lenses, corneal refractive surgery (PRK or LASIK), or IOLs. A multifocal IOL is another option for the correction of presbyopia, either to replace a cataractous lens or as a refractive (clear lens) exchange procedure.
Optical Instruments for Viewing Applications
Published in Abdul Al-Azzawi, Photonics, 2017
The eye focuses on an object by varying the shape of the flexible crystalline lens through an amazing process called accommodation. An important component in accommodation is the ciliary muscle, which is attached to the lens. When the eye is focused on distant objects, the ciliary muscle is relaxed. For an object distance of infinity, the focal length of the eye (the distance between the lens and the retina) is about 1.7 cm. The eye focuses on nearby objects by tensing the ciliary muscle. This action effectively decreases the focal length of the eyelens by slightly decreasing the radius of the curvature of the lens, which allows the image to be focused on the retina. This lens adjustment takes place inside the eye automatically.
Visual fatigue induced by watching virtual reality device and the effect of anisometropia
Published in Ergonomics, 2021
Sang Hyeok Lee, Martha Kim, Hyosun Kim, Choul Yong Park
In this study, the increase of visual fatigue is accompanied by a myopic shift of refractive error and the decrease of maximum amplitude of accommodation. Although the amount of myopic shift was small (≤0.25 D), myopic shift after VR watching was repeatedly observed in all visits. It is previously known that myopic shift is associated with increased visual fatigue (Hayashi et al. 2002). The significant and transient myopic shift after VR watching was previously reported in normal adolescent population (mean age of 14.7 ± 1.3 years) (Ha et al. 2016). Furthermore, the myopic shift after VR watching has an important clinical implication, such that it can aggravate childhood myopia progression. A report shows that about 50% of children showed a myopic shift on the refractive error after watching 3 D television for 50 min (Kim et al. 2015). The decrease in the amplitude of accommodation observed in our study may represent the possible exhaustion of the ciliary muscle after watching VR. The visual fatigue after watching 3 D animation was accompanied by a decrease in the accommodation power (Chang et al. 2015; Lee et al. 2019).
Correlation of accommodation and lens location with higher-order aberrations and axial length elongation during orthokeratology lens wear
Published in International Journal of Optomechatronics, 2021
Jy-Been Liang, Wen-Pin Lin, Richard Wu, Connie Chen
Several mechanisms for the process of myopia have been proposed. The “accommodation lag theory” suggests that axial hyperopic retinal blur (due to accommodation lag during near work) accelerates eye growth.[17] The “mechanical tension theory” postulates that mechanical tension from the ciliary body and lens during accommodation restricts equatorial ocular expansion, thus leading to axial elongation.[18] Another theory suggests that axial elongation is due to accommodation-induced contraction of the ciliary muscle pulling the choroid, and thus shrinking the equatorial circumference of the sclera.[19] Accommodation theory may explain why OK lenses cannot completely control myopia, since providing tension on the surface may not be sufficient to retard axial length elongation. However, myopia progression may result from a number of different processes that may be more or less pronounced in different situations.
Automatic visibility evaluation method for application in virtual prototyping environment
Published in International Journal of Computer Integrated Manufacturing, 2019
Weiwei Wu, Xiaodong Shao, Huanling Liu
According to practical experience, if , the clarity of the observed-object is optimal, and the visual-angle factor is assigned the highest value of one. If (the observed-object is relatively far for the observer under this circumstance), a decrease in the visual-angle (which represents an increase in the visual-distance or a reduction in the object-size) deteriorates the visibility of the observed-object. When the visual-angle is smaller than the critical visual-angle, the observed-object is completely invisible. Therefore, in this angle range, the evaluation value of decreases as the visual-angle decreases. If (the observed-object is too close for the observer under this circumstance), an increase in the visual-angle (which represents a decrease in the visual-distance or an enlargement in the object-size) results in a tension state of the ciliary muscle and visual fatigue of the human eye. Moreover, individuals will have a sense of oppression in the mind and feel dizzy. Therefore, in this angle range, the evaluation value of decreases as the visual-angle increases. The functional relationships between and in the various visual-angle ranges are also constructed through the interpolation method described in Section 5.1 as shown in Figure 11.