Page 6 - Practical-Refraction-English

1.Emmetropia, Ametropia, Emmetropia, Ametropia Presbyopia and their correction When the eye is out of focus, the vision is blurred. There may be several reasons why an eye is not in proper focus, and indeed the reasons why a person develops a refractive error are many and multifactorial, but no matter what the rea- son, the end result is that there is a mismatch between the power of the refracting elements of the eye and the position of the retina (that is, the length of the eye). The eye has a refractive error and the vision is out of focus when the image 1 formed by the refracting components of the eye is located in front of and/or behind the retina, rather that exactly on it. A Emmetropia An eye is said to be emmetropic (from the Greek emmetros = proportionate (measurement) and ops Practical Refraction = eye) when the image of an object located at infi- nity is formed on the retina of the unaccommodated eye. In the emmetropic eye, the retina is conjugate with optical infinity and therefore lies in the image © Essilor International focal plane of the ophthalmic system. The emme- tropic eye sees distant objects clearly, without accommodation. Figure 1: Emmetropic Eye The eye as an optical system: The unaccommodated emmetropic eye can be modelled as an optical system composed of the cor- nea, the aqueous humour, the crystalline lens and the vitreous humour. The characteristics of one such theoretical system (called a schematic eye) are shown in the table below: Anterior Thickness Refractive radius of Posterior radius of (mm) index curvature (mm) curvature (mm) - (single Cornea - 7.80 - surface) Aqueous 3.60 1.336 -- humour Crystalline 3.70 1.422 11.00 -6.48 lens Vitreous 16.79 1.336 -- humour Overall length of eye: 24.09 mm Reference: Bennett and Rabbetts’ Clinical Visual Optics, fourth edition, 2007 A simplified eye can be obtained (Figure 2) by simpli- +42 +22 n=1,336 fying this model; that is, by (i) combining the elements n=1,336 that make up the eye, (ii) considering the cornea and SRL l=24 mm l the lens as thin lenses (as opposed to thick lenses), (iii) using the same index n = 1.336 for the aqueous and vitreous humours and (iv) rounding off the calcu- lations. This simplified eye totals 60 dioptres, is 24 © Essilor International mm in length and is comprised of a transparent sphe- d=5,8 mm re with an optical power of 42 dioptres (the cornea) separating the air from the aqueous humour, and a thin lens with an optical power of 22 dioptres (the Figure 2: Simplified Emmetropic Eye lens) separating the aqueous humour from the vitreous, located 5.8 mm behind the cornea. Although greatly simplified, this model is nevertheless an acceptable optical representation of the human eye (in the unaccommodated state). 6 Copyright © 2008 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.