Ординатура / Офтальмология / Английские материалы / Textbook of Visual Science and Clinical Optometry_Bhattacharya_2009

Ophthalmic Lenses 171

Figs 12-1:A and B: Use of convex lens for correction of hypermetropia

(A) Hypermetropic eye and (B) Convex lens (Cx) Placed infront

How to identify convex lens?

•It magnifies images, i.e. makes them larger.

•When the lens is moved in front of the eye, the object moves in the opposite direction. This is called against movement.

•The lens is thicker in the middle and thinner on the edges.

•The greater the vertex distance (further away from the eye), the stronger the power, i.e. magnification of the lens.

•It is conventionally denoted by plus (+) sign in trial lenses.

Uses: It is used in the treatment of:

•Hypermetropia

•Presbyopia

•Aphakia

•In low visual aids as magnifier.

Types of convex lenses: The power of each surface of a lens add up to arrive at the power of the lens as a whole. So, the total power desired is distributed between the two surfaces. Convex lenses are usually available in following standard forms;

i.Equiconvex: Both surfaces have the same curvature, i.e. same plus (+) power (Fig. 12-2A). Nowadays, this spectacle lens form has become obsolete.

ii.Planoconvex: The surface facing the eye, i.e. back surface is plane and the front surface provides the necessary plus/ convex power (Fig. 12-2B).

iii.Plus/Convex meniscus: This is the usually available form of plus/minus spectacle lenses. Conventionally, the front surface is always convex and the back surface is always concave (Fig. 12-2C), in plus/minus meniscus lenses.

172 Textbook of Visual Science and Clinical Optometry

Fig. 12-2A to C: Different standard forms of convex spherical lenses

(A) Equiconvex, (B) Planoconvex and (C) Plus meniscus

Concave or Minus Lens

Concave lenses are worn by patients who are myopic, i.e. nearsighted. In myopia, the image of an object falls in front of the retina (Fig. 12-3A) and concave lens diverges the parallel rays of light to shift the image behind, on the retina (Fig. 12-3B).

Figs 12-3A and B: Use of concave lens for correction of myopia

(A) Myopic eye and (B) Concave lens (Cc) placed infront

How to identify concave lens ?

•It minifies images, i.e. makes them smaller.

•When the lens is moved in front of the eye, the objects move in the same direction. This is called with movement.

•The lens is thinner in the middle and thicker on the edges.

•The greater the vertex distance, the weaker the power of the lens.

•It is conventionally denoted by minus ( – ) sign in trial lenses.

176 Textbook of Visual Science and Clinical Optometry

than 1.73 are termed as “very high index”. High index lenses are available in both glass and resin lens variety.

Advantages of high index lens:

•Comfortable to wear and cosmetically superior

•Thickness of the lens is reduced

•Weight of the lens is either same or slightly less

•Particularly suitable for myopics due to reduction in edge substance.

•Disadvantages of high index lens:

•Increased transverse chromatic aberration due to lower constringence/Abbé’s number (see chapter-11).

•Higher surface reflectance (see chapter–13)

LENTICULAR LENS

The idea of lenticular lens is to reduce the weight of the lens by decreasing the thickness of edge substance. In lenticular lens, the central area called “aperture”, is responsible for the dioptric strength of the high-powered glass lens or resin lens. Other lenses, i.e. spherical lens, cylindrical lens, toric lens and aspheric lens fall within full aperture category. Lenticular lenses are of two types:

Minus/Concave Lenticular

Minus lenticular lens can be created in both planoconcave glass and resin lenses from normal blank (Fig. 12-6A) by the process of either surfacing or glazing (Figs 12.6B and C). Usually minus lenticular lens are available in single vision lenses. In multifocal lenticular lenses the carrier margin particularly in the lower part may interfere with vision.

Figs 12-6A to C: Transformation of planoconcave blank to minus lenticular lens (A) Planoconcave lens blank, (B) Minus lenticular with a plano carrier margin and (C) Minus lenticular with a convex carrier margin

Ophthalmic Lenses 179

RULES OF TRANSPOSITION

i.Power of the spherical lens–Add the spherical and cylindrical power algebraically, taking into account their signs.

ii.Power of the cylindrical lens–Only change the sign of the cylinder from plus (+) to minus ( – ), or vice versa. The numerical strength of the cylinder remains the same.

iii.Axis of the cylinder–Change the axis by 90º. If the original axis is at or less than 90°, add 90° to it. If the original axis is over 90°, subtract 90° from it.

EXAMPLES

1.If the original prescription is –3.00DSPH with +1.00DCYL 90°↓ Rule i. –3.00 + (+) 1.00 = –2.00 DSPH

Rule ii. +1.00DCYL becomes –1.00DCYL Rule iii. 90°↓ axis becomes 180° axis

Hence, the transposed power is –2.00DSPH with –1.00DCYL 180°

2.If the original prescription is –3.00DSPH with -1.00DCYL 45° Rule i. –3.00 + (–) 1.00 = –4.00 DSPH

Rule ii. –1.00DCYL becomes +1.00DCYL

Rule iii. 45° axis becomes 45° + 90° = 135° axis Hence, the transposed power is –4.00DSPH with +100DCYL 135° axis.

3.If the original prescription is –2.00DSPH with +2.00DCYL 105°

Rule i. –2.00 + (+) 2.00 = 0, i.e. Plano Rule ii. +2.00DCYL becomes –2.00DCYL

Rule iii. 105° axis becomes 105º–90º = 15° axis Hence, the transposed power is –2.00DCYL 15° axis

NEUTRALISATION (DETERMINATION OF POWER OF A LENS)

Neutralisation is the technique for determining the power of an ophthalmic lens. Neutralisation is done by following methods.

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MANUAL METHOD

It is basically a very simple but quick and accurate way of checking the power of glasses. It is done by trial and error with the help of the lenses present in the trial box. In this method, lenses of known power are placed in contact with the lens under verification until the power of the combination becomes zero, i.e. there is no movement of the distant object. A lens under verification will be neutralised by another lens of equivalent power of opposite sign. So, a –3.00DSPH lens will be neutralised by a +3.00DSPH lens {–3.00DSPH + (+3.00DSPH) = 0}.

Neutralisation is based on the identification properties of convex and concave spherical lenses discussed earlier in this chapter. Convex spherical lens exhibit against movement and concave spherical lens exhibit with movement of the object, if they are moved in front of the eye.

RULES OF NEUTRALISATION

•Object/Target should be placed at a convenient furthest distance.

•Ideally a cross with two limbs at 90° should be the target. In the absence of cross, a window frame or other vertical and horizontal intersection may be employed as the target. Conveniently, Snellen’s distant vision chart may also be used.

•Neutralising lens should be placed in close contact with the front surface of the spectacle lens.

•Attention must be paid to the central portion of the cross during neutralisation.

•The lenses should be held in front of the eyes.

PROCEDURE OF NEUTRALISATION

Manual

a.Spherical lenses (Table 21.1)

b.Astigmatic lenses–Here the same principle is applied but it requires further steps. It can be done in two ways—

1st procedure–Here both spherical and planocylindrical neutralising lenses are employed.