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

Frames and Lenses 201

Fig. 14-6: Types of bifocals (depending on shape)

Dot (•) indicates optical centres of the near segment

Fig. 14-7: Split bifocal

•Cemented bifocal–Here a glass button of higher refractive index is placed within the lens which has been split aned cemented. It is basically a three piece bifocal (Fig. 14-8). It is termed “Kryptok” (Greek meaning–hidden) by its inventor John Borsch Sr. This variety of cemented bifocal is also obsolete of now (Fig. 14-9).

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Fig. 14-8: Cemented bifocal (view on section)

Fig. 14-9: Kryplok bifocal lens–Optical centres for distance and near. Near vision centre is 2 mm nasal to and 8 mm below the distance vision center. Distance vision centre = • , Near vision centre = X

•Fused bifocal–Here a button of a glass of higher refractive index imparting higher power for near vision is fused on to the surface of the crown glass (Fig. 14-10). This is the most commonly used variety.

•Solid bifocal (or one piece)–Here the near segment is made by grinding different curvature on one surface imparting higher power. So, solid bifocal glass/resin lens carries two distinct curvatures (Fig. 14-11). They are made from a single piece of material, i.e. either crown glass or resin lens. The lens in which the dividing line of a solid bifocal is made to disappear is called seamless (or blended) solid bifocal. In seamless solid bifocals the two curvatures are joined by a gradual transition zone.

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Fig. 14-10: Fused bifocal

Fig. 14-11: Solid bifocal

Trifocal Lens

This is the lens with three prescriptions of power for distance, intermediate and near vision (Fig. 14-12). The strength of the intermediate addition is usually half of the addition prescribed for near vision. Trifocal lenses are usually of fused (glass)/solid (resin lens) variety. The depth of the intermediate segment usually ranges between 6 to 8 mm. The advent and advantage of progressive addition lenses are gradually declining the market share of trifocals.

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Fig. 14-12: Trifocal lens DS = Distance segment

IS = Intermediate segment NS = Near segment

d = Depth in mm

Progressive Addition Lens (PAL)

This is a spectacle lens with a gradual progressive change in power from distance to near as the patient’s focus moves down the lens with negligible eye movement. It practically offers excellent vision at all distances and superior cosmetic appearance. Features of PAL (Fig. 14-13) are:

•Distance vision area in upper part of the lens.

•An intermediate narrow “transition” corridor of progressive area for distance to near vision.

•Near vision area in lower central part.

•Single vision lens appearance, i.e. absence of dividing lines giving enhanced cosmetic affect.

•Peripheral areas of the lens exhibit unwanted astigmatism and distortion. However, this area is intended for awareness of objects only. Since peripheral vision is disturbed by the optical distortion, eye movements are restricted.

Chronology of Invention of PAL: The conception of progressive addition lens is first conceived by Owen Aves in 1907. However, Henry Oxford Gowlland first made commercially available PAL under the trade name ULTIFO. The VARILUX progressive addition

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Fig. 14-13: Areas of a progressive addition lens (PAL). 1 = Distance vision area, 2 = Intermediate transition corridor, 3 = Periphery and 4 = Near vision area

lens from Essilor is designed by Bernard Maitenaz in 1959. Subsequently various manufacturers have modified and upgraded designs aiming at reduction of unwanted distortion and aberrations in the peripheral area.

Advantages of PAL

•Absence of image “Jump” which is experienced with bifocals and trifocals.

•Lack of a line in the lens which is cosmetically appealing to the wearer due to youthful appearance.

•Natural vision from distance to near.

Progressive addition lenses are supplied by the manufacturers,

with few standard point markings, indicating key areas of the lens. This point marking of key areas helps the dispenser proper positioning and fitting of the lens (glazing) into the frame. Before delivery to the patient the marks are removed usually with thinner.

The centre of the reading area is conventionally inset 2.5 mm in relation to the fitting cross (Fig. 14-14). The fitting cross (+) corresponds to the centre of the pupil.

However, additionally some markings in the form of circles and logos (6, 8 in Fig. 14-14) are engraved on the progressive addition lenses. These engravings do not cause any visual interference and usually the wearer is also unaware of them. They are conventionally placed 34 mm apart across the 180° horizontal line

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Fig. 14-14: Progressive addition lens blank–typical point marks and engravings. 1 = 180° horizontal line, 2 = Prism checking point (optical centre of the lens),

3 = Corresponds to patient’s pupillary centre (fitting cross), 4 = Distance power spherical and astigmatism with axis checking point, 5 = Centre of reading area and power checking point, 6 = Engravings, 7 = Near addition (+2.00) and

8 = Manufacturer’s logo (XYZ)

(6 in Fig. 14-14). Just below them one can locate engraved reading addition on the temporal side and occasionally the manufacturer’s logo on the nasal side (7 and 8 in Fig. 14-14). Proper power dispensing and proper fitting of the progressive addition lenses into the frame for glazing is an essential criteria for optimum visual comfort.

Checking/Verification of power of PAL: Use of lensometer is the correct approach for this purpose. However, in PAL the checking areas are restricted and are designed for this particular purpose (Fig. 14-14).

•Distance power and axis–Area 4 of Fig. 14-14 (ignore any prismatic effects).

•Prism checking point–Area 2 of Fig. 14-14 (ignore power and axis readings).

•Near addition–Area 5 of Fig. 14-14.

Fitting procedure for PAL (specifications):

•Monocular P.D. (interpupillary distance) measurement– Accurate measurement with a pupillometer is essential.

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Fig. 14-16: Binocular PD measurement

(alternative method)–with PD rule

PD Measurement with a Pupillometer

In pupillometer, the examiner moves a vertical line to align with the centre of the corneal reflections. An illuminated target is employed to fixate the patient’s eye at infinity by interpolating a lens in front of the patient’s eye (within the pupillometer). The pupillometer gives an accurate monocular and binocular PD reading in millimetres (mm).

Fig. 14-17: Vertex distance d = Vertex distance in mm

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VERTEX DISTANCE

It is the distance between the back surface of the spectacle lens and the anterior corneal surface (Fig. 14-17). The vertex distance varies between 11 mm and 15 mm (average 12 mm). Vertex distance is important since the power of the glass changes depending on it’s distance from the corneal apex. Hence, a patient’s contact lens prescription differs from his spectacle prescription (See Table 15-2 in Chapter 15).