Ophthalmic Lenses and Dispensing, 3rd edition. Mo Jalie UK: Butterworth Heinemann Elsevier, pages, RRP $ Reviewed by. Request PDF on ResearchGate | On Jan 1, , M. Jalie and others published Ophthalmic Lenses and Dispensing, 3th Edition. OPHTHALMIC LENS ABERRATIONS. .. understanding of ophthalmic lenses (spectacle lenses) . less often performed at the dispensing location.
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But the information presented in this book is designed to provide comprehensive details about the ophthalmic lenses and their dispensing tips. Most of the. THE EYECARE PROFESSIONAL'S BASIC DISPENSING HANDBOOK .. To make ophthalmic lenses more optically correct, and more attrac- tive, the outside . The Training in Ophthalmic Assisting Series and Training in Eye Care .. fields, ultrasonography, contact lens fitting, low vision aids and optical dispensing are.
However, this is not always the case. The lens designer has to deal with varieties of aberrations which prevents perfect image through the lens. There are six types of aberrations that prevent perfect imaging through the spectacle lens: 1.
Chromatic aberration 2. Spherical aberration 3. Coma 4. Marginal aberration 5. Curvature of field 6.
Materials and Methods
Distortion Chromatic aberration is caused because shorter and higher energy wavelengths are bent more readily and to a greater degree than the longer and lower energy wavelengths. The lens designer must deal with this kind of aberration by selecting a lens material with low dispersive value.
Spherical aberration results in a slight blurring of the image due to lenses having the tendency to focus the light rays from the periphery sooner than the rays passing through the central portion of the lens. This results in separate foci which prevent a sharp clear image on the retina. But in general spherical aberration is not a great problem to the lens designer as the small pupillary size is effective in keeping out its effect.
Ophthalmic Lens Material and Design 23 Coma is produced when wider beam of light passes obliquely through the lens. Depending upon the angle of incidence, the rays at the lower end of the beam will bend a different degree than rays at the upper end.
This causes uneven magnification and degrades and defocuses the image. Like spherical aberration, the small aperture of the pupil is effective in limiting the aberrant peripheral rays. Consequently, the lens designer pays little attention to this particular aberration.
Marginal astigmatism is a real concern for the lens designers as it involves narrow beam of parallel rays that strike the lens at an oblique angle.
Pupil does not limit the effect of marginal astigmatism. When a narrow beam of light rays strike a lens obliquely, there is a tendency for the rays in the opposite meridians to focus at different points. The distance between the two points of focus equals the degree of astigmatism created.
The presence of astigmatic error led the lens designer to work upon corrected curve theory. The idea was if the specific curvature can be controlled for specific correction, the effect of marginal astigmatism can be reduced. The curvature of field refers to the phenomenon that the eye viewing an object through a lens does not create a true reproduction of the object. A flat plane may appear to look slightly curved. The effect is dependent upon the refractive index of the lens material and the curvature of the lens surfaces.
Distortion occurs when an extended object is viewed through the periphery of the spherical lens. The edges of the object are distorted. This is due to the increased prismatic effect of the lens periphery which produces uneven magnification of the object. These effects prove a real nuisance to the wearer of high power spectacle lens. The lens which is centered perfectly and also fits closer to the eyes and is able to keep the patients vision within the central area of the lens reduces most of the troubles of these aberrations.
Such a procedure may enable us to control some of the aberrations, but not all. Moreover, the lens designer does not try to concentrate on eliminating lens aberrations, but he intends to find a formulation that balances them out to reduce their effect.
The corrected curve theory is the best form lens for a given prescription. Each lens designer has a slightly different application of the optical principles that control and balance the six lens aberrations. It is out of this optical reality that the theory of base curve selection has its application. Lot of mathematical computations are needed 24 Ophthalmic Lenses Fig. The computations are based on delivering the best optics in any given situation and not just creating the flattest profile.
A flatter lens may look good, but the wearer may complain about compromised optics. Corrected curve theory outlines an effective method for minimizing the effect of lens aberrations. Only one combination of curves will yield the best form and will focus light to the clearest image by eliminating the most amounts of lens aberrations, and if the dispenser combines proper base curve along with good fitting and perfect optical centre, the result is good cosmetics and great optics.
There are many Corrected Curve Theories. The ellipse, a graphical representation developed by Marcus Tscherning shows the best base curve for every prescription to minimize marginal astigmatism. It says that there is a range of power from about Outside this range there is no perfect base curve.
How often are spectacle lenses not dispensed as prescribed?
He demonstrated that there are two recommended best form base curves for each lens power. The flatter form is most commonly employed today. Wollaston is a deep curve form and is difficult to produce. For example— 5. When selecting the proper base curve for a given prescription it is important to follow some rules. To simplify the concept further we can say that as minus power increases the selection of base curve flattens and as plus power increases selection of base curve steepens.
Care has to be taken when changing the lens material from lower index to higher index material. In order to do a professional job, the dispenser must monitor all the options and then make the decision: 1. Determine the old prescription. Determine the old curve what the wearer is used to. Evaluate the new prescription in accordance with theoretically ideal curve.
Determine the best lens design and availability. Check that the design could be fabricated by the laboratory or not. This enables the lens designer to use the combination of bending and asphericity to be employed to improve the off-axis performance.
As a result, the designer can choose any form for a given power and then determine the asphericity required for that particular choice of bending to eliminate oblique astigmatism.
When each surface forms a part of sphere, the lens is called a spherical lens. There are various forms of spherical lens, some of them also have one plane surface. This is acceptable because a plane surface can be taken as part of a sphere with infinite radius. A convex lens causes convergence of incident light, whereas a concave lens causes divergence of incident light Fig.
Figs 3. Since, most of the lenses used in ophthalmology are thin lenses, thickness factor is ignored Fig. Thus, the total power of a thin lens is the sum of the two surface powers. Refraction can be thought of as occurring at the principal plane of the lens. In Figures 3. The point at which the principal plane and the principal axis intersect is called the principal point or nodal point, of the lens which is denoted by N.
Rays of light passing through the nodal point is undeviated. Light rays parallel to the principal axis is converged to or diverged from the point F, the principal focus Fig. There is, therefore, a principal focus on each side of the lens, equidistant from the nodal point.
The first principal focus F1, is the point of origin of rays which, after refraction by the lens, are parallel to the principal axis.
The distance F2N is the first focal length. Incident rays parallel to the principal axis is converged to or diverged from the second principal focus — F2.
The distance F2N is the second focal length. By the sign convention, F2 has a positive sign for convex lens and negative sign for the concave lens. Lenses are designed by their second focal length. If the medium on either side of the lens is the same, i. However, if the second medium differs from the first, e. Therefore, the unit of lens power, the diopter, is based on the reciprocal of the second focal length expressed in metres, gives the vergence power of the lens in diopters D.
So, in the above Figure 3.
And in Figure 3. Thus, the periphery portion of the lens acts as a prism. The refracting angle between the lens surfaces grows larger as the edge of the lens is approached Fig. Thus, the prismatic effect increases towards the periphery of the lens. Use of paraxial portion of a lens to gain a prismatic effect is called decent ration of the lens.
Lens decent ration is frequently employed in spectacles where a prism is to be incorporated. On the other hand, poor centration of spectacle lenses, may produce an unwanted prismatic effect. The increasing prismatic power of the more peripheral parts of a spherical lens is the underlying cause of spherical aberrations.
Spherical Lenses 31 Fig. Spherical lens causes no distortion of the cross. However, when the lens is moved from side to side and up and down along the axis of the cross, the cross also appears to move. Rotation of a spherical lens has no effect upon the image of the crossed lines. The power of a lens can also be found using the neutralization method.
Once the nature of the unknown lens is so determined, lenses of opposite type and known power are superimposed upon the unknown lens until a combination is found which gives no movement of the image of the cross line when the test is performed. To measure this accurately, the neutralizing lens must be placed in contact with the back surface of the spectacle lens. However, with highly curved lenses, this is not possible and an air space intervenes.
It is, therefore, better to place the neutralizing lens against the front surface of the spectacle lens. Neutralization is, thus, somewhat inaccurate for curved lenses of more than about 2. An error of up to 0.
Nevertheless for relatively low power lenses, neutralization is still a very useful technique. The Geneva lens measure can be used to find the surface powers of a lens by measuring the surface curvature Fig. The total power of a thin lens equals the sum of its surface powers. However, the instrument is calibrated for lenses made of crown glass and a correction factor must be applied in the case of lenses made of materials of different refractive indices. Focimeter is used to measure the vertex power of the lens.
The image of the target is seen as a ring of dots when a spherical lens is tested Fig. This sets the scene for addressing more complex concepts in the remaining pages. This book will significantly enhance any reader's understanding of spectacle lenses and it covers areas not found in many, perhaps any, other books; for example, it includes the surfacing of spectacle lenses.
Jalie devotes the last two chapters to this topic and includes a clear explanation of the relatively recent freeform surfacing methods but the book is not merely a theoretical discourse on lenses.
As the name suggests, it also includes advice on dispensing the lenses.
One important piece of advice offered by Jalie is the correct centration of single vision lenses by aligning the principal axis with the centre of rotation of the eye a technique often neglected in practice.
He supports with qualification setting aspheric reading spectacles on distance PD; a suggestion that some may find uncomfortable. Nevertheless maintaining the optical quality of the lenses is more important than the prism induced; prism that young ametropes will always have experienced when reading, if we assume that their constant wear spectacles were made on distance PD. It is pleasing that Jalie raises this option in this potentially controversial area: the weight of his advice will hopefully sway some doubters.
One small point that some Australian optometrists may find a little irksome is that some of the prescriptions are written in plus cylinder form. In a book with so many positive features, this is really of very little consequence and I doubt that many will be too bothered; it does not constitute an error.
Ophthalmic Lenses and Dispensing is an ideal book for any optometrist to keep abreast of knowledge in the area of spectacle lenses, prescribing and dispensing. The author, Mo Jalie, is arguably the world's leading expert in spectacle lenses and his command of his subject is evident. This book should take its place alongside Principles of Ophthalmic Lenses as a standard text and reference, and it should sit within easy reach on the bookshelf of all practices.They should also be advised that while collecting spectacles, they should ask the optician to verify whether the spectacle lenses have been dispensed as prescribed.
Spherical surface has uniform curvature in all its zones, whereas, surfaces other than spherical, the curvature varies in different zones and from one meridian to another. Hence, the axial zone of the lens has greater refractive power than the periphery. The whole idea behind choosing a correct base curve is to provide wider dynamic field of clear vision. Lens thickness has an effect on the density of the tint. Flatter lenses are cosmetically more appealing.
After a dispensing optician has already dealt with any dispensing problems, two to three percent of the patients present with non-tolerance to spectacles.
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