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'Emmetropic' eye is the one which has no visual defects. Vision defects which arise due to the optics of the eye are referred to as "Ametropic. 'These defects can be overcome by the use of opthalmic lenses.

Ametropia may be classified as spherical or astigmatic. The former defects arise either due to the physical distance between the eye lens and the retina not matching its focal length (thus resulting in an ill defined image) or due to the variation in the normal values of the refractive indicies or index of one or the other components or fluids of the 'vision system' The change in cuvature, position of an element or the absence of an element would also result in the same. Astigmatic defects can arise due to the refractive index variations, curvature variations or due to the position of an element.

These physical defects of the eye are clinically classified as myopia, hypermetropia, presbyopia, astigmatism and strabismus.

Myopia is also know as short signtedness. The eye can only see near objects clearly and cannot accommodate for infinity or far distance as the eye converges the objects at infinity into an image plane ahead of the retina. This defect can hence be corrected by divergent lens.

Hypermetropia, also termed as far-sightedness, is the defect of the eye when a person is able to  see only far-away distances, and cannot see near objects clearly as the eye focuses at a plane behind the retina. the defect is thus corrected by a convering lens.

Presbyopia is the loss of power of accommodation of the eye experienced normally in the middle age as a part of the physiology of ageing of the eye. The person is unable to see objects at a particular distance due to the loss of accommodative power. This error is remedied by using an appropriate lens to enable vision within that distance.

Astigmatism is a defect caused by the cornea when its surface is not truly spherical. This defect can be corrected by lenses which posses slightly different curvatures in two sections at right angle to each other e.g. cylindrical, sphero-cylindrical or toric lenses.

Strabismus is the combined defect of the two eyes when they are unable to converge simultaneously at the same object. Prismatic lenses are used to correct this defect. 'Aphakia the lack of the eyelens or a condition caused after the eyelens turns opaque and is removed from the eye (e.g. an eye suffering from cataract), can be corrected by the use of very powerful lenses as the accommodation power is also lost with this defect. It is now possible to introduce plastic lenses in the eye.

The ophthalmic lenses need not to be as accurate as optical quality lenses. The techniques for making spherical surfaces are the same but the methods are somewhat different for toric and bifocal lenses.

                                                      TYPES OF SPECTACLE LENSES

Two main types of spectacle lenses are normally used-single vision and multifocal. Single vision lenses include meniscus and toric types. Meniscus are available from -8.00 D to + 7.00D in steps of ).25D. Further toric lenses combining a cylindrical element are also available in steps of 0.25D upto + 4.00D. Thus by a combination of basic lens forms with the required power differences (up to 4D)a suitable range of lenses has been obtained, resulting in a total of 60 x 16 = 960 variations. All these types are normally available from the bulk manufacturers in semi-finished forms so that these could be worked to the required exact prescriptions.

Multifocal lenses are generally of one piece or fused construction. Here again by grouping of a few basic lens forms and differences in power areas upto say 4D, the range does become manageable, so that almost all bifocals can be supplied in the semi-finished form by a mass manufacturer. Fused bifocal and multifocals have also been introduced. While the above sequence would take care of the maximum demand, it goes without saying that a large number of variations would still be required for specialist prescription. However over a period of time, the type of lenses that got evolved could be summarised as under:-

  1.  'Bi' lenses- These are the usual biconcave or biconvex having the same curvature values on both its sides . Such lenses    give  a clear vision only near the axis
  2. Plano-convex or Plano-concave lenses These lenses have the entire power on one surface while the other is plane.
  3. Periscopic lenses-These are actually the combination of plano concave and plano convex lens. The power of this lens is the algebric sum of the individual plano lenses of which the lenses can be supposed to be a combination.
  4. Meiscus lenses- These lenses, also known as half shell lenses, are either concave-o-convex or convex-o-concave. The most usual bas curvatures are + 6D and 6D. While retaining to base curvature the other side is worked to the prescription.
  5. Point Image Lenses- These are special types of lenses which are used beyond the limit of -6D or + 6D. The two curvatures are specially calculated for each power. As beyond +7D extended Sharp field is not obtained by spherical
    curvatures, one of the surface is made aspheric.
  6. Sphero-cylindrical lens- As the name implies, this lens possesses both a spherical and a cylindrical surface.
  7. Toric lenses- This lens has one surface toric and the other spherical. It can be used to correct the defects in both the meridians

Prismatic Lenses-when one the prism surface is made either spherical, cylindrical or toric the lens is known as prismatic lens.

Meniscus and the toric lenses are the most common that are in use.


An opthalmic optician or an optometrist examines the eyes of a patient to find out the type of defects and the amount of defect. After finding out the error qualitatively and quantitatively he prescribes the spectacles specifications. He person who supplies the spectacles is the 'dispensing optician.'

The Prescriptions contains information regarding the type of lenses for both the eyes, and the axis and the distance between the lenses for fitting the frame. In other words details of spherical and cylindrical powers, axis direction, vertex distance, prism or decentration, tint and also the form and type of lens are mentioned in a prescription.

A set pf standard lenses is used for trial test. Different types of lenses with different powers are available in the trial set. Auto collination devices are also now a das in use for arriving at the correct prescription.

                                                     POWER OF OPTHALMIC LENSES

The power of  a lense is defined as the reciprocal of its focal length measured in meters. The unit of power is Diopters. Ophthalmic crown glass that is mostly used has a refractive index of 1.523 for a plano-convex lens therefore the power would be given by

If the refractive index value is taken as 1.5 and symbols D, f and r respectively indicate power in diopters, focal length and radius of curvature in meters. Now if  a bi-convex lense is thought to be as a combination of two plano-convex lenses then we can write a similar equation for the other side i.e.

ignoring the central thickness and taking the appropriate signs into account.

The Total power therefore is D = D1+D2  It is this this definition that gives an ease of calculation to an ophthalmologist, be it for a convex or concave or for meniscus and toric lenses which are much more in use and avoid large thicknesses. The diptric addition can be extended to cylindrical lenses also and the total value calculated similarly for both the planes i.e., along the cylinder or at right angles to it.

Prismatic diopters are also in use. It is defined as the power of a prism which deviates a ray from its path by an amount of 1cm at a distance of 1 meter, n Diopters of a prism indicates that the ray deviates by n cm. The Prismatic power can also be expressed in terms of angular degrees of the deviating angle. For 1 prismatic diopter, the deviation is 31 minutes while for 1 prismatic diopter the deviation is 34 minutes 23 seconds. Use of prismatic lenses is necessary to correct for strabismus.

                                                                CONTACT LENSES

These lenses are fitted under the eyelid and cover the cornea of the eys. In early days glass contact lenses were used but these were not comfortable due to their weight and tendency to breakage. Besides these lenses were not satisfactory because scleras are rarely spherical and cornea vary in size. Later on methyl methacrylate plastic lenses were introduced in 1938 and were found superior. These lenses are light in weight, almost unbreakable and can be worn with comfort for relatively longer periods. The part of the contact lens which is in front of the cornea is called 'corneal part' and the surrounding annular part is known as 'scleral part.' The scleral part rests on the white portion of the eye.

Contact lenses can correct corneal eye-defects which otherwise cannot be corrected by other lenses, e.g. keratocornea (where in the cornea becomes conical in shape), irregular astigmatism or anisometropy (where the axes of two eyes do not meet at a single point)

                                                                   HAPTIC LENSES

These lenses are made by moulding or taking the help of impression of the eye and are known as moulded scleral lenses or haptic lenses. Haptic lenses are held on the eye by putting a buffer solution between the lens and the eye. This solution helps in lessening the corneal astigmatism and irregularities of the cornea. It also avoids the damage of cornea by the lens when the eye turns or lids are closed. This solution should have its osmotic pressure equal to the liquid in the eye cells or may be to that of the 'tears'. This pressure differs from person to person. If the pressure is not equal it may leas to osmosis and then to the dehydration of tissues and swelling of cells. The buffer solutions are made by a combination of an acid with a salt of that acid, or an alkali with an acid salt to the correct pH value of the tears.

                                                              CORNEAL LENSES

These lenses are better than haptic lenses in their mechanical, optical and cosmetic properties. The inside surface of the lens which is concave has the curvature equal to the corneal curvature while the outer convex curvature is decided by the design requirements. For fitment to the cornea it may be necessary to have one or more radii on the concave surface pf the lens. This lens remains in physical contact with the cornea and moves with eyeball whereas the haptic lens does not move with the eye as it has a slight clearance from the cornea. Even though there may be a film of 'tears' between the corneal lens and the eye, it does not cause any damage as the lens moves with eye.

There are three types of corneal lenses-Tuohy, micro and contour lenses. The shape of tuohy lens is always circular with a standard diameter of 11.5 mm, covering almost the whole of the cornea. Its thickness depends upon the power of the lens. The microlenses are small, 8 to 9.5 mm in diameters, and single curve lenses. Another type of corneal lens with multicurved contour known a contour lens provides, opical clearance with good fitting to the corneal curve. All these lenses are normally made from methyl methacrylate, softer corneal lenses can be produced from hydrophilic materials which may consist of about 62% poly (2-hydroxythyl) methacrylate and 38% water by weight when immersed in a normal saline solution.

Contact lenses have many advantages over spectacles such as a much wider field of vision, less distortion, neutralisation of defects like focusing errors, conical cornea ans astigmatism of the cornea. Astigmatism is removed automatically due to the liquid present in between the lens and the cornea. their manufacture takes special care of the material and tooling required but closely follows the optical techniques of pressing, moulding diamond point turning and preferably high speed polishing.

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