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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:-
- '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
- Plano-convex or Plano-concave lenses These lenses have the entire power on
one surface while the other is plane.
- 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.
- 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.
- 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.
- Sphero-cylindrical lens- As the name implies, this lens possesses both a
spherical and a cylindrical surface.
- 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.
OPTHALMIC PRESCRIPTIONS
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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