LESSON 4

PHYSIOLOGICAL AND OPHTHALMOGICAL OPTICS. OPHTHALMOLOGIC TOOLS

Eye is like a camera. The external object is seen like the camera takes the picture of any object. Light enters the eye through a small hole called the pupil and is focused on the retina, which is like a camera film. Eye also has a focusing lens, which focuses images from different distances on the retina. The colored ring of the eye, the iris, controls the amount of light entering the eye. It closes when light is bright and opens when light is dim. A tough white sheet called sclera covers the outside of the eye. Front of this sheet (sclera) is transparent in order to allow the light to enter the eye, the cornea. Ciliary muscles in ciliary body control the focusing of lens automatically. Choroid forms the vascular layer of the eye supplying nutrition to the eye structures. Image formed on the retina is transmitted to brain by optic nerve. The image is finally perceived by brain. A jelly like substance called vitreous humor fill the space between lens and retina. The lens, iris and cornea are nourished by clear fluid, aqueous humor, formed by the ciliary body and fill the space between lens and cornea. This space is known as anterior chamber. The fluid flows from ciliary body to the pupil and is absorbed through the channels in the angle of anterior chamber. The delicate balance of aqueous production and absorption controls pressure within the eye.

Cornea

Front part of the eye, transparent. Protects front of eye and bends light to form an image on the retina.

Pupil

A black hole in the center of the iris. Allows light to enter into the eye.

Iris

Pigmented (the color of the eyes). Its muscles contract and relax to alter the size of its central hole or pupil. Protects the photoreceptors in the retina from being damaged by too much light.

Retina

The lining at the back of the eye containing two types of photoreceptor cells. It is a screen on which images are formed as a result of light being focused onto it by the cornea and lens.

Lens

Transparent, flexible disc behind the iris attached by muscles. Brings the light entering through the pupil to a focus on the retina.

Macula

The macula contains a high concentration of photoreceptor cells that convert light into nerve signals. Send visual signals to the brain.

Optic nerve

Bundle of sensory neurons at back of eye. Carries signals from the photoreceptors of the retina to the brain.

The Eye as an Optical System

The eye can be considered as an optical system with a positive power of about 58 D. It has two main refractive elements, the cornea and the lens. The cornea bulges in the front of the eyeball, and because its first surface is in contact with air, it bears most of the power of the eye (about 45 D). The eyeball has a mean length of 24 mm, and the image is formed at the interior of the back side, where the retina is found. The aqueous humor, which has a refractive index of 1.336, is located in the midst of the cornea and the lens. The volume behind the lens is filled with vitreous humor, with index 1.337. The power of the lens is not fixed, and it can expand surface curvature and power via the ciliar muscles surrounding it. This process is called accommodation, and serves to bring near objects into focus. The pupil of the eye is located 3 mm behind the front vertex of the cornea. Its diameter ranges from 2 to 8 mm, and it is influenced by lighting conditions, age, drugs, and health status.

Visual acuity is defined as the inverse of the smallest angle subtended by two points so they can be resolved.

Visual acuity is not constant along the field imaged by the optics of the eye. The highest density of photoreceptors, and therefore the highest visual acuity, is located in a very narrow region known as the fovea. We distinguish colors, read (among other highly specialized tasks), and scrutinize the world by using this small part of the retina. Therefore, when we say the image is focused onto the retina, we really mean that it falls on the fovea, where it can be better detected, recognized, and identified by the visual cortex in the brain. The outer retina develops other important missions in our spatial perception of the surrounding visual environment.

In order to extend the small angle subtended by the fovea to a wide field with high resolving power, the eye rotates in different directions within different ranges. The location of the center of rotation relates with the anatomic structure of the eyeball and the surrounding muscles.

Although each movement has its own center of rotation, it is possible to define a mean center of rotation, located, on average, 15mm behind the corneal apex.

Ammetropies and Refractive Error

Emmetropia is defined as the condition for which the relaxed eye (without accommodation) images a distant object onto the retina (fovea).

We say that the eye suffers from refractive error when it fails to bring into focus the image of a distance object. The condition in which a refractive error occurs is called ammetropia.

Let us define the remote point as the conjugate point of the fovea in a relaxed eye. The refracting error, R, is then defined as the inverse of the distance from the principal plane of the eye to the remote point. The conjugate point of the fovea moves when the eye accommodates. It reaches the so-called near point when the eye attains its maximum accommodation. The range between the remote and near points is called the range of clear vision. All points within it are accessible via accommodation. Amplitude of accommodation is defined as the difference between the refractive error and the inverse of the distance between the principal point of the eye and the near point. Ammetropia can be classified into four different types: two forms of spherical ammetropia (myopia and hyperopia), astigmatism, and presbyopia.

Myopia: The image of a distant object forms before the retina because the eye is too powerful, too large, or both. Refractive error is negative and the remote point is located in front of the eye. Near point is also located in front of the eye, at a smaller distance from it. Glasses or contact lenses with concave lenses will correct the eye's error and bring the images of far-off objects into sharp focus on the retina. Concave lenses curve inward, like the inside of a bowl.

Hyperopia: The image of the distant object forms behind the retina as a result of any of the followingreasons: 1) the eye does not attain enough power, 2) the eye is too small, or 3) combination of both. In case the refractive error is smaller than the amplitude of accommodation, the hyperopic eye may bring the images of distance objects into focus via accommodation. The remote point of the hyperopic eye is located behind (to the right of) the same. The near point can be located at the right of the remote, when amplitude of accommodation is smaller than the refractive error. Otherwise, it is located at a finite distance in front of the eye. When blurred vision occurs, an ophthalmologist may prescribe glasses or contact lenses, with convex lenses to reinforce focusing power. A convex lens is rounded outward, like the outer surface of a globe.

Astigmatism: The eye shows different powers at different meridian planes. We will assume that the meridian planes with maximum and minimum power are orthogonal (regular astigmatism). These are called the principal meridians. The power difference between them is termed the astigmatism of the eye. When characterizing astigmatism, aside from the value of the difference between powers, it is also necessary to provide the orientation of any of the principal meridians. The condition can be treated with cylindrical lens glasses or with hard contact lenses. A special "toric" soft lens is also available now. Cylindrical lenses are shaped like slices from a tube and compensate for the defects of the eye by bending the light rays inward.

Presbyopia: The amplitude of accommodation reduces with age and along with it, the ability to focus near objects. When amplitude of accommodation reduces be- low 3 D (which usually happens between 40 and 50 years of age in Europe), comfortable reading at 33 cm is no longer possible, and ophthalmic compensation is necessary for activities using near vision. This is a common condition and simple convex lens reading glasses are used to correct it.

When the eye rotates, the remote point, locked to the visual axis, also rotates. Assuming a well-defined rotation center, the locus of points that correspond with remote points for different sight directions is a sphere called remote point sphere. The center of this sphere coincides with the rotation center of the eye. As the eye accommodates, the radius of the remote point sphere changes accordingly with eye ammetropia. For myopic eyes, the radius decreases as accommodation increases. The same occurs for hyperopic eyes with accommodation values greater than the refractive error. For hyperopic eyes with accommodation values smaller than the refractive error, the radius of the remote point sphere increases with accommodation. In any case, the accommodation always causes the remote point sphere a movement to the right (light travels from left to right). When accommodation reach its maximum, the sphere of conjugate points turns into the near point sphere. In astigmatic eyes, there are two remote and two near points, one for each of the principal meridians. Accordingly, there are two remote point spheres and two near point spheres. Fig. 1 shows the situation of the remote point and the near point for a myopic and a hyperopic eyes.

LENSES

Patients normally come to an ophthalmologist to determine their refractive status of their eyes either diagnostic or treatment. So, ophthalmic dispensing is certainly a significant part of ophthalmology. The purpose of this article is to provide a brief resume of the types of spectacle lenses available to the patient for better vision (visual needs). The medical practitioners should know about the ophthalmic lenses to able to manage the patient’s requirements.

History

The use of spectacles has its origin in ancient history. Marcopolo noted the first authentic record, when he visited China in 1270. He noted that convex lenses were used by the aged to read the fine print. In Europe, the famous English Franciscan monk Roger Bagon was the first to recognize the value of +lens.

In 1784, Benjamin Franklin invented the first bifocal by dividing his lenses for distance and near. Cemented bifocals, were invented in 1884, the fused bifocals in

1890, and solid (or) one-piece types followed in 1906.

Materials used in spectacle lens
Spectacle lenses are made from three different sources of materials. In spectacle lens, there are many materials used. Natural media, quartz (or) rock crystal, semi-precious stones (i.e. Topaz, Ruby, etc) were widely used for making lenses.

Glass materials

Now-a-days spectacle lenses are made from either plastic or a high quality glass material. Although many types of glass materials are used in optical industry, crown glass (1.523) material is extensively used for making single vision ophthalmic lenses. It is a soda-lime-silica material that contains about 70% silica,

12% calcium oxide and 15% sodium oxide and some other materials in smaller percentages like potassium, borax, arsenic etc. Flint glass, material (1.620) is used in the making of bifocal or achromatic lens. It contains 60%, lead oxide, 30% silica, 8% soda and potash and small percentage of arsenic. The lead oxide material increases the refractive index of glass as well as specific gravity and weight. This material is used for making the bifocal segment and also it produced high chromatic aberration. In later days, Flint glass material was replaced by barium crown material, which has no chromatic aberration. It contains 35% Barium Oxide, 30% silica and small percentages of lime, zinc, zincorium, aluminium, boran etc. It increases the refractive index of the glass. Now-a-days almost high quality bifocal lenses are made only from Barium-crown glass material. The high-index glass named Hidex (1.806), which is used for making high refractive power lenses in a remarkably thin form. High-index glass is very suitable with antireflection coating, giving a superior effect to the appearance of crown glass. Such glasses are obviously not suitable in fused bifocal form.

Plastic materials

Plastic lenses are generally made from two different materials. They are:

1. Original plastic lens made of (PMMA) Polymethylmethocrylate

2. Modern hard resin lens from allyl diglycol carbonate (CR 39) which is harder and more resistant to scratches than other plastic lens materials.

Plastic lenses are made from a very high quality material as glass. Plastic lenses are about half the weight of glass and are highly impact-resistant. With a center thickness of 3.0mm without special hardening process. Plastic lenses have a thicker profile than glass, get scratches more easily and do not protect the eye from UV rays unless properly tinted. Glass lenses unlike plastic, must be treated to resist breakage. They can be hardened by chemical or heat processes.

Corrective lens

In optical lab each manufacturer has an individualized series of base curves for toric meniscus lenses. In theory, the inside Base Curve should have the same radius coincides with that of the rotating eye ball, that maintains the proper vertex distance between cornea and the spectacles to avoid distortions. A corrected lens is a compromise designed to avoid or minimize the distortions through the edges of the lens. Corrected lenses are in different designs but mainly in meniscus lens form, which are usually ground on 6.00D Base curve.

A toric lens is curved like a meniscus lens but also contains a cylinder that is ground on a convex surface in single vision lens and on a concave surface in bifocal lens. In plastic lenses the cylinder is ground on concave surface, most modern plastic cylinder lens have the cylinder on concave side and are referred to as “cyl” lens.

Multi focals designs

There are two basic types of multifocal lenses used.

1. In one-piece (or) solid type designs, the same material (glass or plastic) is used throughout the lens and changing the curvature of lens varies the power. (Fig.1)

The Executive bifocal (glass or plastic) is a modern version of the original Benjamin Franklin bifocal which has two lenses in each eye of which the lower half is used for closer view and the upper half for distance.

2. The fused multifocal lenses are made of two or more glass materials with different refractive indexes when the segment with higher indices is fused into the main lens; the surfaces of fused lens have no change of curvature.

(Fig. 2) Falling into a category between one piece and fused lenses are cement bifocals. Two lenses of the same type having the same index of refraction are attached together to form a lens with the special features of the one-piece lens. The original cement used was Canada balsam and choser because it has a refractive index close to that of glass. Cemented segments are useful because it can be made of any power ranges and positioned anywhere into the main lens. These types of lenses are particularly helpful for a patient with low visual acuity who needs the high add powers (+20.0 DS). A high powered lenticular lens can be constructed using the thermally cemented segment process. There are now numerous designs of

Multifocals available in market. Bifocals have an inherent disadvantage which is the image jump when there is a change in the direction from distance zone to near zone. Effort is made to minimize the image jumps/ displacements by incorporating additional segments with optical centers designed to produce

a compensating prismatic effect.

Progressive addition lens

Among the various types of multifocal designs progressive addition lens has become very popular now-a-days. Over 150 PAL designs have been introduced since 1984 with more than 70 PAL designs currently available in the market.

The progressive addition lenses gradually increases in power as the line of sight comes downwards through the lens. The main difficulty in any lens that gradually increases in power is that vision on either side of a vertical line through the optical centre produces unwanted cylinder causing great distortion, which occurs in the lower part of both sides of the lens. Progressive addition lenses are suitable for the person who need the intermediate vision (e.g. computerist) and for the constant bifocal wearers. The progressive additive lens is recommended for the presbyope to provide clear distance, intermediate and near vision without visible separating lines.

High power lenses

In high-powered lenses a strong distortions would occur through the edges of the lens inherently. To avoid these distortions special lens have been designed to minimize the distortion and the weight of these lenses.

The types most commonly used are

(i) Lenticular lens

(ii) Aspheric lens

Lenticular lens

The lenticular lens may be described as a small in diameter or circular and mounted on a longer diameter, thin planocarrier which is edged to fit into the frame. The main disadvantage of lenticular lens is that it gives a bull’s eye effect making it more conspicuous than the other lens.

Aspheric lens

An Aspheric lens is particularly designed to eliminate the “pincushion” distortion in the (aphakia) high plus lenses.

A standard “+” lens suitable for a patient who has had a cataract operation may be optically correct through its center but it gives a increased strong effect as the patients vision moves away from the center to the edges of the lens. This increase in ‘+’ power towards the edges resulting from an increase in vertex distance, causes the pincusion distortion as well as blurred vision in these areas.

Fresnel lenses

Fresnel lenses are sheets of Polyvinyl chloride, and it was designed by Augustine Fresnel. The Fresnel lenses are used for various purposes. It is used in ships and lighthouses as a “light-condensing lens”. As Fresnel lens is thin and weightless it would make an ideal cataract lens but it shows a pattern of fine concentric circles, which gives poor cosmetic appearance to the wearers.

Safety lens

The risk of damage to the eye from broken glasses is minimized by the use of safety glass. It is however advisable to use it for those who are engaged in industrial works and sports.

Plastic hard-resin lenses

These are safety lenses with no additional treatment, because they will take abuse much greater than that required to shatter a standard glass lens. A shattered hard-resin lens does not have the sharp splinters typical for broken glass. Hard resin lenses are superior to hardened glass for welding for if not metal may splatter on the lens.

Another type of safety lens is the laminated lens in which a sheet of plastic is sandwiched between two pieces of glasses. If the lens is shattered, the glass particles adhere to the plastic.

Polycarbonate lens

First introduced in plano safety goggles in industry, polycarbonate lenses are one the most impact-resistant lenses now available in the market. In this regard they out perform plastic and glass heat-treated or chemically treated and thus easy to scratches.

Polycarbonate is now being moulded into ophthalmic Rx lenses that are coated to substantially reduce their tendency to scratch.

Heat-treated impact-resistant lens

The polished lens is heated an oven almost to its melting or softening point, removed and then rapidly cooled by blasts of cold air on both surfaces simultaneously. The surfaces cool and contract quicker than the interior of the lens. Eventually the interior of the lenses cools and contracts and brings the surface of the glass lens into a compression condition. All heat-treated glass lenses may be identified by the fact that they un polarize the polarized light. A popular type of sunglass in the market is the polarized sunglass that are usually made of plastic but sometimes are found in a laminated form in which the polarizing filter is sandwiched between two sheets of glass. Polarized sunglasses are available in prescription form.

Some glasses in the market are named ‘toughened lens’. Another type of impact-resistant lens is a chemical treatment lens which is popularly employed and is far superior in impact-resistant, qualities to those produced by the heat-treating method, this type of lens consists of placing the finished glass lens in a hot solutions of potassium salt for about 14 hrs.

Meanwhile the sodium ions in the glass are replaced by potassium ions. While cooling the potassium ions place the surface in a state of compression, which give impact resistant properties to the lens. Unlike the heat-treated lens, polarized lights cannot delete this process. Failing other indications that the lens has been chemically treated.

Safety lenses are recommended mainly for children, with refractive errors and for those who are engaged in sports and industrial works.

Tinted lenses

By using of the appropriate chemicals, white lens may be dyed with colour or mirrored evenly. The popular tints are available as green, neutral gray, pink, brown and transparent one-way mirror surfaces. Tinted ophthalmic lenses give an even colouring across the whole surface of the lens whether it is a strong plus or a strong minus lens. The coating, whether it is anti reflection mirror or not, colour may be removed chemically in about 10 seconds, should this be necessary.

Almost all sunglasses are white lenses coated with colour. However, coloured-glass lenses are still available that are perfectly satisfactory for plane and weak power prescriptions. In high-powered lenses, such tinted lenses are not suitable for perfect vision. The tinted lens usually reduce the actual illumination level so that such lenses are not suitable for high power lens as well as Bifocals.

Neutral gray tint has been the most popular tint for sunglasses. Because of its neutral absorption of all colours of the visible spectrum, light intensity is reduced without colour distortion or imbalance- a very important factor when proper colour perception is concerned. Such a gray lens can be recommended for one who wants perfection from “intensive light” or glare without loss of colour patterns.

A green tint lens will absorb most of UV & IR light and transmission peaks roughly at the same point as the luminous curve of the eye.

Green lenses are recommended for situations with high amount of reflected (UV) light. In industry green lenses of various densities are used for welding and other high light and heat situations. They also have the psychologics effect of “coolness” during hot weather and thus provide comfort to the wearer. For many years, brown tints were very popular. Brown lenses will absorb almost all of the UV light and have a very even progressive curve throughout the visible spectrum. This type lenses are most useful made moderate-to-cold-climate radiations with the added benefit of creating a “warm” visual environment.

Yellow tint lenses are good absorbers of UV, violet, and blue suppressing this area of the spectrum will enhance contrast in the rest of the visible spectrum. A yellow lens therefore is preferred to increase contrast in marginal light conditions, such as twilight and foggy condition, but should not be worn to protect against excessive light.

Other tints, such as pink, purple and mauve are deviants of the aforementioned tints and are used mainly as fashion accents. Cheap sunglasses are made from flat, coloured glass sheets of low quality.

Photochromic lens

Photochromic lenses have the chamelon-like ability to change from light to dark and back again. The silver halide microcrystals in the glass, which gives this changeability never wear out. The halide becomes darken when exposed to UV light or blue end of the spectrum.

The cycling of the modern photochromic lens requires 60 seconds to become darken. The range of dark to light is greater in cold weather than in hot.

The clear photo-chronic lens darkens only to the point where 15% of the light is filtered out and is not really dark enough when compared with standard sunglasses. It is available in three different colours; grey, pink and brown.

In dispensing tinted lenses it is important to know about light conditions and environment the lenses are going to be used in. In dark areas, tinted lens will dilate the pupil and reduces the visual acuity. Providing a medium tint with a gradient mirror will help the patient to select the density according to the common light conditions.

Ultraviolet and blue-blocking lenses

Through-and-through tinted lenses that filter out over 98% of blue and UV light were developed. The latter is made in CR-39 plastic which is in colours that rang from amber to red. For the normal individual these lenses provide comfort from glare, reduce haziness and create sharper vision; such light-colour lenses may help persons with developing cataracts and the darker red lenses may improve the functional visual acuity for such conditions like Retinitis Pigmentosa albinism, aniridia and intense photophobia.

Mirrored sunglasses

In mirrored sunglasses a one-way mirror surface is placed on a white or coloured lens to convert it in to a sunglass for special purpose.

There are situations in which patients do not wish to trouble their eyes (perhaps because of disfigurement). Mirrored sunglass give a cosmetic protection and the patients has no trouble seeing through the mirrored lenses.

Anti-reflection coating

Artificial powerful lights (i.e.) halogen lights in cars and trucks, and computer monitors can cause reflections in untreated glass and produce ghost images. It occurs mainly in high index glasses. When light passes through spectacles some light rays are reflected by front and back surface of the lens. Streetlights in the driver’s field of view may be duplicated or triplicated. AR coated lens prevent these images and only one image is seen.

Magnesium fluoride a material 1/4th of wavelength of yellow-green light, which is heated and fumed on the lens surfaces. The coating material is very tough and usually lasts througout the life of the lens. It will be seen on the surface of the lens. All camera lens and ophthalmic instrument lenses are coated to filter out internal reflections and permit greater light transmission. Almost all lens coatings are not multicoatings. These coatings, which are almost invisible, eliminate the red and blue end of the spectrum. Such multi coated lens should be cleaned with proper special cleaning solutions. AR coated lenses are readily available in market and it can be coated separately too.

CONTACT LENSES

A contact lens (also known simply as a contact) is a corrective, cosmetic, or therapeutic lens usually placed on the cornea of the eye. Leonardo da Vinci is credited with describing and sketching the first ideas for contact lenses in 1508, but it was more than 300 years later before contact lenses were actually fabricated and worn on the eye. Rigid ones were produced and marketed first. Modern soft contact lenses were invented by the Czech chemist Otto Wichterle and his assistant Drahoslav Lím, who also invented the first gel used for their production.

Some soft contact lenses are tinted a faint blue to make them more visible when immersed in cleaning and storage solutions. Some cosmetic lenses are deliberately colored to alter the appearance of the eye. Some lenses now have a UV protection surface treatment to reduce UV damage to the eye's natural lens.

It has been estimated that 125 million people use contact lenses worldwide (2%),including 28 to 38 million in the United States and 13 million in Japan.The types of lenses used and prescribed vary markedly among countries, with rigid lenses accounting for over 20% of currently-prescribed lenses in Japan, the Netherlands and Germany but less than 5% in Scandinavia.

People choose to wear contact lenses for many reasons, often due to their appearance and practicality.When compared with spectacles, contact lenses are less affected by wet weather, do not steam up, and provide a wider field of vision. They are more suitable for a number of sporting activities. Additionally, ophthalmological conditions such as keratoconus and aniseikonia may not be accurately corrected with glasses.

Types of contact lenses

Corrective contact lenses

A corrective contact lens is designed to improve vision. For many people, there is a mismatch between the refractive power of the eye and the length of the eye, leading to a refraction error. A contact lens neutralizes this mismatch and allows for correct focusing of light onto the retina. Conditions correctable with contact lenses include myopia (near or short sightedness), hypermetropia (far or long sightedness), astigmatism and presbyopia. Contact wearers must usually take their contact lenses out every night or every few days, depending on the brand and style of the contact. Recently, there has been renewed interest in orthokeratology, the correction of myopia by deliberate overnight flattening of the cornea, leaving the eye without contact lens or eyeglasses correction during the day.

For those with certain color deficiencies, a red-tinted "X-Chrom" contact lens may be used. Although the lens does not restore normal color vision, it allows some colorblind individuals to distinguish colors better.

ChromaGen lenses have been used and these have been shown to have some limitations with vision at night although otherwise producing significant improvements in color vision. An earlier study showed very significant improvements in color vision and patient satisfaction.

Later work that used these ChromaGen lenses with dyslexics in a randomised, double-blind, placebo controlled trial showed highly significant improvements in reading ability over reading without the lenses. This system has been granted FDA approval in the USA.

Cosmetic contact lenses

A woman wearing a cosmetic type of contact lenses; the enlarged section of the image shows the grain produced during the manufacturing process. As the lines of printed dots are curved, these lenses were manufactured by printing onto a flat sheet and then shaping the sheet.

A cosmetic contact lens is designed to change the appearance of the eye. These lenses may also correct the vision, but some blurring or obstruction of vision may occur as a result of the color or design. In the USA, the Food and Drug Administration frequently calls non-corrective cosmetic contact lenses decorative contact lenses. These types of lenses tend to cause mild irritation on insertion, but after accustoming to the lenses, the eyes are typically well tolerated. As with any contact lens, cosmetic lenses carry risks of mild and serious complications, including ocular redness, irritation, and infection. All individuals who decide to wear cosmetic lenses should check with an eye care provider prior to first use, and periodically over long term use in order to avoid potentially blinding complications.

Theatrical contact lenses are a type of cosmetic contact lens that are used primarily in the entertainment industry to make the eye appear confusing and arousing in appearance,most often in horror film and zombie movies, where lenses can make one's eyes appear demonic, cloudy and lifeless, or even to make the pupils of the wearer appear dilated to simulate the natural appearance of the pupils under the influence of various illicit drugs.

Scleral lenses cover the white part of the eye (i.e. sclera) and are used in many theatrical lenses. Due to their size, these lenses are difficult to insert and do not move very well within the eye. They may also hamper the vision as the lens has a small area for the user to see through. As a result they generally cannot be worn for more than 3 hours as they can cause temporary vision disturbances.

Similar lenses have more direct medical applications. For example, some lenses can give the iris an enlarged appearance, or mask defects such as absence of (aniridia) or damage to (dyscoria) the iris.

A new trend in Japan, South Korea and China is the circle contact lens. Circle lenses appear to be bigger because they are not only tinted in areas that cover the iris of the eye, but tinted prominently in the extra-wide outer ring of the lens. The result is the appearance of a bigger, wider iris.

Although many brands of contact lenses are lightly tinted to make them easier to handle, cosmetic lenses worn to change the color of the eye are far less common, accounting for only 3% of contact lens fits in 2004.

As a specialist's tool, in the hands of the untrained general public, nonprescrition cosmetic contact lenses may represent a health risk.

Therapeutic contact lenses

Soft lenses are often used in the treatment and management of non-refractive disorders of the eye. A bandage contact lens protects an injured or diseased cornea from the constant rubbing of blinking eyelids thereby allowing it to heal. They are used in the treatment of conditions including bullous keratopathy, dry eyes, corneal ulcers and erosion, keratitis, corneal edema, descemetocele, corneal ectasis, Mooren's ulcer, anterior corneal dystrophy, and neurotrophic keratoconjunctivitis. Contact lenses that deliver drugs to the eye have also been developed.

Types of Contact Lenses

Daily-wear soft lenses (Yearly)

The most popular type of lenses.

Made of soft, flexible plastics that allow oxygen to pass through to the eyes.

Short period of adaptation.

More comfortable and more difficult to dislodge than RGP lenses.

Available in bifocals and colors.

Ideal for active and sportive lifestyles.

Lens Care Products are very simple to use.

Daily disposable soft lenses

No lens Care products are required.

Clean, fresh and sterile lenses replaced every day.

Ideal for active lifestyles.

Monthly disposable soft lenses

Clean, fresh and sterile lenses when replaced every month.

Available in most prescriptions.

Useful as spare lenses.

3 Months disposable soft lenses

All the benefits of yearly contact.

For improve health.

Available in most prescriptions lenses

Gas Permeable (GP)

Made of harder plastic materials that do not contain water.

Made of slightly flexible plastics, not as flexible as soft contacts, but they allow more oxygen to pass through to the eyes than do soft lenses.

Comfortable for most people after a short period of adaptation

Sharper vision than with soft contact lenses

Causes less infection than soft lenses.

Relatively long life (3-4) years

Available in bifocals or multifocals

Daily-wear and extended-wear designs available

Disadvantage:

They may slip off the center of the eye more easily

Once you don't wear these lenses for about week, it needs an adaptation period before they're comfortable again.

Ortho-K

A vision correction therapy for patients with moderate myopia and low amounts of astigmatism.

It involves wearing a contact lens retainer while you sleep to gently and painlessly corrects the surface of your eye.

This process is reversible and non-surgical.

Daytime free of contact lenses and spectacles.

Ideal for sports, swimming and for dusty or dirty environments.

CARE

While daily disposable lenses require no cleaning, other types require regular cleaning and disinfecting in order to retain clear vision and prevent discomfort and infections by various microorganisms including bacteria, fungi, and Acanthamoeba, that form a biofilm on the lens surface. There are a number of products available for cleaning:

Multipurpose solutions - The most popular cleaning solution[clarification needed] for contact lenses; these are suitable for rinsing, disinfecting, cleaning and storing lenses, and in most cases eliminate the need for protein removal enzyme tablets. Some multipurpose solutions are not effective at disinfecting Acanthamoeba from the lens. In May 2007, one brand of multipurpose solution was recalled due to a cluster of Acanthamoeba infections. Newer generations of multipurpose solutions are effective against bacteria, fungi, and acanthamoeba and are designed to condition the lenses while soaking.

Saline solution - Used for rinsing the lens after cleaning and preparing it for insertion. Saline solutions do not disinfect.

Daily cleaner - Used to clean lenses on a daily basis. A few drops of cleaner are applied to the lens while it rests in the palm of the hand, and the lens is rubbed for about 20 seconds with a fingertip (depending on the product) on each side. Long fingernails can damage lenses.

Hydrogen peroxide solution - Used for disinfecting the lenses, and available as 'two-step' or 'one-step' systems. With 'two-step' products, the peroxide must be rinsed away with saline before the lenses may be worn, because hydrogen peroxide is an irritant and strong oxidizer.[47]

Enzymatic cleaner - Used for cleaning protein deposits off lenses, usually weekly, if the daily cleaner is not sufficient. Typically, this cleaner is in tablet form. Protein deposits make use of contact lenses uncomfortable, and may lead to various eye problems.

Ultraviolet, vibration or ultrasonic devices - Used to both disinfect and clean contact lenses. The lenses are inserted inside the portable device (running on batteries and/or plug-in) for 2 to 6 minutes during which both the microorganisms and protein build-up are thoroughly cleaned. Saline solution is typically used as multi-purpose solutions are not necessary. These devices are not usually available in optic retailers but are in some electro-domestic stores.

Some products must only be used with certain types of contact lenses: it is important to check the product label for lens compatibility. Water alone will not adequately disinfect the lens, and can lead to lens contamination and has been known in some cases to cause irreparable harm to the eye. Proper lens cleaning is important in warding off biofilm formation.

The cleanliness of the cleaning products themselves is important, and to counteract minor contamination of the product and kill microorganisms on the contact lens, some products contain preservatives such as thiomersal, benzalkonium chloride, and benzyl alcohol. In 1989, thiomersal was responsible for about 10% of problems related to contact lenses: because of this, many products no longer contain thiomersal. Preservative-free products usually have shorter shelf lives. For example, non-aerosol preservative-free saline solutions can typically be used for only two weeks once opened. The introduction of silicone-hydrogel soft contact lens materials in 1999 made selection of the proper disinfecting solution more important.

OPHTHALMOLOGICAL DIAGNOSTIC EQUIPMENT

Torch light

Description and purpose: A torch light is the first instrument an ophthalmologist uses to examine the eye of a patient. A good torch light should give a circular patch of light of nearly uniform brightness.

Several models of torch light are available in the market. Any one of them would suit the purpose. The light from a torch light that uses two of the regular 1.5V dry cells and a bulb of 1 or 2 watt rating is sufficient for the initial examination. If the light is such that it uses the dry type D cells it can be used for several weeks for normal usage before the cells are to be replaced. Torch lights with rechargeable cells are also available.

The front glass cover, the bulb, the concave reflector, the switch, the cells and the barrel are the main parts of the torch cell

Ophthalmoscope

Description and Purpose: Ophthalmoscopes are of two kinds direct and indirect. Direct Ophthalmoscope which is usually referred to as ophthalmoscope, and sometimes briefly as the ‘scope’, is a very handy instrument for the examination of the retina around the fundus. Light from a bulb is reflected at right angles and projected as a spot through the iris of the patient to illuminate the retina. This reflection is achieved using a front silvered mirror or partially silvered mirror or a total reflecting prism in different scopes. The illuminated retina is seen directly by the doctor through the iris of the patient. A disc with lenses of different powers is provided in the instrument and a lens of required power can be brought in the line of sight to correct any refractive error of the patient or of the doctor himself if he does not look through his spectacles. The doctor looks just above the front silvered mirror and the reflecting prism or through the partially silvered reflector mentioned earlier. An image magnified nearly fifteen times is seen.

Description of major subsystems: The Ophthalmoscope has two major subsystems.

1. An electrical system

2. An optical system also known as the head.

The electrical system: This consists of either dry cells or a rechargeable battery, used to light a bulb through a switch and a rheostat (a variable resistor) that controls the current through the bulb for changing its brightness. Some ophthalmoscopes run on the main electric supply through a step down transformer. The tube in which the cells (rechargeable battery) are kept also serves as the handle for the instrument. The instrument that works on mains supply is provided with a solid handle. As in the case of torch light described in chapter-4 it is possible to convert locally a battery operated ophthalmoscope into an mains voltage operated one using a suitable step down trans- former and an associated regulator circuit.

The optical system (the head): This is fitted on the handle with a spring loaded locking or screw mechanism. It consists of (1) A system of condensing and focusing lenses and a reflector (a front coated mirror or a total reflecting prism) to produce the spot of height and (2). The viewing system consisting of a disc with lenses of different powers usually ranging from -20 to +20D. In some instruments a high positive or a high negative lens can be brought in by a sliding adjustment. In some instruments that have a much wider range of powers (-40 to +40D), two wheels carrying the lenses are provided and a combination of two lenses give the required power. The power of the lens used for viewing is indicated on the disc and can be seen through a window in the head. The reading is illuminated using a partially reflecting plate held in the path of the main light team.

There exist provisions in the head for changing the spot size or for obtaining a semi- circular spot or for reducing it to a streak or to provide concentric circles on the spot using different stops in the path of the light beam. Also there is be provision for introducing filters (red free or polarizing) in the path of the light for special applications.

Retinoscope

Description and purpose: Different types of retinoscope are used. One of them, the streak retinoscope described here, is the most common instrument. It is used in the objective evaluation of the power of the spectacles needed to correct the refractive error of patients and also in determining the axis and cylindrical power needed for patients with astigmatism. As in direct ophthalmoscope (chapter-5) light from a bulb is reflected at right angles and is projected on to the eye of the patient. The light is either a rectangular patch (known as plane mirror mode) or a streak of light of variable width (concave mirror mode). The instrument is generally used in the cubicle of refractionist/optometrist which is usually dark.

Description of major system: The streak retinoscope has the two major subsystems like direct ophthalmoscope

1. An electrical system

2. An optical system known as the head.

Several suppliers supply a streak retinoscope head that fits onto the handle of the direct ophthalmoscope that holds the electrical system. Thus one can get a two-in-one set and save some cost.

All streak retinoscope are almost identified in construction. A picture of the retinoscope head with labelled parts is as shown in figure

The electrical system: Some as the one of direct ophthalmoscope except that streak retinoscopes use a bulb with straight filament (The bulbs of ophthalmoscopes and retinoscopes are not interchangeable).

The optical system: The distance between the focusing lens of the instrument and the bulb is movable. This is done by moving a sleeve up or down. This produces either a patch of light or a streak of light of variable width. The bulb is positioned in such a way that it can be turned about the axis of the instrument usually by turning the same sleeve.

This results in the rotation of the patch of light or the streak about the line of sight of the user. The reflector used is always a front silvered mirror and is much wider. In some instruments the reflector has a peep hole at the centre of the reflector. In other instruments the user has to look just above the reflector. The lens disc present in ophthalmoscope is not present in streak retinoscope.

Indirect Ophthalmoscope

Description and Purpose: The modern Indirect Ophthalmoscope functions as the eye piece of a stereomicroscope for which a hand held high positive aspheric lens (17D, 20D or 30D) serves as the objective. When viewed properly, a magnified image of the retina is seen. Some of the advantages of the instrument as compared to the direct ophthalmoscope are (1) stereoscopic view (2) greater field of view (3) increased illumination and (4) reduced distortion. An additional advantage is that the doctor is at a distance from the patient. However, the final image seen is inverted and the magnification is much lesser than in a direct ophthalmoscope.

Description of major subsystems: An indirect ophthalmoscope has four major subsystems.

1. An illumination system

2. An electric system

3. A stereoscopic viewing system (vision box)

4. A head band that supports the illumination system and the vision box

All models of the indirect ophthalmoscopes are similar in construction. A picture of Indirect Ophthalmoscope is shown in figure.

The Illumination System: This consists of a tungsten filament lamp or a halogen lamp and a front silvered concave reflector suitably positioned behind the lamp. Two condensing lenses are placed in front of the lamp. The lens close to the lamp is fixed while the other lens could be moved forward or backward and fixed in position with the help of a spring loaded screw. There is provision for introducing filters of required characteristics in the path of the light. The light coming through the second lens is reflected using a front silvered mirror to provide the illumination at the eye of the patient. The mirror could be tilted and fixed in any required position for easy examination. The size of the spot could be varied by pushing in stops of different sizes in the path of the light near the bulb.

The Electrical System: This consists of a step down transformer provided with a switch, a rheostat, a fuse and a sufficiently long connecting cable. The transformer is either fixed on the wall near the examination table or kept in the box of the instrument.

The Vision Box: This has two eye pieces. They can be moved laterally in the vision box to match with the inter pupillary distance of the doctor. The hand held high power positive aspheric lens gives a real inverted image of the patient's retina in space in front of the lens. The light from this image meets a 90° wedgeformed by two mirrors in the vision box. The wedge divides the beam into two beams which are further reflected by two 45° mirrors (or total reflecting prisms) before they reach the eye of the doctor through the eye pieces. The ray diagram indicating the working of vision box is as shown in figure.

The vision box is rigidly attached to the illumination system. This is usually well sealed so that no dust enters the box. In some instruments, a pair of semi-silvered reflectors can be fixed in the vision box. These are known as teaching attachments. They are useful for assistants (students) to look at what the doctor is looking at. The picture of an indirect ophthalmoscope with this attachment is shown in Figure.

The Head Band: The illumination system and the attached vision box are attached to a head band that a doctor can wear conveniently. The cable for the lamp is also attached to the head band. The illumination system, vision box combination could be tilted and fixed at the desired position using screws on the head band. While in use, the eye pieces are as close to the doctor's eyes as possible to give a wide field of view.

The illumination system and the vision box are also attached to a spectacle frame.

The box provided for the instrument is such that when not in use the instrument together with the head band and the cables could be kept in marked positions in the box. Sometimes, the instrument with the head band is kept hanging on a hook on the wall near the examination area. Since the examination using indirect ophthalmoscope is done without any external light, one can arrange such that when the ophthalmoscope is removed from its hook the room light is turned off and the light is turned on when the instrument is back on its hook.

Slit Lamp

Description and purpose: The slit lamp is one of the most commonly used diagnostic instruments of an ophthalmologist of today. It provides illumination and magnification for examining the eye and its various parts. The light is projected as a bright slit, thereby enabling detailed examination of the eye in small segments. It is used in the examination of the anterior segment of the eye, including the crystalline lens. With supplementary lenses the slit lamp is useful in the examination of the posterior region of the eye and the chamber angle, the fundus and good part of the retina. A number of accessories can be added to a slit lamp to convert it into a measuring instrument as well. One can measure intraocular pressure, the curvature of the cornea, the thickness of the cornea, the distance between the cornea and the lens, the anterior chamber volume, the opacity, etc. using different attachments. Some slit lamps can have camera attachment for photographic recording. Slit lamps are also used to deliver laser beam spots at any required place in the eye for treatment.

Description of major subsystems: A modern slit lamp consists of three major components:

1. An illumination system - light source, mirrors & prisms

2. A magnification system - the bio microscope

3. A mechanical system that links the magnification with the illumination system and provides vertical and lateral movements to focus the light on the desired part of the eye.

Generic models: There are essentially two types of modern slit lamps that are in use. One has the illumination system at the top while the other has it at the bottom. The other variation is in the magnification system as described below. The instrument with illumination system at the top can be tilted about a horizontal axis so that light is projected up or down at an angle. In the other the light is projected horizontally at all times. Pictures of thetwo slit lamps with the parts labelled are shown in Figures.

The Illumination System: The illumination system gives a well defined bright streak of light with sharp edges. The slit width is variable. The slit can be tilted as required. It has a source of light, a filament lamp or a halogen lamp used in some models for brighter light. The position and orientation of the filament is very important to avoid the image of the filament in the light projected from the illumination unit on to the eye of the patient. In some models there is a provision for tilting the illumination system forward through an angle of 20 degrees in steps of 5 degrees.

The electrical connection for the lamp is just a series connection through switches and pilot lamp. The voltage supplied to the bulb is adjustable in two or three steps. This is done by turning a knob, designated as brightness control knob (The brightness may have two steps low or high, or three steps low, medium and high).

There is a main switch for the slit lamp. The pilot lamp glows when the power is turned on.

The Magnification System: The Biomicroscope is a stereo microscope that provides a stereoscopic view of the object (part of the eye) under focus, which is achieved by moving the microscope towards or away from the object after ensuring that it is at the proper height. The microscope is always focused on the slit image formed by the illumination system.

(Inclined optics) The most common type has the axis of the two tubes forming the stereo microscopes inclined at 13° to each other, which constitutes essentially two microscopes, each with its own objective. There are usually two sets of objectives available for two different magnifications. The switching from one objective to another can be done with a small handle provided near the objective.

(Parallel optics) In another type, two beams of light are picked from two diametrically opposite points of objective lens and two parallel beams are taken through two apertures in the instrument. In such instruments, with combinations of two pairs of lenses, forming a galilean telescope, and a rotating device, five different magnifications can be achieved. The eye pieces generally give a magnification of 10X or 12.5X. A second set of eye pieces with magnification 15X or 16X are usually supplied with the instrument.

The alignment of the optics in the microscope generally remains unaltered as long as the microscope is not subjected to a mechanical or thermal shocks. If the alignment gets disturbed it is a specialist's job to set it right. A rod is provided with the slit lamp to be used as object while carrying out the alignment of the microscope.

Mechanical System: This consists of three major parts.

Part I: This includes the table on which the slit lamp is mounted. The table is provided with castor wheels for ease in moving the equipment from one location to another. The castors also have a locking mechanism to fix the equipment in a location. A manual or motor driven mechanism is provided for the movement of the table top up or down. Motor driver one is activated using a hand or foot switch. The transformer mentioned earlier is fixed to the table. A drawer is provided in the table for storing the extra eyepieces and other accessories.

Part II: A chin rest mounted on a vertical stand is provided for placing the patient’s head in the correct position. A head rest with a head band is available for securing the patient’s head to the stand if necessary. The height of the chin rest could be adjusted manually to bring the chin rest to the level of patient’s chin.

Part III: A joy stick (a lever) is provided to move the illumination system and the microscope together up or down, left or right, forward or backward as needed during observation.

Besides these, the illumination system alone can be rotated relative to the microscope and also tilted to provide the light from different directions for the observation.

Lensometer:

It is used to measure the focal powers of lenses (spherical, cylindrical and spherocylindrical) It can also determine the decentration in the lens. There are two generic models of the instruments. One in which the target seen through the eyepiece of the instrument consists of a number of bright points forming a circle, and another in which the target has a set of three wide lines with wide spacing between them and another set of three narrow lines with smaller spacing between them. These two sets of lines intersect at right angles. The equipment comes in different shapes. A picture of a typical lensometer, also known as focimeter, is shown in figure 9.2. The equipment has a clamp (C) for mounting the lens whose power is needed. There is provision for making ink dots on the lens at the desired points. For measuring the power of the lens, a calibrated disc (D) is turned till a clear and sharp image of the target is seen through the eye piece (E). For measuring the focal power of cylindrical and spherocylindrical lenses that have different powers in different meridians, the optics of the equipment can be rotated around the axis. The angular position can be seen on a circular scale(s) on the instrument.

Keratometer:

It is used to measure corneal power of the eyes. A picture of a keratometer is as shown in figure 9.1. The telescope like part (T) of the equipment can be raised or lowered by timing a knob (K1). It can be turned left or right manually by rotation around a vertical axis and fixed in any desired position by turning a knob (K2). Small left right movement in this fixed position is possible by turning another knob (K3). The telescope like part (T) of the equipment can be turned around a horizontal axis by hand. The angular position can be read on a circular scale (S). Knob (K4) is used for focusing on the cornea. While making the measurements, two drums (D) provided on either side of the equipment are turned to get the coincidence in the pattern seen through the telescope. The drums are calibrated in diopter units of power of the cornea. The instrument has a bulb that provides the necessary illumination. A chin and head rest are provided in the equipment for use by the patients. Keratometers of different manufacturers look alike.