Ophthalmic Lenses PDF

OPHTHALMIC LENSES Lens & Types of lenses Lens: a piece of a refracting medium bounded by two surfaces, at least one of which is a curved surface. Types can be divided on the basis of: ❑Correction ❑Form ❑Work ❑Materials ❑Coating 1. Correction: ▪A corrected lens is a form: to avoid or minimize the...

OPHTHALMIC LENSES Lens & Types of lenses Lens: a piece of a refracting medium bounded by two surfaces, at least one of which is a curved surface. Types can be divided on the basis of: ❑Correction ❑Form ❑Work ❑Materials ❑Coating 1. Correction: ▪A corrected lens is a form: to avoid or minimize the distortions through the edges of lens. ▪In theory the base curve should have: same curvature, which coincides with the corneal curvature of the rotating eyeball, Spherical lenses In general there are two types of spherical lenses. 1. Convex 2. concave Convex lenses Lenses formed by binding two spherical surfaces bulging outward are known as convex lenses Convex lens converge parallel rays of light to bring the image forward on retina. For correction of hypermetropia presbyopia aphakia  Identification of convex lens Convex lens is thick in the center and thin at the periphery. Types of convex lenses biconvex Plano convex Convex meniscus Image formation by convex lens When an object is placed at infinity, the real image is formed at the focus. The size of the image is much smaller as compared to that of the object  When an object is placed behind the center of curvature, the real image is formed between the center of curvature and focus. The size of the image is smaller as compared to that of the object. When an object is at the center of curvature, the real image is formed at the other center of curvature. The size of the image is same as compared to that of the object. Concave lenses Lenses formed by binding two spherical surfaces such that they are curved inward are known as concave lenses Concave lens diverge the parallel rays of light to shift the image behind, on retina Identification concave lens Thin in the centre and thick at the periphery. Minifies images When lens is moved the object seen through it moves in same direction (with movement) Uses of concave lense For correction of Myopia diagnostic purposes for fundus examination with slit lamp. Types of concave lens biconcave Plano concave Concave meniscus Image formation by concave lens ◦When an object is placed at infinity, a virtual image is formed at the focus. The size of the image is much smaller as compared to that of the object. When an object is placed at a finite distance from the lens, a virtual image is formed between pole and focus of the concave lens. The size of the image is smaller as compared to that of the object. First principal focus: First principal focus of a lens is that point on the principal axis of the lens at which if an object is placed, the image would be formed at infinity. Second Principal Focus: Second principal focus of a lens is that point on the principal axis of the lens where the image is formed when the object is at infinity Cylindrical lens ❑One plane surface ❑Other forms part of a cylinder. ❑one meridian the lens has no vergence-axis of the cylinder. ❑meridian at right angles to the axis, the cylinder acts as a spherical lens. Cont… ❑formation of a line image of a point object ❑focal line. ❑parallel to the axis of the cylinder. ❑Plus cyl front curve ❑Minus cyl back curve  Cont… Types of corrective lenses : Single-vision lenses, which provide correction for only one distance (that is, far or near) Cont.. Multifocal lenses (bi-focals, trifocals, and progressive-addition lens), which combine two or more corrections in a single lens to provide sharp vision at more than one distance. 2.FORM The total power of a lens can be achieved by combining different types of curved surfaces (concave or convex) and this is called the form of lenses. Depending up on the form lenses could be of two types: Flat Lenses Curved Lenses Cont.. Flat lenses: Both its surfaces have got same type of curvature,e.g. biconcave or biconvex One surface is flat and the power is grounded on the other surface, e.g. Plano-concave and Plano-convex. Cont… If both surfaces are convex or both concave, the lens is biconvex or biconcave. If one surface is Plano and the other an outward curved plus surface (i.e., a convex surface), the lens is referred to as Plano convex. Cont.. If one surface is Plano and the other curved inward for minus power (i.e., a concave surface), the lens is Planoconcave. Cont.. Curved lenses: A lens is said to be curved when there is a convex curve on one surface and a concave curve on the other surface. Curved Lenses are of two types: Meniscus Toric Cont… Meniscus are curved lenses where both the surfaces are spherical Anterior surface is convex and posterior surface is concave. Introduction of these lenses has made mass production of lenses easier. Cont… Toric are curved lenses where one surface is spherical and the other surface is toroidal in shape. Toric lenses are used where a cylinder is also present in the prescription. Lenses can be made in a variety of forms, with many forms possible for a lens of the same power 3. Work Driver: It is common for the vision standard to be at least 6/12 on the Snellen chart with both eyes open or in the only eye if monocular it is highly recommended that users have an anti-reflection coating applied to reduce the effects of glare. Plastic lens materials should be given Cont… Pilots: The entry requirements for the aviation pilots require a visual acuity, with spectacles or contact lenses, atleast to be 6/9 in each eye with an ability to see 6/6 Generally grey or brown tint should be given,but not darker than 80% Cont… Polarized filter is introduced allowing the light in one direction To deal with those who are presbyopic progressive lens allows a pilot to see at distance, intermediate and near Plastic frame will be preferred Cont… Factory workers: Many people such as engineers, processing, cleaners, craftsmen etc. will work in a factory environment The lenses supplied will be CR39,polycarbonate or Trivex Cont… Office Workers: Anti reflective coated glasses are preferred Plastic lens material will be preferred Progressive lenses are preferred if the patient is old Cont… Lecturers: Progressive lenses should be first consideration in the classroom Cont… Outdoor Activities: For people such as gardeners, traffic, police, road workers etc., photochromic lenses and plastic frame should be considered When photochromic lenses are exposed to UV rays they darken and provide visual comfort in direct sunlight. Cont… Sport Person Polycarbonate is a material usually associated with lenses Shop workers: Progressive glass will be more useful than bifocal glass Glass materials should be given in metal as well plastic frame Cont… Boating: Glare from the water Use a polarizing lens, which also blocks UV light Use a sunglass lens that cuts out all UV light up to 400 nm. Use thermosetting frame materials and plastic lens materials  Cont… Mountain Climbing: Increased exposure to UV radiation because of less atmospheric absorption Brightness of outdoor environment is more Use UV-absorbing goggles 4.Materials Glass Plastic  Glass 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. CONT.. Flint glass, material (1.620) is used in the making of bifocal or achromatic lens. This material is used for making the bifocal segment.  Cont… Flint glass material was replaced by barium crown material, which has no chromatic aberration. It increases the refractive index of the glass. Now-a days almost all high quality bifocal lenses are made only from Barium-crown glass material. Cont… The high-index glass named Hidex (1.806), is used for making high refractive power lenses, which are remarkably thin. Glass lenses are high scratch resistant but not impact resistant. Glass lenses must be treated to resist breakage. They can be hardened by chemical or heat processes.  Plastic Original plastic lens made of (PMMA) Polymethylmethacrylate Modern hard resin lens from allyl diglycol carbonate (ColumbiaResin# 39) is harder and more resistant to scratches than other plastic lens materials Cont.. Plastic lenses are made from a very high quality material as glass. Plastic lenses are about half the weight of glass Highly impact-resistant, but get scratches more easily and do not protect the eye from UV rays unless properly tinted.  5.Coating 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. CONT… Mirrored sunglass gives a good cosmetic appearance to patients with facial disfigurements’ and at the same time they can see clearly through the mirrored lenses.  Cont… Ultraviolet and blue-blocking lenses Tinted lenses that filter out over 98% of blue and UV light are available. They are made of CR-39 plastic which is coloured amber to red. CONT.. For the normal individual these lenses provide comfort from glare, reduce haziness and create sharper vision. The lightcoloured lenses may help persons with developing cataracts to see well. The darker red lenses may improve the functional visual acuity in conditions like Retinitis Pigmentosa albinism, aniridia and intense photophobia.  Cont… Anti-reflection coating (AR) Artificial powerful lights (i.e.) halogen lights in cars and trucks, and computer monitors can cause reflections in untreated glass. When light passes through spectacles some light rays are reflected by front and back surface of the lens producing ghost images. AR coated lens prevent these images and only one image is seen. CONT… The coating material is very tough lasts through out the life of the lens. It will be seen on the surface of the lens. These coated lenses should be cleaned with proper special cleaning solutions. AR coated lenses are readily available in market and it can be coated separately too.