Ophthalmic Lenses PDF

In simple terms, ophthalmic lenses are the lenses that are used in the field of optometry and ophthalmology. Ophthalmic lens is the portion of the transparent medium bounded by two refracting surfaces at least one of which is curved. It is a lens for correcting vision in person with visual impairm...

In simple terms, ophthalmic lenses are the lenses that are used in the field of optometry and ophthalmology. Ophthalmic lens is the portion of the transparent medium bounded by two refracting surfaces at least one of which is curved. It is a lens for correcting vision in person with visual impairment where the focal point of eye does not hit the retina. Various ophthalmic lens materials: ================================== Ophthalmic lens refers to all the materials used during its manufacturing. They are: - - - NATURAL MEDIA - - - GLASS - - - Properties - - - - *Composition* Raw materials used to make modern glass are: Quartz sand(silica) Soda (as sodium carbonate) Lime (as carbon oxide) Quartz sand provides silica which is the principle ingredient of glass. Glass has following two properties that makes it suitable for optical uses. a. b. varieties of optical glass ========================== Optical glass is optically homogenous glass, free from defects such as striae and bubbles, which is used with optical functionalities, for example in the form of lenses or prism. Main varieties include following: - - - Other specific varieties of glass includes: - - - - OPHTHALMIC CROWN GLASS - - Ingredients: - - - - Properties: - - - - - - - - - - - - - - - - - - - - Disadvantages: - - - - - FLINT GLASS - - Composition: - - - Advantages - - - Disadvantages - - Types Refractive index Abbe value Specific gravity ------------- ------------------ ------------ ------------------ Dense 1.649 33.8 3.90 Extra-dense 1.69 30.90 4.23 #### BARIUM CROWN GLASS - - - Composition: - - - Its main use is for segments of Nikrome's series of fused bifocals. It is used as a material for optical parts in optical instruments such as optical lenses, prism and cylindrical lenses. Types Refractive index Abbe value Specific gravity -------------------- ------------------ ------------ ------------------ Light barium crown 1.573 57.4 3.21 Dense barium crown 1.616 55.1 3.26 Advantages - Disadvantages - #### High index glass - - - - - - - ABSORPTIVE LENSES - - - - Advantages: - - PHOTOCHROMIC LENS - - - Advantages: - - - - - Disadvantages: - - - PLASTIC ======= - - - a. b. Thermoplastic/ Thermosoft materials - - - - - Advantages: - - - - - Disadvantages/limitations: - Examples of thermoplastic materials are as mentioned below: - - - - - - - - Thermosetting materials - - - - - - - - Advantages: - - - - - Disadvantages: - - - Examples of thermosetting materials includes: - - - - - Various commonly used plastic lens materials are: 1. 2. 3. 4. 5. Optical properties of plastic lens: Material Refractive index Abbe value Specific gravity Scratch resistance Impact resistance UV-cut off(nm) -------------------- ------------------ ------------ ------------------ -------------------- ------------------- ---------------- CR-39 1.498 58 1.32 1.0 Fails 340 Polycarbonate 1.586 30 1.20 0.2 Passes 380 Trivex 1.530 43 1.11 0.66 Passes 400 High index plastic 1.57-1.71 32-42 1.30-1.40 0.5 Fails 380 MANUFACTURING OF LENS MATERIALS: Manufacturing of optical glass \`Optical glass is manufactured in multi-step process that begins with the selection of raw chemical elements that will be measured and combined to create the glass formulation. The process may vary somewhat based on the specific type of optical glass being manufactured and its intended application, but viewed from a general perspective, the steps used to fabricate optical glass can be summarized into following process. 1. - 2. - Both of these processes include following operations: - - - - - - - [BATCH PROCESS] - - - - - - - - [CONTINUOUS PROCESS] - - Manufacturing of plastic materials CR-39 - - Manufacture: - - - - - - - - - - - - - - - - - - - - POLYCARBONATE - - - - Manufacture: - - - Advantages: - - - - - - - Disadvantages: - - PMMA - - - - - - Advantages: - - - - Disadvantages: - - - IMPACT RESISTANCE ================= - - - - Impact resistance testing There are three methods for testing impact resistance of spectacle lens materials. 1. 2. 3. BALLISTIC TEST - DROP-BALL TEST - - - - - - - - - STATIC LOAD TEST - - - - - - - - - Impact resistance of polycarbonate lens: - - Impact resistance of CR-39 lens: - CLASSIFICATION OF LENS EYEWEAR ON THE BASIS OF IMPACT RSISTANCE: Eyewear can be broadly divided into following three categories: Strength of lens material TEMPERING - - - - - - Chemical tempering - - - - - - - A. B. **References:** - -