Type of Spectacle Lenses PDF

# Type of Spectacle Lenses ## Refractive Index (n) Increasing the refractive index reduces the volume of the lens making the lens flatter. In minus lenses it reduces edge thickness, and in plus lenses, it reduces central thickness. The lenses with higher index are known as (Hi) index lens. ## Len...

# Type of Spectacle Lenses ## Refractive Index (n) Increasing the refractive index reduces the volume of the lens making the lens flatter. In minus lenses it reduces edge thickness, and in plus lenses, it reduces central thickness. The lenses with higher index are known as (Hi) index lens. ## Lens Materials: - **Glass** (K & F): This is the most common used lens. (glass) - **Plastic lenses** (CR-39): - CR-45 - CR-79 - Plastic lenses have a lesser refractive index (n) than glass. - The specific gravity is more than glass. ## Polycarbonates (n = 1.591) ## Plastic Lenses ### CR-39 Index 1.499 - Single Vision - Round-Shape Bifocal - Flat-Top Bifocal - Progressive - Polarized... etc. ### Index 1.56 Lenses - Single Vision - Progressive - Photo chromic Single Vision - Photo chromic Flat-Top Bifocal - Photo chromic Progressive. ### Hi-Index 1.61 Lenses - SPH & ASP Single Vision Lenses. ### 1.591 Polycarbonate Lenses - Single Vision Lenses. # Stages of Making Glass Lenses: 1. **Annealing**: This process consists of cooling the material of the lens very slowly through certain ranges of temperature. 2. **Blank**: After annealing, the lenses are made into a rough model of approximately the required thickness 3. **Grinding**: A process applied to the lens blanks, using electrically driven tools of finely grained cast iron of appropriate curvature. 4. **Polishing**: After grinding, the lenses are subjected to polishing whereby the lenses become smooth and shiny by substituting the power with cloth or wax. 5. **Uncut lenses**: This is the product of annealing, grinding, and polishing, and is kept in stock for ordinary use. 6. **Marking**: This process determines the focusing, tilting, optical center, and the axis of the cylinder by using a foci-meter. Then, the size and the shapes of the lenses have been fixed. 7. **Cutting of the lens**: The lens is then cut to the proposed shape by a machine which engraves a deep line in the lens. The lens is then broken off at this line by nippers. 8. **Edging**: After cutting, the lens is subsequently edged by using a hand edger or carborundum wheel, which grinds it to receive the frame into which it is now fitted. 9. **Glazing**: The lenses are edged into the frame’s rim by hand, or by using a glazing machine. # Glass *Type: K7 or F3* 1. **Annealing**: From 1273°C to room temperature 25°C, very slowly. - SiO₂ + Al₂O₃ + Fe₂O + etc. (diff. powder) 2. **Blank** (required thickness + shape) by using a press machine. - Material: SiO₂, etc. 3. **Generation** (grinding), using cast iron (tools) with Emery to remove 5 mm with high speed machine to get ≈ R (curvature). - CT = Center thickness in (mm) The image shows a diagram of the stages of making glass lenses. It includes descriptions of the stages and some of the materials and tools used.