Podcast
Questions and Answers
Which of the following is NOT an advantage of polycarbonate lens materials?
Which of the following is NOT an advantage of polycarbonate lens materials?
CR-39 lenses exhibit higher impact resistance than polycarbonate lenses.
CR-39 lenses exhibit higher impact resistance than polycarbonate lenses.
False
Name one testing method for impact resistance of spectacle lens materials.
Name one testing method for impact resistance of spectacle lens materials.
Ballistic test
The process of chemical ____ is used to enhance the strength of lens materials.
The process of chemical ____ is used to enhance the strength of lens materials.
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Match the type of lens material with its characteristic:
Match the type of lens material with its characteristic:
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What is one of the primary purposes of ophthalmic lenses?
What is one of the primary purposes of ophthalmic lenses?
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Optical glass can contain defects like striae and bubbles.
Optical glass can contain defects like striae and bubbles.
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What are the three raw materials used to make modern glass?
What are the three raw materials used to make modern glass?
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Ophthalmic lenses are made from _______ materials.
Ophthalmic lenses are made from _______ materials.
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Match the type of glass with its specific property:
Match the type of glass with its specific property:
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Which of the following is NOT a property of glass that makes it suitable for optical uses?
Which of the following is NOT a property of glass that makes it suitable for optical uses?
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The refractive index of extra-dense Flint glass is 1.69.
The refractive index of extra-dense Flint glass is 1.69.
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What is one disadvantage of using ophthalmic crown glass?
What is one disadvantage of using ophthalmic crown glass?
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What is the refractive index of Dense barium crown glass?
What is the refractive index of Dense barium crown glass?
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Plastic lens materials generally have a higher Abbe value than glass lens materials.
Plastic lens materials generally have a higher Abbe value than glass lens materials.
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What is an example of a thermoplastic material used in lens manufacturing?
What is an example of a thermoplastic material used in lens manufacturing?
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The Abbe value of CR-39 plastic is _____
The Abbe value of CR-39 plastic is _____
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Match the following plastic lens materials with their corresponding UV-cut off values:
Match the following plastic lens materials with their corresponding UV-cut off values:
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Which material has the highest specific gravity?
Which material has the highest specific gravity?
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Photochromic lenses are designed to darken in response to UV light.
Photochromic lenses are designed to darken in response to UV light.
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Name one disadvantage of using high index glass in lens manufacturing.
Name one disadvantage of using high index glass in lens manufacturing.
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Study Notes
Ophthalmic Lenses
- Ophthalmic lenses are used to correct vision in people with visual impairments.
- These lenses are designed to focus light onto the retina, which is the light-sensitive layer at the back of the eye.
- The lenses are made of various materials like glass, plastic, and natural media.
Glass Lens Materials
- Optical glass is a specific type of homogeneous glass, free from defects, used for optical purposes.
- There are various types of optical glass including:
- Crown glass
- Flint glass
- Barium crown glass
- High index glass
Crown Glass
- Crown glass is a type of glass used in ophthalmic lenses.
- It is made from:
- Silica (quartz sand)
- Sodium carbonate (soda)
- Calcium oxide (lime)
- It is known for its low refractive index and high Abbe value.
- It refracts light less and has less chromatic aberration.
- It is a soft material, susceptible to scratches, but has better clarity than flint glass.
Flint Glass
- Flint glass has a higher refractive index and a lower Abbe value than crown glass.
- It is made from:
- Silica
- Sodium carbonate
- Lead oxide
- Flint glass refracts light more and has higher chromatic aberration.
- It is harder than crown glass and less susceptible to scratches.
- It is used in high-power lenses where a small size is desired.
Barium Crown Glass
- Barium crown glass has a higher index than crown glass but lower than flint glass.
- It is made from:
- Silica
- Sodium carbonate
- Barium oxide
- It is commonly used in bifocals.
- It can be used for optical lenses, prisms and cylindrical lenses.
- Its higher index allows for thinner lenses, and it has good chromatic correction.
High Index Glass
- High index glass has a refractive index greater than 1.6.
- It allows for thinner and lighter lenses compared to traditional crown glass.
- It is more expensive than other types.
- It is used for stronger prescriptions.
Absorptive Lenses
- Absorptive lenses block certain wavelengths of light.
- They can absorb UV radiation, blue light, or certain colors of the spectrum.
- They are used to protect eyes from harmful UV rays and blue light from digital devices.
Photochromic Lenses
- Photochromic lenses darken in response to sunlight.
- They contain molecules that change their structure when exposed to UV light.
- They transition between clear and dark states, providing both clear vision and protection in different light conditions.
Plastic Lens Materials
- Commonly used plastic materials for ophthalmic lenses include:
- CR-39
- Polycarbonate
- Trivex
- High index plastic
- Plastics are popular for their:
- Light weight
- Impact resistance
- Ease of manufacturing
- Thermoplastic materials can be easily melted and reformed.
- Thermosetting materials harden and cannot be re-melted.
CR-39
- It is a lightweight thermoplastic material widely used for ophthalmic lenses.
- It is known for its scratch resistance and its high index.
- It has limited impact resistance and UV protection.
Polycarbonate
- Polycarbonate is a strong and lightweight thermoplastic material with high impact resistance.
- This material is often used for safety glasses and children's lenses.
- It is known for its good UV protection.
- Commonly used for high impact sports and work environments.
Trivex
- Trivex is a thermoplastic material, known for its high impact resistance.
- It is commonly used for children's glasses and sports eyewear.
- It has good UV protection.
High Index Plastic
- High Index plastic is known for its low refractive index.
- It allows for thinner and lighter lenses than CR-39.
- It has a lower Abbe value compared to CR-39.
- It is not as scratch-resistant as CR-39 but is known for its superior impact resistance.
Manufacturing of Ophthalmic Lenses
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General Process*
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The manufacturing process starts with the selection of raw materials.
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They are then melted and shaped into lenses using a specific manufacturing process.
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The lenses are then polished and treated with coatings.
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Optical Glass Manufacturing*
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The manufacturing process involves:
- Melting the raw materials
- Forming the glass into specific shapes
- Annealing to relieve stress and increase durability
- Grinding and polishing to achieve the desired optical properties
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Plastic Lens Manufacturing*
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Plastic lenses are manufactured using:
- Casting: pouring liquid monomer into a mold
- Injection Molding: injecting molten plastic into a mold
- Extrusion: pushing molten plastic through a die to create a continuous profile
Impact Resistance
- The impact resistance of ophthalmic lenses is crucial for eye safety.
- Commonly tested using:
- Ballistic test: simulating a projectile impact
- Drop-ball test: dropping a weighted ball onto the lenses
- Static Load Test: measuring the force required to break the lens
Tempering
- Tempering is a process to strengthen glass lenses by inducing compressive stress on the surface.
- This makes the lens more resistant to impact.
- This process can be achieved physically via heat treatment or chemically using a bath of ion exchange.
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Description
This quiz covers the basics of ophthalmic lenses, their materials, and specific types like crown and flint glass. Test your knowledge on how these lenses function to correct vision and the characteristics of various optical glass types. Get ready to dive into the world of optical materials!
