Podcast
Questions and Answers
Which of the following is an advantage of CR39 plastic lenses compared to crown glass lenses?
Which of the following is an advantage of CR39 plastic lenses compared to crown glass lenses?
- Better impact resistance, reducing the risk of shattering upon impact. (correct)
- Higher index of refraction, resulting in thinner lenses for the same prescription.
- Greater scratch resistance, making them less prone to surface damage.
- Less pliable material, which doesn't warp when mounted into frame.
Crown glass lenses provide better UV protection compared to CR39 plastic lenses.
Crown glass lenses provide better UV protection compared to CR39 plastic lenses.
False (B)
What characteristic of CR39 plastic makes it less prone to fogging compared to crown glass when transitioning from a cold to a warm environment?
What characteristic of CR39 plastic makes it less prone to fogging compared to crown glass when transitioning from a cold to a warm environment?
CR39 plastic doesn’t heat up as quickly as crown glass.
Due to its lower index of refraction, a CR39 plastic lens will have a greater ______ or edge thickness than a crown glass lens with the same prescription.
Due to its lower index of refraction, a CR39 plastic lens will have a greater ______ or edge thickness than a crown glass lens with the same prescription.
Match the lens material with its corresponding property:
Match the lens material with its corresponding property:
Which of these factors contributes to CR39 plastic lenses being more susceptible to warpage compared to crown glass lenses?
Which of these factors contributes to CR39 plastic lenses being more susceptible to warpage compared to crown glass lenses?
Crown glass lenses are generally lighter in weight compared to CR39 plastic lenses.
Crown glass lenses are generally lighter in weight compared to CR39 plastic lenses.
Why is it important to properly dispense CR39 plastic lenses?
Why is it important to properly dispense CR39 plastic lenses?
Flashcards
CR39 Plastic
CR39 Plastic
A plastic lens material offering good optics and impact resistance.
Crown Glass
Crown Glass
Traditional glass lens material offering excellent clarity.
Impact Resistance of CR39
Impact Resistance of CR39
CR39 plastic is less likely to shatter compared to crown glass.
Weight Comparison
Weight Comparison
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Fogging Resistance
Fogging Resistance
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Tinting Options
Tinting Options
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UV Absorption
UV Absorption
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Scratch Resistance
Scratch Resistance
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Study Notes
- CR39 plastic lenses are often wrongly considered inferior to crown glass lenses.
- When properly dispensed, CR39 plastic lenses can perform as well as, or even better than, crown glass lenses.
Impact Resistance
- CR39 plastic is more impact resistant than crown glass.
- Crown glass can be heat or chemically treated to increase impact resistance, but it can still shatter.
- When crown glass shatters, it breaks into sharp shards.
- CR39 plastic is more difficult to shatter, and when it does, it breaks into large chunks.
Weight
- CR39 plastic has a lower specific gravity (1.31) than crown glass (2.54).
- CR39 plastic is about half the weight of crown glass.
Resistance to Fogging
- CR39 plastic heats up less quickly than crown glass.
- CR39 plastic does not fog up as much as crown glass when going from a cold to a warm environment.
Tinting
- CR39 plastic can be tinted any color and to any degree.
- Crown glass has limited colors and degrees of color.
- Surface coating for crown glass tinting is also limited.
UV Absorption
- CR39 plastic absorbs UV radiation up to 350 nm.
- Crown glass absorbs UV radiation up to 290 nm.
- CR39 plastic offers greater UV protection than crown glass.
Pitting
- CR39 plastic does not heat up as quickly as crown glass.
- Welding splatter does not embed itself into the lens surface as easily with CR39 plastic compared to crown glass.
Scratch Resistance
- CR39 plastic is a softer material than crown glass.
- CR39 plastic scratches more easily than crown glass.
Thickness
- CR39 plastic has a lower index of refraction (1.498) than crown glass (1.523).
- For the same prescription, decentration, lens shape and size, CR39 plastic lenses will have greater center or edge thickness than crown glass lenses.
Warpage
- CR39 plastic is more pliable than crown glass.
- If a CR39 plastic lens is edged too large and mounted into a frame, the pressure can cause it to warp.
- Warpage can affect the patient’s vision.
- Warpage does not occur with crown glass lenses.
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Description
Explore the properties of CR39 plastic lenses compared to crown glass. CR39 plastic offers superior impact resistance and is half the weight of crown glass. It also fogs less and allows for versatile tinting options, making it a practical choice for lenses.
