Lens Materials Flashcards
What indicates how much the materials will refract or bend lights called?
Refractive index
What happens when there is a higher index of refraction?
- The higher index of refraction, the more the material can bend/refract light
- Higher index lenses can be ground thinner
- Shows it has a greater ability to bend light
What is specific gravity in lens materials?
- Refers to the density of a lens material and compares its mass to the density of water.
- The higher the specific gravity of a lens, the higher the density resulting in a heavier lens.
What happens if two lenses have the same power and size?
- The one with the higher specific gravity will be heavier
- The lower the numbers the lighter the lens
- Thinner doesn’t necessarily mean lighter
What is ABBE value?
The chromatic aberrations in a lens material
Why do chromatic aberrations occurs?
The chromatin aberration occurs because color components of light have different wavelengths.
Which color wavelength is shorter than the other?
Blue wavelength is shorter than red wavelengths and bends at a greater angle than longer wavelength
What happens to the chromatic aberration when there is a higher ABBE value?
- The higher the ABBE value, the lower the amount of chromatic aberration
- Generally speaking, higher index lenses have a greater chromatic aberration, and a lower ABBE value number
What is Reflectance?
- Describes the percentage of light reflected from the highly polished surface of a lens material
- Higher index lenses tend to reflect more light than plastic or glass lenses as you can see in this chart of common materials
What is transmittance?
- Describes the amount of light that will pass through that material.
- Usually specified for a given waveband like the UV cut-off for the materials seen on the chart
- This is the point where UV light begins to be transmitted though the lens material
What is CR-39 (resin)?
- Plastic
- Widely used lens material with good optics
- These lenses are weight and more impact-resistant when compared to glass lenses
- The lenses can be hard-coated for scratch resistance and are easy to tint and UV coat.
- Plastic lenses are appropriate for patients who fall within this prescription range. Good for Rx’s less than +/-2.00D
What is Polycarbonate?
- Is one of the most impact-resistant material available
- Thinner and lighter than CR-39.
- Provides good optics in lower powers.
- In higher powers, over +/-4.00D, consider the possibility that some patients may experience reduced clarity due to chromatic aberrations
- Has an inherent ability to block the ability to block UV
Is polycarbonate easy to scratch?
- May scratch more easily than plastic lenses this is solved with a scratch resistance coating.
- Scratch resistance features can make lenses more difficult to tint but it’s not as difficult today as it was a few years ago.
What is trivex?
- Newer plastic lens materials that are strong, light, and impact-resistant similar to polycarbonate
- Can be made to a minimum center thickness of 1.0mm
- Built-in UVA and UVB protection
- Can be used without scratch-coating
- Easily tinted and anti-reflective coated.
When do you recommend trivex or polycarbonate?
For Children, patients with good vision in one eye only, active in sports, and professions that might pose a danger to the eye.
What is Duty to Warn?
- The duty to warn responsibility means that patients must be provided with enough information to make informed choices regarding lens materials
- Instructing patients about eyewear safety is one of the important services opticians provide.
What are high-index materials?
- Index of refraction higher than 1.58
- Thinner and lighter than plastic lenses
- Good optic
- Built in UV protection
- Base curves can be ground flatter reducing lens thickness
- Easy to tint
- Needs anti-reflective coating
What to recommend with patient Rx greater than +/-2.00D?
- Consider the mid-range index (trivex)
- High index material between 1.60 and 1.70
- All things being equal, a 1.66 index will be thinner than the 1.56 material
What to recommend with a patient Rx greater than +2.00D?
- Consider thinner, lighter high-index lens material along with an asperic front surface.
- This is not only minimizes the lens weight and thickness but also reduces the visible lens magnification.
What are glass materials?
- Also known as crown glass
- Provides good optics and scratch resistance
- They are heavy and shatter more easily
- Rarely used today
- Flint glass is another type of glass with typically higher refractive index and higher dispersion value and it is also not used today.
