Properties of Lens Materials Flashcards
(85 cards)
What is chromatic aberration?
Type of aberration that causes light of different wavelengths (colors) to be refracted differently through optical system.
The ___________ is the most commonly used number for identifying the amount of chromatic aberration for a given lens material.
Abbe Value
When the Abbe value of a material is high, the chromatic aberration present is ______.
Low
When the Abbe value of a material is low, the chromatic aberration present is ______.
High
Abbe’ value is the __________ of ω (dispersive power) and is symbolized by the Greek letter nu or ν.
reciprocal
Abbe’ Value equation
Abbe value (nu or v) = 1 /dispersive power (w)
What is an abberation? When does it occur?
The degradation of an image.
Occurs when a point source of light does not result in a single-point image after going through the lens or lens system.
What is an antireflection coating? What is its purpose?
A thin layer or series of layers of material is applied to the surface of a lens to reduce unwanted reflections from the lens surface and thus increasing the amount of light that passes through to the eye.
Antiscratch Coating (AKA Scratch Resistant Coating)
A thin, hard coating is applied to plastic lens surfaces to make them more resistant to scratching.
What are the two types of chromatic aberration?
1) Transverse / Lateral chromatic aberration
2) Longitudinal / Axial Chromatic Aberration
What is Lateral/ Transverse Chromatic Aberration? How do you find it? What are the units?
An aberration that produces images of slightly different sizes at the focal length of the lens, depending upon the color of the light.
TCA= power of lens x cm off center / Abbe number
Units are in prism DIOPTERS
What is Longitudinal / Axial Chromatic Aberration? How do you find it? What are the units? What lenses is this more of a problem with?
Occurs when a point light source that is composed of several wavelengths (such as white light) forms a series of point images along the optical axis by looking laterally. Each of these images is a different color, and has a slightly different focal length. Red closest to center, blue furthest away.
- ACA = Power of lens / Abbe number
- Units in DIOPTERS
Crown Glass Characteristics (n, specific gravity, Abbe number, materials, traits)
- n=1.523
- Specific gravity= 2.54
- Abbe = 59
- Main ingredient is silica
- Very stable
- Best optics (optically superior)
- low chromatic aberration
- least warpage
- scratch resistant
- heavy
- low refractive index
- fog easily
- shatters easily
- test individual lens for impact resistance
CR-39 (plastic) Characteristics (n, specific gravity, production, Abbe number, traits)
- n= 1.5
- specific gravity = 1.32
- Abbe = 58
- Columbia Resin 39
- made by thermosetting
- better impact resistance than glass
- lighter than glass
- scratches easier than glass
- shatters less than glass
- tested by impact resistance in batches
What does chemical tampering do? How is it done?
(AKA Chemtempering, Chem Hardening)
Ion-exchange method
The process of increasing the impact resistance of glass lenses by immersing them in a bath of molten salt. Exchanges the smaller ions in the glass (K+) with larger ions in the salt bath (K+). Then it is compressed with a temperature much less than thermal tempering. This results in less warpage.
-Resurfacing and re-edging are possible
-takes a lot longer than heat tempering
-MORE IMPACT RESULTS THAN HEAT TEMPERING
What is does heat treating do? How is it done?
AKA Air Hardening, Heat Hardening, Heat Tempering
The process of hardening a glass lens.
First heat it in a kiln, then quickly cool by blowing forced air against both front and back surfaces.
-no resurfacing or re-edging
-much faster than chemical
What is drop-ball test? What is used? Who developed this law? What is the regulation called?
- A test to determine the impact resistance of ophthalmic lenses.
- Either a 5/8th inch or 1-inch steel ball (0.56 ounces) is dropped onto the front surface of a lens from a height of 50 inches.
- Developed by the FDA
- FDA regulation 21 CFR 801.410
Mid/High Index plastic characteristics (pros/cons)
Pros:
- considered for high prescriptions
- thinner overall
- flatter curvature
- higher index of refraction
- lighter weight than other plastics
Cons:
- expensive
- high distortion
- more reflection
- more brittle
___________ is an aberration that is present even when light is made up of only one wavelength (one color).
Monochromatic aberration
A _________ lens changes its transmission characteristics when exposed to light.
photochromic
Polycarbonate Characteristics , Pros/Cons (n, specific gravity, production, Abbe number, traits)
- n = 1.586
- specific gravity = 1.20
- Abbe = 30
Pros:
- very impact resistant (known for its strength)
- made from thermoplastic
- high refractive index
- low weight
- less fogging
Cons:
- more dispersion than glass/CR-39
- scratches easily (soft)
- not resistant to chemicals like rubbing alcohol and acetone
What kind of glasses material would you give a child?
polycarbonate
Thermosetting materials and characteristics. What lens uses thermosetting materials?
-once hardened cannot be softened
-supplied as liquid monomers
-other ingredients added (dyes, etc.)
-The mixture is heated to produce polymerization, then cured
stable, rigid, hard, flame resistant, insensitive to heat and chemicals
CR-39 is thermosetting material
Thermoplastic materials. What kind of lens uses thermoplastic materials?
-A material that will bend when heated, but does not return to its original shape when reheated because it does not have a “plastic memory”.
-soften when heated
-can be stretched, pressed, or molded without changing chemical structure
may be injection molded to form ophthalmic lenses
- less stable and more susceptible to deformation by heat
Polycarbonate
