Lens Material And Design Flashcards
Does glass have a low or high abbe number?, what does this mean regarding chromatic aberrations
High abbe number=low chromatic aberrations
What are 4 advantages of glass?
- highly transparent and colorless
- resistant to heat and chemicals
- high scratch resistant
- high abbe number= low aberrations
What are 5 disadvantages of glass?
- weight
- poor impact resistance
- poor UV protection
- cant be tinted after manufacturing
- lengthy processing time
What are the 2 types of plastics?
- thermoplastic (polycarbonate)
- thermosetting (CR-39)
This plastic can be softened when heated?
Polycarbonate
This plastic when hardened cant be softened even at high temps?
CR-39
Does CR-39 have a love or high abbe number?
High
What are the 5 advantages of CR-39?
- lighter weight than glass
- high abbe number
- inexpensive
- tintable
- many designs
What are 4 disadvantages of CR-39?
- scratches and chips easily
- thicker than hi index plastic
- warpage
- not for children
T/F: the higher the index, the thinner a lens can be and still produce the same power
True
This is type of lens material is good for high prescriptions
Hi-index plastic
Does hi index plastic have a low or high abbe number?
Low
Since hi-index plastic has a low abbe number=more aberrations, what type of coating does it need?
A/R coating
What is the refractive index of glass/crown glass?
1.523
What is the refractive index of CR-39?
1.498
What is the refractive index of polycarbonate?
1.586
What are the 3 advantages of polycarbonate?
- excellent impact resistant
- UV protection
- many designs
What are 4 disadvantages of polycarbonate?
- not great for optics
- not great for tinting
- soft surfaces, scratches easily if not treated
- must avoid exposure to heat
This type of lens material is used in children’s glasses or sportswear
Polycarbonate
What is the refractive index of trivex?
1.53
What is the order of refractive indexes from low to high?
- ) CR39
- ) glass
- ) trivex
- )polycarbonate
- ) hi index plastic
What are 5 advantages of trivex?
- tintable
- good UV protection
- very lightweight
- extremely high impact resistant
- fewer heat issues that polycarb
This lens material is low cost and has excellent optics
CR-39
This lens material is chemically resistant and not commonly used
Glass
This lens material is expensive, has chromatic dispersions, scratches easily, and is used for strong rx’
Hi-index plastic
These lens materials are used for children, sports, hazardous jobs, and thinner/light lenses
Polycarbonate and trivex
A point on the datum line halfway between the 2 vertical lines which are tangent to the edges
Geometrical center
Horizontal length of the box when referring to the frame or lens
Eye size or lens size (A)
The vertical length of the box
B distance (B)
Shortest horizontal distance between the lenses
Bridge size (DBL)
Horizontal distance between the geometrical centers of the 2 lenses
Frame PD
The longest diameter of a lens
Effective diameter (ED)
Point on the lens through which the line of sight passes (this would correspond to the optical axis if no prism power were needed)
Major reference point (MRP)
The plus power in a multifocal lens segment that is added to the power in the distance portion of the lens
Near addition
Distance is measured at ~___ft or ~____m
20ft or 6m
Computer/intermediate is measured at ____
Arms length
Reading is measured at ___cm
40cm
If asphericity is applied to the front surface of a plus lens, the surface will become ___ away from the center.
Flatter
If asphericity is applied to the back surface of a plus lens, the surface will become ___ away from the center.
Steeper
If asphericity is applied to the front surface of a minus lens, the surface will become ___ away from the center.
Steeper
If asphericity is applied to the back surface of a minus lens, the surface will become ___ away from the center.
Flatter
Designed for one rx correction
Single vision lenses
A lens where blurring in parts around the periphery results in a lack of clarity
Spherical lens
A lens with peripheral blurring improves, resulting in enhanced clarity
Aspheric lens
Longest horizontal dimension of the segment
Seg width
Longest vertical dimension of the segment
Seg depth
Distance from the lowest point on the lens to the top of the segment
Seg height
Vertical distance between the MRP and the top of the seg
Seg drop
T/F: the distance portion of the lens is decentered from the GC to correspond to the wearers PD
True
the distance from the Geometric Center (GC) to the Major Reference Point (MRP)
Distance decentration
the inset accounting for the near PD (because PD is different for near vision). The distance from the MRP to the center or the seg
Seg Inset
The inset of the seg as measured from the Optical Center of the lens. Distance from the GC to the center of the seg
total inset
First one-piece bifocal. Designed by starting with required near power and adding a second curvature to upper portion and back of surface.
Solid upcurve
Small segment cemented to back surface of lens. Segment could be replaced as add changed.
Cement
components fused together by heat instead of cement.
Fused kryptok
One-piece back surface bifocal
Ultex
What are the 7 types of bifocals
- straight top/flat top
- rounded
- blended
- executive
- curved top
- B or ribbon
- ultex
What are the 2 advantages of a round segment?
- least visible
- light
What are the 2 disadvantages of a round segment?
- takes a while to get down to a useful width
- most jump, especially in larger segs
What are the 3 advantages of a flat top?
- Immediate broad field
- minimal jump
- no prism
- light
What an disadvantage of a flat top bifocal?
Visible ledge in larger segs
What is an advantage of an executive bifocal?
Widest field
What are 6 disadvantages of an executive bifocal?
- thick
- heavy
- ugly
- collects debris
- chips
- no jump
has a smooth continuous building of power from the distance to the near through a “corridor”.
PAL
Do PAL have an image jump?
NO
process by which a PAL can be made thinner by removing some base up prism. This leaves residual BD prism and a substantially thinner lens
Prism thinning
the cylindrical power lateral to the umbilic of a progressive surface increases twice as rapidly as the addition power
Minkwitzs theorem
Vertical meridian of a PAL that is spherical at every point
Umbilic
T/F: lens designs with shorter corridors produce more unwanted cylinder power in the periphery or narrower viewing zones.
True
Design characterized by wider area of stable optics at distance and near along with a shorter, narrower corridor (steeper gradients); Better for sustained viewing tasks
Hard designs
Designs have narrower areas of optimal power at distance and near, but offer a longer, wider intermediate zone (shallower gradients); Better for dynamic viewing tasks
Soft designs
Many lenses today are multi-design meaning what?
Softer with decreasing add power and harder with increasing add power
PALS with small B measurements
Short corridor PALs
uses computer- aided design and processing to create high-level, customized lenses unique to your prescription; makes you see better and across more of your lens
Free-form processing
What are 4 advantages of PALs?
- no image jump
- cosmetically appealing
- more youthful and fashionable
- uninterrupted vision from distance to near
What are 3 disadvantages of PALs?
- expensive
- adaptation time
- unwanted Astigmatism is periphery
