Review Flashcards
What are the characteristics of an ideal lens?(6)
- High index of refraction
- High ABBE
- low specific gravity
- Low UV transmittance
- high light transmittance
- high impact resistance
What type of lens materials are available to recommend to patients?
- glass
- plastic
- high index
Different type of plastic lenses include:
Thermosetting
Thermoplastic
Trivex
Different kinds of high index lenses include
Plastic and glass
Physical requirements that lenses must meet before they are able to be worn?
-meet standard impact resistance requirements
Physical requirements for dress eyewear:
No minimum thickness requirement
Lens must not fracture on resistance test
Who supplies the standards that ophthalmic products should follow?
ANSI: American national standards institute
n for CR39
1.50
n for crown
1.523
n for poly
1.586
n for 1.7 index
1.701
Specific gravity for 1.7 index
2.99
Specific gravity for crown
2.54
Specific gravity for cr39
1.32
Specific gravity for poly
1.20
Abbe value for crown
58.9
Abbe value for CR39
57.8
Abbe value for 1.7 index
30.8
Abbe value for poly
30
Has exceptional impact resistance
Polycarbonate
How does the reflectance of a lens material vary with its index of refraction?
Based on the equation: higher index means higher reflection
Why do optometrist care about the light reflected from ophthalmic lenses?
The reflection can cause ghost images, it can be aesthetically frustrating in photos, cause haze or decrease contrast
Which lens materials are/is thermosetting?
CR39
Which material is/are thermoplastic
Polycarbonate
What is the difference between thermosetting and thermoplastic?
- Thermosetting cannot be melted back while melting and cooling are the cycles used in thermoplastic lenses
- thermosetting uses cross linking while thermoplastic so dont
Compare and contrast trivex and polycarbonate
- trivex abbe value = 43-45 far better than poly carb = 30
- trivex rivals poly in terms of impact resistance
- trivex is very lightweight just like poly
- 2x more scratch resistance over poly
What is a lens blank
Circular ophthalmic lenses at any stage (unfinished, semi, or finished) prior to being edged
What is lens surfacing?
When a prescription is ground onto the surface of a lens blank
What is lens edging
Cutting a lens blank to fit an ophthalmic spectacle frame
Compare semi-unfinished with finished lenses
- semi-finished: unfinished back surface, typically used for multi focal and high-powered single vision lenses
- finished: finished optical surfaces on both the front and back -> ready for edging
What lenses would you recommend to patients with high prescriptions and/or requesting thin/ner lenses
High index
What ophthalmic lens material is a patient likely viewing through if they “see rainbows”?
Some type of low ABBE value meaning there is more chromatic aberration (poly)
How does curvature relate to the radius of a surface
The curve has an inverse relationship to the radius
Compare minus cylinder and plus cylinder form lenses
Plus cyl= toric portion on the front of the lens
Minus cyl= toric portion on the back of the lens
Tsercherning ellipse
The locus of points showing the relationship between the power of a surface and the power of the lens for which oblique astigmatism is eliminated
WTR
150-180 and 180-030
ATR
060-120
Oblique
030-060 and 120-150
What is the optical axis
Line going through the center of the lens
Optic center
Literally a point of the center of the lens
Refract light ray to meet at one common point
Spherical
Refract rays to a focal line
Cylindrical
Refract light rays to form 2 line images perpendicular to each other
Spherocylindrical
For a cyl lens the axis meridian has most___ power and power meridian has most ___ power
Plus
Minus
How do we determine the power of a obliquely crossed toric lenses
The sin squares law of curvature
If alpha is 30 degrees make sure to go 30 degrees from the axis meridian
Generalizations about the resultant power of obliquely crossed toric lenses
- both cyl with same sign= axis is between the original axes
- 2 cyl are of equal power = axis is between the 2
- if the 2 cyl are of unequal power= axis is shifter toward the stronger of the 2 cylinders
Pantoscopic tilt
180 degree axis
Face form tilt
090 axis
How is the optic center adjusted for purposely pantoscopic lens tilt
Optical center must be lowered from the pupillary level an amount in mm - equal to one half the pantoscopic tilt in degrees
