Handout 2 Flashcards
Chromatic dispersion, aberration, and disperse power all affect
ABBE value
Travel more SLOWLY through material (undergo the greatest refraction)
Shorter wavelengths (violet and blue)
Who has the highest index of refraction between blue and red
Blue
Who has the higher index of refraction short or long wavelengths
Shorter
Travel faster (undergo the least refraction)
Longer wavelengths
(Fraunhofer D/yellow) Relationship between D and n of a lens/prism?
D of a lens/prism is based on the n that the material has for this chosen standard reference wavelength
Index of refraction equation
n= velocity of light in air or vacuum / velocity of light in material
Different colors have different ___ in the same material
Index of refractions (n)
Based on Fraunhofer D/ yellow He line wavelength is
587.56 nm
Different wavelengths of light have different n for the same material. This is called:
Chromatic dispersion
Chromatic aberration
Because of the chromatic dispersion of the lens the lens fails to focus all colors to the same convergence point
The mean dispersion of a material is
The difference in the refractive indices of red light (656nm) and blue light (486nm) for that material
Fraunhofer C line
Red
Fraunhofer F line
Blue
Dispersive power
Dispersion of a material is more commonly related to the refractive index of 587.6 nm (nD) (yellow) by the number known as the dispersive power
The dispersive power equation will tell us
How great is the color spread by this material
Low dispersive power
Little to no chromatic aberrations
High dispersive power
More chromatic aberration
The reciprocal of dispersive power
Abbe value
Low dispersive power will have ___ chromatic aberrations
Little to none
ABBE value desired for best optics
High abbe value
What is specific gravity
Ratio of the weight of a substance to the weight of water with the same volume
Other names for Abbe value
Nu value, v, or constringence
Abbe value for poly
30, low abbe value
Reflectance
The amount of light reflected at each surface
Reflectance produces
Produce ghost images, falsification of image position, haze, decreased contrast
Reflectance effect on cosmesis
They obscure the wearers eyes with glare and draw attention to glasses
Reflectance equation is
IR equation on equation sheet
Lens is reflected from what portion of the lens
Front and back
Aka lens refractive index
A typical, non coated, ophthalmic lens reflects
8% or more of incident light
Transmittance (t) is
The fraction of incident light passing through lens
Transmittance varies for
Wavelengths, especially for tinted lenses
Major forms of absorptive lenses
Tinted, glass coating, photochromic and polarizing lenses
Absorptive lenses act as filters by
Reducing the amount of transmitted light (tinted or colored glasses)
Color lenses absorb
Different wavelength
Increase in index of refraction = ___ light reflected and ____ light transmitted.
High reflectance
Low light transmitted
(2) have hard coatings that incorporate special UV absorbers
Poly and high index
(2) do not provide adequate protection and require UVtreatment
Glass and CR39
Lenses without UV protection can cause harm because
They cause pupils to dilate, allowing more harmful UV radiation to enter eye
Uv radiation transmitted to ocular surfaces
200-380nm
- invisible to human eye
- contains more energy than visible light
Lens that meets impact safety requirements described by z87.1 means
- it’s at least 2.00mm
- it passes the high velocity impact test (1/4” steel ball, traveling at 150ft/s)
The drop ball test is ___. What determines if pass or fail?
FDA regulation for impact resistance. Pass= lens must not fracture
What exactly is a fracture in a drop pall test
Lens cracks into 2 or more pieces or lens material detaches from surface
Drop ball test was designed for
Glass, plastic lenses can be batch tested
Lens material that is not inherently impact resistant and therefore must be treated “tempered” to pass FDA test
Glass, they are individually tested
What does tempering mean
Place the outer surface of the lens in a state of compression. Used for glass lenses. Either thermal or chemical
Thermal tempering
Lens is heated to ~650 degrees then rapidly cooled-> places outer surfaces in a state of compression and the inner portion in a state of tension-> causes the glass to crumble into small grain chucks when broken instead of sharp pieces
What is chemical tempering
Heated salt bath (~400 C) larger ions from slat exchanges w small on lens surface-> surface is compressed, inner portion is in tension. = small granular chunks instead of sharp pieces
Polycarbonate is __ more impact resistant than CR39
21x
Effects of AR coating/ or scratch resistance on the lense’s impact resistance
decreases Impact resistance
Surface starching and pitting i lenses reduces
Impact resistance
(2) are intrinsically impact resistant (different amounts tho)
CR39 and poly
Glass lenses advantages
Optical clarity, resists scratching, not easily affected by environmental factors
Glass disadvantages
Weight and not impact resistant
Semi finished lens
Back surface remains unfinished
Finished lens
Back surface is ground and polished
Ophthalmic crown glass, flint glass, barium crown glass
Types of glass lenses
Ophthalmic crow glass is used for
Distance portion of glass bifocal and trifocals lenses
Flint glass index of refraction
1.580-1.690 (higher than crown)
Flint glass used for
Fuse bifocal segments (higher n than distance lens)
Flint glass was historically used for __ until replaced
High powered lenses
Barium crown glass used for __ its high index is __?
Fused bifocal segments (higher index than distance)
1.541-1.616 (higher than crown)
“Organic material”
Plastic
Hard resin lenses that are made by “casting” where they begin as liquid and cured in oven where cross linking occurs and cant be melted back down to the liquid monomer resin
Thermosetting / CR39
Most common type of plastic lens
CR39
CR39 block __ of incident UV
55%
Can be melted and cooled back and forth from liquid to solid states “injection- molding”
Thermoplastic/ poly
Material recommended for monocular, kid and safety/sports wear
Polycarbonate
Lens that block 98% of incident UV light
Poly
Whats a disadvantage of poly
“Soft” required antiscratch coating, unable to tint
Why is trivex so names
Because of its 3 desired qualities, good ABBE, excellent impact resistance, and extremely light weight
Lens that would reflect most light if untreated
Flint
Combines both thermosetting and thermoplastic monomers giving it superior tensile strength
Trivex
Material of choice for drill mountings
Trivex
Is trivex able to be tinted
Yes
Advantages of high index lenses
- Bend light more efficiently and therefore lenses can be thinner and light
- thinned is limited by impact resistance standards and lens stability
- can use glass or plastic
High index lens from glass material
Flint (1.6)
Barium (1.541)
High index plastics
Polycarbonate (1.59)
Trivex (1.53)
Disadvantages of high index
Lower ABBE
Increased reflections
Power of the front surface of a spectacle lens. May be used to refer to flatter meridian of a toric surface
Base curve
FDA regulation that is the law and must be followed
CFR801.410
Impact resistance
Z80.1
Recommendations for prescription ophthalmic lenses
Z80.3
Non prescription sunglasses and fashion eyewear requirements
Z80.5
Dress ophthalmic frames
Z87.1
Occupational and educational eye and face protection
-federal requirement
Z80.1 and z80.3
FDA mandates the impact resistance of lenses
ANSI standard thats a federal requirement
Z87.1
Z87.1 has 2 levels of safety requirements
Non impact and impact
Non impact requirement of z87.1
- 3.00mm minimum thickness except more mm can have 2.5 edge thickness
- drop ball test
- manufactures logo on lens
- should tell patient that it meets basic impact requirement but should not be relied up for high impact exposure
impact requirement of z87.1
- 00 mm thickness
- high velocity impact test
- manufactures logo with a + on the lens
