Optics Lecture 8/9 Frame materials

The electromagnetic spectrum

All of the frequencies or wavelengths of electromagnetic radiation

Visible light

400-700nm

visible to the eye

stimulate photoreceptors and send a signal to the brain

Blue

higher energy

slower

shorter wavelength

Red

lower energy

faster

refraction

The bending of a wave as it passes at an angle from one medium to another

change of direction and speed

the extend to which it changes depends on the angle the light ray makes with the surface of the substance (------) and the ----

angle of incidence

curvature

Properties of ophthalmic lenses

Transparency

refractive index

chromatic abberation

ABBE

Specific Gravity

Transparency

shorter wavelengths (ultraviolet) contain relatively ______ levels of enervy

higher

longer wavelengths (infra red) contain relatively ______ levels of enervy

lower

Higher energy, higher frequency, slower

blue

amorphous solids

A solid made up of particles that are not arranged in a regular pattern

rigid but brittle

transparent but nonconductive

refractive index

ratio of the velocity of light in air to its velocity within a given medium

efficiency of a lens' ability to refract or change the speed of the traveling light

as light moves from air to a medium of higer refractive indiex, it

slows down

the higher the index of refraction,

the more efficient the medium

the slower the wave travels

chromatic abberation

different wavelengths of electro-magnetic energy (experienced as color) react differently when passing through various materials

all wavelengths are not focused at the same point

--- wavelengths are more efficiently refracted compared to longer wacelengths

shorter (blue)

longer (blue)

opthalmic materials can either----- or ----- chromatic abebration

limit or exacerbate

the effect of chromatic abberation on the visible spectrum is lnpown as

disperions

why do phoropters have glass lenses

least amount of dispersion

dispersion

every wavelength of light hits at a different spot because of different levels of energy

Abbe Value

ratio of differences between performan in the center of the visual spectrum comapred to the ends

quantifies a materials abillity to limit chroatic abberation

abbe is

GOOD

Specific Gravity

ratio of the density of a given material to that of water

high specific gravity

heavy

•Big difference in specific gravity between glass and plastics- but relatively small differences between different plastic materials

Crown glass

optical grade mineral glasses with high refractive index and low dispersion

(high abbe)

crown glass is made by

pouring molten glass into molds resulting in a blank

Plastic

transparent organic (carbon) polymers

Polymers

large compound formed from combinations of many monomers

polymerization can occur through

chain polymerization: a chemical reaction to form a long chain or polymer

condensation polymnerization: formed with water as.a bbyproduct

Thermosetting polymers (resins)

products manufactored usinga. chemical process

molded

not reverseible

Thermoplastics

products manufactured using an external thermal process

injection molding

recycled

CR39 is a

thermosetting polymer (resin)

There is no industry standard (ANSI or FDA) which regulates what

can be marketed as high index

Low index

<1.53

mid index

1.60 - 1.61

high index

1.66 - 1.70

Ultra high index

1.74

Why is the same power lens relatively thinner with higher index material?

high index material is more effective in slowing down the speed of light

atoms!

Optical Crown Glass

best optical properties

alavilable through 1.9 index

heat and scratch resistance

needs to be tempered

heavy

CR39 Allyl Diglycol Carbonate

1.49 index

rx sph +/- 4.00 and below

rx cyl < +/- 2.00

not impact reistant

Not 100% UV protection

Avoid larger frames or excessive decentration in higher powers

lowest cost

Polycarbonate

1.58 index

+/- 4.00 to 6 sphere

-2.00 and less for cyl

impact resistant

100% UV

thiner and lighter than CR39

heavier but thinner than trivex

lower ABBE than trivex

more than CR39 less than trivex

Polyurethane (trivex)

1.53

rx sph +/-3.00 and below

cly +/- 2.00

impact resistant

100% UV protection

thinner and lighter than CR39

lighter but thicker Polycarb

high ABBE

mid range cost (more than polycarb)

For a child or pt blind in one eye, legally have to prescribe

polycarbonate

polyurethane

UV protection

Mid Index

1.60 - 1.61

4.25 - 6..75

Cly 2.25 - 3.00

100% UV protection

Thinner and lighter than CR39 and Polycarb

apheric designs to minimize distortions

mid range cost

Higher index lens

1.67 - 1.70

7 - 9

3.25 - 4 cyl

100% UV

Thinnest and lightest

qwpheric design

more expensive

ultra high index

1.74

9.25 and above

4.24 - 6 cyl

100% UV

Thinner and lighter

apsheric design

MOST EXPENSIVE

plastic/glass possesses superior resistance to scratching and thermal damage (abrasives, welding debris, heat stress)

glass

American National Standards Institute

ANSI

nonprofit

NOT A GOVERNEMT AGENCY

ANSI standards are enforced by

FDA

not OSHA

z80.1 General Standard for Ophthalmic Materials

ANSI standard

Covers tolerance for power, prism, base curve, polarization, center thickness, heat resistance and basic impact resistance

Dropped Ball test (z80.1)

lens must withstand a free fall impact of a 5/8 inch diamter steel ball (16gm) from a height of 50 inches at its GC

Z87.1 Safety Lenses and Safety Eyewear Standards

lens and frame must withstand impact of 1 inch steel ball from a height of 50 inches

minimum thickness 3mm

2.5 mm for powers >3.00D

ANSI Z87+

High Impact standard (lens and frame tested as a system)

lens and frame must withstand a free fall impact of ponted steel missin (500g) from 50 inches

lens and frame must withstand prpelled imapct of 1.4 inch steel ball (44gms) at a velocity of 167 ft/sec

Lens and frame must resist penetration of 44gm steel ball dropped from 50 inches

coverage of eyes from sides

American Society for Testing and Materials

Sports

lenses and frames

NOT ANSI