9.3 Absorptive Lenses

What lens material is absorption of light minimal for?

crown glass

How do we increase the amount of light absorbed by a lens?

tinting a lens

How do you find T(surface)?

by finding reflection

How do you find T(media)?

by using lamberts law

How does thickness affect transmittance for a tinted material lens?

increased thickness=decreased transmission

Why does fabricating a lens from tinted material cause bullseye appearance?

the thicker a lens is the less transmission, aka the darker the tint will be, depending on prescription the thickness of the lens will vary from center to periphery

A patient with tinted lenses comes in and the lenses are darker in the center, are they myopic or hyperopic?

hyperopic

A patient with tinted lenses comes in and the lenses are darker at the edges and lighter in the center, are they myopic or hyperopic?

myopic

For tinted material cyl lenses, the lighter meridian is the?

minus cyl axis

What is opacity?

reciprocal of transmission

What is optical density often used to describe?

safety eyewear

What type of lenses are commonly described by shade #?

welding lenses

How is optical density related to shade number?

as OD increases, shade number increases

How is transmission related to shade number?

as transmission increases, shade number decreases

How is optical density related to transmission?

as optical density increases, transmission decreases

What are absorptive lenses used for?

reduce the amount of transmitted light

What do selective absorption lens colors mean?

color of lens is the result of the light that the lens transmits the most

(ex: green lens transmits more green light than any other color)

How does depth of color relate to selective absorptive lenses?

the deeper the color the less transmission of the visible spectrum, does not tell us UV or IR

What are the advantages of tinted glass lenses?

low production cost

no transmission change due to surface scratches

no reflections

no special equipment needed

What are the disadvantage of a tinted glass lens?

variation in transmission from center to edge

variation in transmission between eyes

permanence of tint

What does the density of tint depend on for coated glass lenses?

thickness of coating

What is the main disadvantage to coated glass lenses?

may need AR coat to reduce reflections since the index of the coating is higher than index of glass

What is the best color tint for plastic lenses?

neutral density grey

What are the advantages of neural density grey tint?

uniform absorption across spectrum while keeping color rendition

What are the advantages to tinted plastic lenses?

uniform density across lens surface

tint density and color can be changes

What are the disadvantages to tinted plastic lenses?

lenses from different manufacturers may absorb tint @ different rates

What plastic lens is not good to tint?

polycarbonate lenses

What does a brown tint absorb? transmits?

absorbs shorter λ (cooler colors) and selective UV

transmits longer λ (warmer colors)

What does a yellow tint absorb?

shorter λ and some UV

What is an advantage to yellow tint?

absorbs shorter λ which are scattered more by the atmosphere, therefore decreasing scattered light

What are the "advantages" of a yellow tint (that are not backed by science)?

improve performance in fog

shooter lens

decreases glare at night

What are cosmetic tints?

pink or soft lite tints

Why do patients get cosmetic tints?

increases comfort

improves appearance of lens

helps hide bifocal segment

What do cosmetic tints absorb?

little UV + IR (more than clear lenses)

What lenses must never be used for night driving?

NO TINTS

Which lens materials do not provide adequate UV protection?

crown glass + CR39

According to ANSI, a sunglass lens must transmit what portion of light?

≤40%

For a sunglass lens used for bright day light conditions, what should transmission be?

10-20%

What should you make your patients aware of before they get photochromic (transition) lenses?

darken more and faster in the cold

don't darken much in the vehicle

break-in period

lenses darken faster than they lighten

What kind of light is natural direct sunlight (unreflected)?

polarized

What kind of light is reflected sunlight?

polarized

How is sunlight polarized?

parallel to its reflecting plane

Vertically oriented polarized spec lenses blocks?

horizontally polarized light

Horizontally oriented polarized specs block?

vertically polarized light

What is Brewsters angle?

maximum polarization of reflected light from a surface

What is Brewsters angle for water?

53º

What is bestirs angle for crown glass?

57º

What is Malus' Law?

zero transmission of light occurs when the angle between absorbing meridian of lens and orientation of polarized light is 0

(Max absorption)

How are polarizing lenses made?

many layers of stretched plastic film with UV protection in them

What should you tell your patients before they get polarized lenses?

they will see:

more visible stress patterns on windshield

issues with some screens

problems for advanced skiiers

may cause problems with pilots

What are occupational absorptive lenses?

specifically designed lenses for specific occupations

General purpose sunglass should transmit what?

8-40% of visible light

UVA transmittance ≤ visible

UVB transmittance ≤ 1/2 visible or 5%

Why must we require lower UV transmittance than visible spectrum transmittance when prescribing absorptive lenses?

decreased visible light causes our pupils to dilate, letting in more UV

Cosmetic use sunglasses should transmit what?

>40% visible light

UVA ≤ visible

UVB ≤ 1/2 visible or 30%

Special purpose sunglasses should transmit what?

3-8% visible light

UVA ≤ 1/2 visible

UVB ≤ 1%

Lenses with what transmittance should not be used for ANY driving?

transmittance < 8%

What are the characteristics of ideal sunglasses?

protect the eyes from excess ambient luminance

eliminate portions of spectrum harmful to eyes

maintain optimal dark adaptation + night vision

provide adequate impact resistance

dont change color perception

What type of absorptive lenses should be used for daytime driving?

grey tint

photochromic lenses

polarized lenses

What type of lens should be used for night driving?

clear lens with AR coat

NO TINTS

What type of lens should you use for very bright outdoor conditions

grey tint

photochromic lenses

polarized lenses

What type of lens should you used for excessive UV conditions?

UV400 tint

What tint should be used for furnace operator/glass work?

blue/green tint

IR protection