Absorptive Lenses Flashcards
Bleaching
The lightening of a photochromic lens from exposure to red light, infrared radiation, or heat.
Brewster’s Angle
The angle of incidence at which reflected light from a refractive surface is completely polarized
Disability glare
Glare that reduces visual performance and visibility and may be accompanied by discomfort.
Discomfort glare
Glare that produces discomfort but does not necessarily reduce visual resolution
Glare control lenses
A lens that absorbs wavelengths toward the blue end of the spectrum in an attempt to reduce glare and increase contrast.
Fresnel equation is the formula used to determine
The amount of light that will be reflected from an uncooked lens surface based on the index of refraction and lens material.
Infrared
Invisible rays having wavelengths longer than those at the red end of the visible spectrum. Shorter than radio waves.
Lambert’s Law of absorption predicts what
Predicts how the amount of light transmitted will change based on a change in thickness of the absorbing material
Laminated lens
An ophthalmic lens that is made up of more than 1 layer. ex: polarized lens
Malus Law
Law of physics that predicts how much polarized light will be transmitted by an obliquely oriented polarizing filter.
Trade name for a brand of photochromic lenses
Transitions
UV
Rays with wavelength shorter than those at the violet end of the visible spectrum
Absorptive lenses are designed to do what 2 things
- Prevent certain wavelengths from entering the eye
OR - Reduce intensity of certain wavelengths that do enter eye
Visible spectrum in nm
380-760nm
UVA range
380-320nm
UVB range
320-290nm
UVC range
290-200nm
What kind of UV does crown glass allow through
Not a good UV blocker. Lets thru some B and all A.
CR39 transmission curve compared to Crown glass
Shifts to the right. Better UV blocker than crown glass but not optimal. UVA can still get thru.
Polycarb transmission compared to CR39 and crown glass
Excellent UV blocker. Do not need additional UV coating. Slightly less transmittance than CR39 and crown glass due to increased index of ref.
what 3 things occur when light strikes a lens
reflection
absorption
transmission
light transmission is determined by calculating which 3 things
light lost by reflection on front surface
light lost by reflection on back surface
Light lost by absorption
Transmission =
incident light - absorption - reflection
Lamberts law of absorption: For an absorptive material, layers of equal thickness ____
Absorb equal quantities/percentages of light regardless of the intensity of the light.
*must also know thickness of material
Transmittance factor= q ex: q= 0.5 per mm
If a problem says to ignore reflection, you can assume
lens has AR coating
If q= 0.5 per mm and the lens is 4mm thick, what does that mean
In the first mm, the intensity of the light leaving the first mm is 0.5 the intensity of the light that entered.
Cut 100% in half 4 times.
100-50-25-12.5-6.25% transmitted. Pretty dark lens.
or
(0.5)^4= 0.0625=6.25%
IR= (n’-n)^2 / (n’+n)^2 (I)
What do the variables stand for
IR= amount of light reflected n' = index of lens n= surrounding index. Usually air. (^ flip these two when calculating back surface) I= incident light on the surface
In an example considering reflection and absorption. What are the steps
- considering light reflected on the front surface using fresnel’s equation
Rfront= 0.040 = 4% 1-0.040= 0.960= 96%
- Take into account the incident light when starting the absorption calculation.
- 960(0.50)^4 = 0.060 = I = Incident light on the surface
- Apply the fresnel equation for the back of the lens using Incident light I = 0.060
Amount of light reflected on the back= 0.0024
- Total transmission= step 2 - step 3
Total transmission= incident light on the surface - amount of light reflected on the back
0.060- 0.0024 = 0.0576 = 5.8% total transmission
Ultimate transmission
If light passes through a number of lenses, one after another, the ultimate transmission (Tu) is found by multiplying the separate transmission of each of the lenses in decimal form.
Tu= (T1)(T2)(T3)
Find ultimate transmission of lens 1= 80% and lens 2= 80%
Tu= (0.8)(0.8)= 0.64= 64%
Opacity is the ___
Reciprocal of transmission.
O= 1/T
T= transmittance in decimal form
Ultimate opacity
Similar to ultimate transmission, multiply the separate opacities of each of the lenses.
Ou= (O1)(O2)(O3)
Find ultimate opacity of lens #1= 2 and lens #2= 1.5
(2)(1.5)= 3
If opacity is 3, what is transmission
3= 1/T T= 0.33 or 33% transmission
Optical density is a term in connection with ___
Absorption
Opacity is connected with ___
Optical density is connected with ___
Transmission
Absorption
Formula for Optical density (D)
D= -log(Transmission)
Transmission in decimal form
Role of absorptive lenses
Reduces the amount of transmitted light or radiant energy. Acts as a filter.
Uniform/neutral absorptive lenses
Absorbing light of all wavelengths equally
Selective absorptive lenses
Absorbing light of certain wavelengths more than others
Difference between uniform/neutral and selective absorptive lenses.
Absorbing light of all wavelengths equally vs absorbing certain wavelengths more than others.
Tinted solid glass absorptive lenses
- introduces _ or _ during manufacturing.
- How are spectral transmission characteristics controlled?
Problems
- Metals or metallic oxides during manufacturing.
- Controlled by the quantities of the metals used.
Transmission is greatly reduced with tint.
Different areas of lens (thick/thin areas) will have varying shades.
Tinted solid glass problem: Different areas of lens (thick/thin areas) will have varying shades.
How can you fix this?
Add surface coatings. Thin metallic oxide is deposited on the surface of the lens-usually the back surface. Requires high temp.
How are plastic lenses tinted?
Tinted by dipping into a dye. Dye penetrates lens surface to a uniform depth- therefore, no change in density with changes in lens thickness from center to edge.
How to reverse over-tinting of a plastic lens?
Dip into a bleaching solution.
Most popular tint for sun protection and benefits
Gray. Even transmission through entire spectrum- decreases all wavelengths almost equally. Colors seen close to natural state “relative” to one another.
Gray tint is ideal for which settings
Bright or overcast weather, nearly all sports (Golf, skiing)
Brown tinted lenses
Alters color perception a little. Not as true as grey tho.
Higher absorption of shorter visible wavelengths (decreases blue light/glare)
Brown tinted lenses are ideal for which settings
Bright or overcast weather, shooting, baseball, soccer, tennis.
Yellow tinted lens
Alters color perception a little. Not as true as grey tho.
Higher absorption of shorter visible wavelengths (decreases blue light/glare)
Enhances contrast of targets against sky
Yellow tinted lenses are ideal for which settings
Overcast weather, fog, twilight, sport performed in low light- shooting, snow sports, night driving.
Green tinted lenses
Roughly approximates color sensitivity curve of human eye. does alter color perception.
Increases depth perception, enhances green objects, enhances contrast between brown and green.
Green tinted lenses are good for which settings
Bright weather, golf, some shooting, maybe tennis
Red tinted lenses
Fashion color, alters color perception. May enhance contrast between orange/red.
Good for bright weather conditions, trap shooting, possibly skiing.
Pink tinted lenses
Minimal color distortion for wearer. May be used to negate poor indoor lighting conditions. Computer use? Better to just use AR.
How much tint (AKA how much transmission) is appropriate for sunglasses?
15-30% transmission is normal for sun lenses.
8-40% transmission for a general purpose lens per ANSI
Sunglasses.
Transmission too high?
Transmission too low?
Too high: may not help average wear enough in full sunlight
Too low: VA reduction in dimly lit conditions
Photochromic lenses contain
Silver halide crystals that darken when exposed to long wavelength UV radiation.
Photochromic darkening rate is ___ dependent
Temperature
UV transforms silver halide crystals into
Silver and halogen atoms.
Photochromic lenses work best in what temp
Will darken quickest in: Cold temp with high UV. Best on top of a mountain.
Will clear up with heat: put under warm water to clear.
Do photochromic glass lenses wear out?
No, but may need to be “broken in.”
Corning photochromic filter (CPF) lenses - glass
RELIEVE GLARE for patients with severe light sensitivity. Filter rout the shorter (blue) wavelengths. Selective! Ex: block all wavelengths under 511 nm
Ex: retinitis pigmentosa. Bad glare issue, albinism, aniridia.
Can you use silver halide for photochromic lenses in glass or plastic?
Glass only. Cannot be used in plastic lenses.
Photochromic compounds used in plastic lenses
Spiropyrans. UV breaks the bond between the spiro carbon and oxygen. The new open compound strongly absorbs light in the visible region. Reverse when the UV source is removed.
Disadvantage to plastic photochromic lenses
May wear out/fatigue over time. Ex; Couple years
Imbibed photochromic plastic lenses
Darkens consistently across the lens, regardless of rx. Available in wide range of material/design.
In mass photochromic plastic lenses. Benefits
Molecules never “wear out” or fatigue.
Darkens up to 50% in the car.
Scratches do not affect performance.
Exterior 1.5mm of lens activated preventing uneven darkening.
Difference between transitions Gen 8 and XTRActive transitions
xtractive have superior outdoor darkness with a slight indoor tint. Moderate activation behind a car windshield.
Can you polarize lenses that have photochromic?
no
Advantages/disadvantages to photochromic motorcycle visors or helmet shield
Advantage: convenient
Dis: Will the visors change quick enough when you are going 60mph into a dark tunnel?
What is glare and what are the 4 types
Relatively bright light which interferes with optimal vision or produces discomfort
Distracting, discomfort, disability, reflected
Distracting: Annoying. Caused by lens reflections- when you look at someone and you see yourself reflecting off their glasses. Fix with AR.
Discomfort: Sensation of irritation or pain from sources of light in the field of view. Stray light that causes visual discomfort but DOES NOT interfere with resolution.
Fix by changing environment aka get out of sun
Disability: Causes objects to have a lower contrast than they would if there was no glare. INTERFERES with resolution. Background brightness is increased and object brightness decreases.
Ex: turning on lights while watching a movie.
Light sources within the central 10 degree of the visual field contribute to this glare
Reflected: Caused by reflected light sources.
Ex: Glare off a shiny page in a book when holding it at the wrong angle.
Fix with polarized lenses.
Distracting glare
Annoying. Caused by lens reflections- when you look at someone and you see yourself reflecting off their glasses. Fix with AR.
Discomfort glare
Sensation of irritation or pain from sources of light in the field of view. Stray light that causes visual discomfort but DOES NOT interfere with resolution.
Fix by changing environment aka get out of sun
Disability glare
Causes objects to have a lower contrast than they would if there was no glare. INTERFERES with resolution. Background brightness is increased and object brightness decreases.
Ex: turning on lights while watching a movie.
Light sources within the central 10 degree of the visual field contribute to this glare
Reflected glare AKA blinding glare
Caused by reflected light sources.
Ex: Glare off a shiny page in a book when holding it at the wrong angle.
Fix with polarized lenses.
