Midterm 1 Flashcards
(124 cards)
1
Q
Absorptive Lenses
A
Lenses designed to:
-Prevent certain wavelengths from entering the eye
OR - Reduce the intensity of certain wavelengths that do enter the eye
2
Q
Electromagnetic Radiation
A
Emitted by the sun, composed of a continuous range of radiations differing in frequency and therefore wavelength
3
Q
Optical Spectrum
A
Visible spectrum: 380nm-760 nm
UV spectrum: wavelengths a little shorter, UVA is closest to the visible spectrum. UVA 380-320, UVB 320-290, UVC 290-200
Infrared spectrum: 760-1 mm
Although IR and UV can be a problem and can cause damage, we worry mostly about UV, it is shorter with more energy, and the biggest concern for human tissues
4
Q
Transmission characteristics- Crown glass
A
Crown glass is not a good UV blocker, it lets most of UVA and some UVB through
5
Q
Transmission characteristics- CR-39
A
CR39 is a better UV blocker than crown glass, but not optimal
6
Q
Transmission characteristics- polycarbonate
A
Untreated, uncoated UV blocker just due to the material
7
Q
When light strikes a lens, it is:
A
Reflected, Absorbed, or Transmitted
8
Q
Light Transmission through a lens is determined by calculating:
A
percentage of light lose by reflection of the front surface
percentage of light lost by absorption
percentage of light lost by reflection of the back surface
9
Q
Amount of light reflected by a lens surface (formula)
A
lr= (n'-n)^2/(n'+n)^2 (l) l= incident light on front of lens, n=the index before the interface, n'=the index after the interface
As index increases, reflection increases. If you have a high index lens with no antireflective coating, you will have a huge loss: won’t look good, won’t see good
10
Q
Transmission= ? (formula)
A
Incident light-absorption-reflection(front)-reflection (back)
11
Q
Lambert’s law of absorption
A
For an absorptive material, layers of equal thickness absorb equal quantities (percentages) of light regardless of the intensity of the light
12
Q
Transmittance factor (q)
A
Given- but need to make sure units are the same as the lens units
As light gets to each layer of lens, the amount of light will be decreased by the transmittance factor
13
Q
Amount of Absorption by a lens surface (formula)
A
Transmission= q^number (mm) of thicknesses
14
Q
Transmission (many lenses)
A
If light passes through a number of lenses, one after another, the ultimate transmission is found by multiplying the separate transmission of each of the lenses
15
Q
Opacity
A
reciprocal of transmission o=1/T
16
Q
Optical density
A
Used in connection with absorption, stated for a given thickness and is the negative log of transmission. It is an easier way to describe transmission that makes a very small transmission
17
Q
Types of Absorptive lenses
A
Reduces the amount of transmitted light or radiant energy
Acts as a filter
May be uniform or neutral, absorbing light of all wavelengths equally
May be selective, absorbing light of certain wavelengths more than others
Major forms of absorptive lenses
-Tinted solid glass lenses
-Glass lenses with surface coatings
-Tinted plastic lenses
-Photochromatic glass/plastic lenses
-Polarizing lenses
-Other
18
Q
Tinted solid glass lenses
A
Introduction of metals or metallic oxides during manufacturing
Spectral transmission characteristics are controlled by the quantities of metals used
Color imparted to lens by addition of an absorptive substance is of no significance other than cosmetic
19
Q
Glass lenses with surface coatings
A
Thin, metallic oxide is deposited on the surface of the lens, usually the back surface, it’s not through the entire lens
Requires high temperature
20
Q
Tinted plastic lenses
A
Tinted by dipping into a dye, dye penetrates the lens surface to a uniform depth, therefore, no change in density with changes in lens thickness from lens center to edge, overtinting a lens can be reversed by dipping the lens into a bleaching solution
21
Q
Common tinted lens colors
A
Pink: no color distortion for wearer, occasionally used to try and negate poor indoor lighting conditions. Usually very faint. Used for computer vision syndrome and eye strain
Red: Bold fashion color
Yellow: myth and speculation, possible applications (shooting, night driving, poor visibility glasses), may reduce scatter somehow, may enhance contrast of things against the sky
Green: approximates color sensitivity curve of human eye, may increase depth perception, for golf
Brown: popular for sunglasses, higher absorption of shorter visible wavelengths
**Gray: most popular tint for sun protection, approximately even transmission through entire visible spectrum, colors seen in NATURAL state relative to one another
22
Q
Photochromic Lenses (Glass)
A
Lenses that change darkness in different environmental conditions
-Developed by corning in 1964, glass contains SILVE HALIDE cystals, lenses darken when exposed to long-wavelength UV radiation, UV transforms silver halide crystals into silver and halogen atoms, glass keeps it together so it can combine again. Darkening rate is temperature dependent, works best at low temperatures. Fading back to clear works best when in the heat, the more exposed to UV, the darker it gets. Glass photochrome never wears out
23
Q
Corning Photochromic Filter Lenses
A
Relieve glare for patients with severe light sensitivity, filter out the shorter (blue) wavelengths, marketed for low vision patients, these patients can have significant glare issues and photophobia, these lenses can relieve this light sensitivity
24
Q
CPF Lenses-Glass
A
Created to relieve glare for patients with severe light sensitivity problems arising from: developing cataracts, aphakia/pseudophakia, macular degeneration, retinitis pigmentosa, albinism, aniridia
25
Photochromic Lenses- Plastic
Problem facing the photochromic industry is that silver halide chemistry is impossible in plastic. Compounds known as spiropyrans were created to produce the photochromic effect in plastic. UV light breaks the bond between spiro carbon and oxygen. The new open form compound strongly absorbs light in the visible region, reduced light transmission occurs through the lens
Two main types: In-mass technology- throughout lens (corning sunsensors, roden stock) and Imbibed technology- surface coating (transitions)
26
In-Mass plastic photochromic lenses
Molecules never wear out or fatigue, if surface molecules don't work, you have back up
Darkens up to 50% in car, scratches do not affect performance, exterior 1.5 mm of lensactivated preventing uneven darkening
27
Imbibed plastic photochromic lenses
Transitions- Darkens consistently across the lens, regardless of prescription, available in a wide range of materials and designs
28
Wide Jam angling plier
Adjustment pliers, used for pantoscopic angle adjustments, heavy bridge and endpiece corrections
29
Lens Axis aligning plier
Lens pliers, used for lens axis aligning, they can grasp the lens on front and back and twist the lens in the frame a little which will change the axis
30
Flat Nylon/Round metal plier
Adjustment pliers, used for shaping eyewires, endpieces, and bridges that require a rounded shape. The nylon allows you to grasp the frame so you don't scar it
31
Long snipe nose plier
Snipe/Chain nose pliers, used for adjustment of pad arms and thin, stainless steel and titanium eyewear
32
Pad arm adjusting plier
Nose pad/pad arm pliers, used for screw on, push-on, and clip on type nose pad assembly adjustments, they have a recessed area to grasp nose pad assembly
33
Chappel cutting plier
Cutting pliers, used for lens screw assemblies, reduces risk of damaging lenses, cuts titanium, stainless steel, and nickel materials
34
Optical screwdriver
has a head that can be flipped to a different one
35
Hex wrench
has a hexagonal tip
36
Prescription alignment gaze
indicated if lens is off-axis, inspection of optical centers, DBL, seg ht. etc.
37
Double Nylon pliers
gripping pliers, used for multi purpose adjusting tool for frames with delicate finishes, doesn't scratch frame surfaces
38
Spring Hinge screw replacement
Tool helps you engage a spring hinge to get the screw back in
39
Steps in standard alignment
1. Horizontal alignment
- Check for a rotated lens, check for a skewed bridge
2. Vertical alignment
- Check for x-ing, check for variant planes (or lenses (or lenses out of coplanar alignment)
- Makes sure the two lenses are equal distance from the eyes
3. Open temple Alignment
- Check the temples for straightness of the shaft, check the angles of the temple when fully opened for symmetry
4. Temple parallelism
- check for bent endpiece, check for loose or broken rivets or loose hidden edge, check for bend in the temple shaft
5. Alignment of the bent-down portion of the temple
- Check for equality of downward bend, check for equality of inward bend
6. Temple-fold angle
- Check for central crossing of the temple shaft when folded, check for a fold that permits insertion of the spectacles into a standard cases
40
Glare
Relatively bright light which interferes with optimal vision
- Distracting glare
- Discomfort glare
- Disability glare
- Reflected glare
41
Distracting glare
caused by lens surface reflections. Can be eliminated with AR coating
42
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, physiologic basis appears unknown. Everyday bright lights, best corrected by changing environmental conditions, but fixed tints would help
43
Reflected glare
Blinding glare, glare caused by reflected light sources, the glare off a shiny page in a book when held at the wrong angle when reading, if all surfaces were diffusely reflecting and all lighting indirect, reflected glare would not be a problem. Waves hit a reflective surface, and come scattered or polarized back. This is considered visual noise, but is a big deal. The only thing that will work here is polarized lenses
44
Polarizing lenses
Designed to eliminate glare from flat surfaces that have resulted in polarized or partially polarized light to the eye. Done with a sheet that only allows a certain orientation of light through
45
Two axes of polarizing filter
Absorption axis blocks all polarized light incident at that axis
Transmission axis transmits all polarized light incident at that axis. In any particular polarizing lens, the two axes are oriented PERPENDICULAR to each other, eliminates specularly reflected from horizontally polarized light
46
Polarizing filters on lenses
Eliminates specularly reflected horizontally polarized light. These arise from a horizontally oriented surface. Improves VA and restores the natural balance of light intensities, helpful for motorists, fishermen, and skiers for reducing glare from water and snow. Materials that best polarize light by reflectance are generally nonconductors called dielectrics such as glass, pavement, sand, and snow
47
Dielectric
A substance that is thermally and electrically neutral, like sand, water, and pavement. They are oriented horizontal, and when light hits these, partially polarized light comes in as reflected glare
-Light reflected from a dielectric is completely polarized at a specific angle of incidence (Brewster's angle)
48
Brewster's Angle
Angle of incidence, the angle between the normal to the surface and the ray**
49
Formula of Brewster's angle (i)
tani=n' (i- angle of incidence for which the reflected light is most completely polarized, n=index of the medium to which the light is incident)
50
Malus Law
Determines the intensity of light that passes through two perfect polarizers (that comes through to the other side of out lens)
-If the 2 polarizers are 90 degrees apart (transmission axes perpendicular), the polarizers are crossed and no light is transmitted. If the two polarizers are coincident (transmission axes are aligned) the polarizers are uncrossed and 100% of light is transmitted
51
Malus Law (formula)
Io=Icos^2 theta (theta=angle between the transmission axis of first polarizer and the transmission axis of the second polarizer, I=intitial intensity of light, Io=light passing through two polarizer series)
52
Aviation Study
Appropriate aviation sunglasses are free from distortions and imperfections, transmit at least 15% overall light, do not alter color perception, employ large lenses in a wrap-around design
Sunglasses that are NOT appropriate are POLAROID sunglasses (they reduce or eliminate the visibility of instruments in the cockpit, interfere with visibility through an aircraft windscreen due to striations in some laminated materials, can mask the sparkle of light that reflects off shiny surfaces such as another aircraft's wings, fuselage, or windscreen)
53
Melanin lenses
Melanin provides the added advantage of blocking not only UVA, UVB, and glare, but also what it calls HEV (High Energy Visible) radiation
54
Combination Lenses
Transitions drivewear, combines photochromics by transitions and glare protections by Nupolar. They have polarized photochromics that darken behind the windshields
55
Modern Antireflective system
Combination scratch coat and anti-reflective layers per side, includes quartz-like layers, outer hydrophobic layer. about 1% reflection per surface, with a very good multi layer, you get 99 percent transmission, remember, woth polarized, we were dealing with reflected glare, and now we are dealing with distracting/annoying glare
56
Antireflective Coat
Purpose: increase the amount of light transmitted through the lens, this is visual: less reflected means more transmitted. Reduce unwanted reflections.
This is largely cosmetic, and these reflections can be a problem for someone looking at the patient. It is noticeable and disturbing. And the wearer will have issues with ghosting
57
Reflections from lens surfaces
Majority of light striking a lens surface is refracted, small portion of reflected light can cause ghost images, falsification of image position, haze, and loss of contrast
58
Methods for reducing surface reflections
Base curve selection, adjusting the pantoscopic tilt, changing the vertex distance, selecting a smaller lens size, antireflective coatings (the best option of all 5)
59
Antireflective Coatings 2
Work on the principle of interference, whenever two waves of equal intensity or amplitude are superimposed by a monchromatic source neutralization occurs if the path differency is an ODD number of half wavelengths (NEGATIVE interference)
Reinforcement occurs if the path difference is an EVEN number of half wavelengths (POSITIVE interference)
60
Light striking the front surface of a lens with AR coating
Creates two reflecting waves -->Front surface of the lens coating, interface between the coating and the lens. Two reflecting waves cancel one another if waves are of equal amplitude (amplitude condition), AND if waves are a half wavelength out of phase (path condition)
61
Standard Lens without AR
The higher the refractive index of the lens material, the greater the incidence of reflections and glare. You can lose up to 15% on a very high index lens. High index= Need AR coating
62
Amplitude condition
Determines the index of refraction for the coating, intensity of the reflection from the air-coating interference must equal the intensity of the reflection for the coating-lens interference. Typicaly use magnesium fluoride.
63
AR coating index
You put AR coating on top of a lens, the index of refraction for the coating needs to be different from the lens, or nothing will happen
nc=square root nL (nc=index of coating, nL=index of lens) index of coating, is less that the index of the lens
64
Path Condition
2 waves out of phase by a half wavelength at the point of interference. Determines the thickness of the AR coating.
The thickness doesn't need to be one wavelength- this would actually increase, not decrease because it has to go through the coating and the lens before it can enter the eye. So one half wavelength is achieved by allowing the wave to travel one fourth through the coating before it goes to reflect on the front of the lens and back into the coating. So it goes in and out of the coating to make a total one half- the initial wage going in cancels the returning wave that we followed and you get no reflection
65
A Good AR coating:
Has to stick to the lens, be reasonably hard, cannot be water soluble. Properties that help make the coatings good: Hydrophobic properties, repel water and makes lens surface easier to clean. Oleophobic properties, repels oil, dirt and dust and helps reduce fingerprints, smudges and grease. Anti-particulate properties, repels dust, dirt and debris by giving the lens surface anti-static properties
66
AR Production sequence (Glass)
Thorough lens cleaning, lens heated in vacuum to 300 degrees C, Magnesium fluoride heated to 2500 degrees C causing vaporization and deposition on the lens surface. Process takes 4-10 hours, microscopically thin layers of low, medium, and high index materials are alternatively applied applied, final thickness of the AR coating is about 1/5000th width of human hair
67
AR production sequence (plastic)
Thorough lens cleaning, lenses baked for 2 hours to remove H20, lens heated in vacuum to 100 degrees C, thin layer if hugh refractive index, sodium dioxide used with electron beam evaporation for outer layer, Process takes 4-10 hours, microscopically thin layers of low, medium, and high index materials are alternatively applied applied, final thickness of the AR coating is about 1/5000th width of human hair
68
Problems with AR coatings
Soft and scratch easily, display grease and smudges (old lenses, much better now)
69
Hydrophobic coatings
Layer of silicone added as outermost layer, causes water and grease to form smaller droplets; therefore, less noticeable, increases hardness of lenses surface, also, anti-smudge
70
How many layers is the typical AR coating
Most are 5-6 layers. Some new ones are 11-layers (which adds hardness without brittleness)
71
Mirrored Coatings
instead of reducing, they enhance reflections. Mirrored effect is created by CONSTRUCTIVE interference at the lens surface. Usually applied over a dark tinted lens but can be over any base color, even clear. Difference is in the thickness of the layers. Created by depositing powdered oxides, metals or compounds onto the surface of a tinted spectacle lens.
72
Two mirror treatments
Metallic- scratches easily
| Dielectric- non conductive, very durable-ceramic type. Put over the top of the metallic and you get a good combo
73
Metallic treatments
Metallic mirrors- Reduces visible light transmission by 25-35%, solid reflective one-way mirror, can't see wearer's eyes. Usually gold, silver, or blue
Flash Mirrors- less metallic coating, reduces visible light transmission by 8-12%, used more for fashion
74
Dielectric treatment
Dielectric, produced by metallic oxides being deposited onto the lens surface, reflects light without absorption
75
Scratch-Resistant Coatings
Finished lenses usually have both front and back surface scratch-resistant coatings, may be purchased with SRC only on the front surface for semi-finished lenses, avoid exposure to excessive heat
76
Superior Scratch perfomance
AR layers actually enhance scratch resistance by including three SiO2 layers.
77
Lens coatings and impact resistance
Scratch resistant and AR coating will decrease impact resistance. Both coatings are harder than the plastic lens, if it were hit by an object, the lens may flex but the coating may crack.
78
Frame
The part of spectacles that holds the lenses containing the ophthalmic prescription in their proper position in front of the eyes
79
Frame Fronts
Front consists of two eyewires (around lenses), bridge (connects eyewires), Endpieces (where the temples are connected)
80
Mounting
Name given to a "frame" when lenses are held in place without the aid of an eye wire (or nylon cord). Attaching of lenses to a rimless or semirimless frame. Usually held in place by screws, posts, clips, etc. Type: Ballgrip/Ilford, Numont, Wils-edge
81
Bridge
the area of the frame front between the lenses
Types: Keyhole- rests on side of the nose, not the crest. Saddle- smooth curve attempts to follow bridge of nose. Comfort- clear plastic saddle type nose piece, Semisaddle bridge- from the side, you can see the permanent nonadjustable nose pads that extend back. Metal saddle- Sits on the crest of the nose. Pad bridge- has nose pads
82
Nose Pad Arms
Metal pieces that connect adjustable nosepads to the front of the frame (guard ars). Types: Inverted U-shaped (most common), Question mark style, Hybrid
83
Nose pads
plastic pieces that rest on the nose to support the frame (via a pad arm). Screw on, push on, clip on, twist on. Come one different sizes and shapes- oval, circular, 9-20mm
84
Endpiece
one of the two outer areas of the frame front to the extreme left and right (when looking at the frame from the front) where the temples attach. Types: American, English, French, Butt-type, mitre-type, turn-back
85
Hinges
Part of the frame that both holds the temple to the front and allows the temple to be closed. Many are spring hinges- take the screw out and its hard to get back in. Types in plastic frames- Riveted: Apparatus characterized by rivets attaching to shield plate. Shield visible from frame front. --Rivet: a permanent fastener; a short metal pin for fastening two or more pieces together having a head at one end and the other end fixed after passing through hole in each piece.--Shield on aplastic frame, the metal piece to which rivets are attached to hold the hinge on place.
Hidden: Apparatus anchored directly into plastic, no shield visible from frame front, the hinge is sunk into plastic
86
Temples
the part of a pair of spectacles that attaches to the frame front and hooks over the ears to hold the spectacles in place
87
Skull Temples
most commonly used in prescription spectacles, bend down behind ears and resting evenly against the contour of the skull, rests evenly against the contour of the skull
88
Library Temples
Straight and do not bend down behind the ears, held in place by light, even pressure against the side of the head
89
Convertible temples
Can be used as either skull or library temple, depending on how they are bent
90
Riding bow temples
Curve around the ear and level with the ear lobe, used mostly on children's, athletic, and industrial safety frames
91
Comfort Cable temples
constructed from a flexible, coiled metal cable, similar to riding bow temple
92
Anisometropia
One eye differs significantly in refractive power from one eye to another, anisometropia is considered to exist when the spherical equivalent refraction of the two eyes differs by 1.00 D or more
93
Uncorrected aniso refractive error
Impossible to have sharp focus on both eyes simultaneously, image size differences may be present, can lead to suppression and amblyopia
94
Corrected aniso refractive error
Can cause image size differences, vertical imbalance in the reading position, horizontal vergence problems, possible problems when correcting anisometropic refractive errors: Accommodative (both eyes will always respond with equal amounts of accommodation), Vergence (induced prism whenver the patient looks away from optical centers of the spectcle lenses), Image size (the greater the image size difference between the two eyes, the less chance of comfortable BV)
95
Secondary Anisometropia
Can occur due to: axial differences between the eyes, refractive differences between the eyes (corneal, or lenticular)
96
Aniseikonia
the relative differences in the sizes and/or SHAPES of the ocular images of the two eyes, produced because of the mag properties of lenses. Cyl can have different mag in one or more meridians. Can be broken down into symmetrical or asymmetrical size differences
97
Symmetrical Aniseikonia
Overall- the ocular image of one eye is increased or decreased in size equally in all directions as compared to the other eye. Meridional- the ocular image of one eye is increased or decreased in size in one meridian compared to the other eye
98
Asymmetrical Aniseikonia
A progressive decrease or increase in the image size in one meridian or all directions from the visual axis (barrel/pincusion distortion)
99
Developing asthenopia from aniseikonia
To produce a single image, images from the two retinas must be fused, the more alike the easier to fuse. Asthenopia can develop because the patient can't tolerate fusing two different images anymore. Their eyes feel funny, there's eyestrain, headaches, photophobia, nervousness. If these symptoms are relieved by occluding one eye, aniseikonia should be suspected
100
Spectacle Magnification
Determining the amount of aniseikonia in a spectacle prescription. Formula SM= retinal image size in corrected eye/ retinal image size in the uncorrected eye. Composed of two factors: the shape factor and the power factor
101
Knapp's Law
"When a correcting lens is so placed before the eye that its second principal plane coincides with the anterior focal point of an axially ametropic eye, the size of the retinal image will be the same as though the eye were emmetropic"
102
Correcting Refractive and Axial Ametropia according to Knapps Law
You have refractive (cornea) and axial (lengthening/shortening of the eye), most of the time you don't the extent of either. The size of the image produced on the retina depends on the amount of each. Desired change in the image size will occur in AXIAL ametropia with SPECTACLE correction. Minimal change in the image size will occur in REFRACTIVE ametropia with CONTACT LENSES. You are trying to get the retinal image size the same for each eye. The goal with these is to get the retinal image size the same as it would be for the emmetropic eye.
103
Refractive or Axial?
For ametropia less than +/- 4.00D, the refractive components are within emmetropic ranges. For ametropia greater than +/- 4.00D, the axial length almost always beyond emmetropic norms
104
What image size difference will result in the loss of binocularity?
Image size difference between the two eyes is greater than 5%
105
What was the gold standard for measuring aniseikonia?
The Dartmouth eikometer
106
What is the rule of thumb between anisometropia and aniseikonia?
For every 1.00 D between the eyes, there is a 1% image size difference
107
Frame materials- general
Most frames are plastic or metal
| There are other materials, but they aren't used much
108
Plastic
Advantages: inexpensive, easily moldable, many colors.
Disadvantages: easy to break, sunlight and time can degrade, colors can fade.
They all have their own personality and many different properties, such as some will shrink with overheating, and some will expand. You can't treat them all the same.
109
Zyl (cellulose acetate or zylonite)
Most popular material for plastic frames. Thermoplastic. Advantages: cost-effective, wide variety of colors, generally color fast/stable, can be worked easily, allows some margin for sizing errors, lightweight, hypoallergenic.
Disadvantages: Can become brittle with age, can lose surface luster with age, may fade with exposure to UV, body oils, overheating can result, sensitive to solvents, a little heavier than other plastics.
110
Proprionate
Cellulose acetyl-proprionate. Nylon based. Advantages: reasonable variety of colors (not as many as zyl), less surface dulling, reasonable color fast, good flexibility, extremely lightweight, extremely hypoallergenic.
Disadvantages: difficult to dye, heat sensitive (at 105-110 degrees), not well suited to stretching or shrinking, sensitive to solvents.
111
Optyl
Made from the epoxy resin, Thermoelastic.
Advantages: wide variety of fashion shapes, colors, patterns. Very lightweight, excellent shape and adjustment retention. Hypoallergenic. Extremely heat resistant. Resistent to most solvents.
Disadvantages: adjustments more difficult, require heat to work, will not shrink.
112
Metal
Metals are all different, but you treat them the same. You don't use heat as they have a built in malleability, the best combo is rigid front and a bendable temple.
113
Aluminum
Used on a limited basis, softer but combined with other metals to make it harder.
Advantages: lightweight, strong, corrosion resistant. Low density (lightweight) allows for large frames. Long lasting. Natural silver allows a high luster. May be anodized allowing for variations in color.
Disadvantages: difficult to achieve good lens fit without gapping. Difficult to adjust, limited fitting versatility due to stiffness. Complicated manufacturing. Poor color finish durablity. Not hypoallergenic
114
Monel*
Alloy of Nickel (largest component) and other metals like copper, zinc, iron, manganese. Most widely used metal/alloy.
Advantages: Cost-effective and efficient. Extremely malleable and has good resistance to stress. Corrosion resistant with proper plating/coating
Disadvantages: May be difficult to reshape. Medium corrosion resistant unless plated. Not hypoallergenic, lightweight or strong like other metals.
115
Titanium
"perfect frame material" Synonymous with high-quality, value-added, frame product
Advantages: great strength and durability, lighter than traditional metals. Excellent surface hardness. Impervious to corrosion. Hypoallergenic. Available in colors and styles
Disadvantages: Expensive. Not flexible. Requires special welding equipment in frame fabrication
Beta titanium- 70% titanium with no nickel, to allow for flexibleness, may be better than pure titanium.
116
Flexon
Titanium based alloy (40%) From manifacturer Marchon. Memory metal, smart metal, smart alloy.
Advantages: extraordinarily flexible. Hypoallergenic, lightweight, strong, and corrosion resistant.
Disadvantages: Not available in wide variety of colors.
117
Berylium
Lower cost alternative to titanium
Advantages: Generally lightweight, strong, flexible, and corrosion resistant. Very flexible, available in colors
Disadvantages: Not hypoallergenic
118
Stainless steel
Alloy of steel and chromium
Advantages: generally lightweight, strong, hypoallergenic, strength allows stable, thinner construct. Resistant to abrasion and heat. Reasonably priced and easy to keep clean
Disadvantages: adjustments are difficult
119
Other frame materials
Bone, wood, buffalo horn
120
Nose pad materials
Acetate, hard and non flexible
Vinyl- flexible, but slides down
Silicone- tends to stick
121
Frame accents
Silver, gold, leather, velvet, semi-precious, precious stones, rhinestones, feathers etc.
122
Frame coatings
Often seal surface, usually contains a UV inhibitor to keep from fading.
123
How much tint is appropriate in sunglasses?
15-30% is normal transmission for sun lenses. If its too high, may not help average wearer enough in sunlight, if it's too low, may result in VA reduction in dimly lit conditions
124
Disability glare
Causes objects to appear to have lower contrast than they would have if there were no glare. Increases the brightness of the background and lowers the brightness of the object thereby interfering with resolution. Similar to turning room lights on when watching a slide show
