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Materials Test 2

Quiz yourself by thinking what should be in each of the black spaces below before clicking on it to display the answer.

What are three main types of ophthalmic glass?   Crown, Flint, and Barium  
What are the three critical properties of ophthalmic glass?   Refractive Index (n); ABBE number (Dispersion) (V); Specific Gravity  
Why is it important to know what the refractive index is?   To know how much it's going to bend light  
What is the ABBE number?   The reciprocal of the relative dispersion expression  
An optical medium having a high ABBE value has a ____ dispersion?   low  
An optical medium having a low ABBE value has a ____ dispersion?   High  
What is specific gravity?   The ratio of the weight of a substance to the weight of water with the same volume.  
Higher the ABBE number, the ________ the material is.   heavier  
Why do we use ABBE value for optical reasons?   We use specific gravity to compare between different lens materials in the same Rs and similar optical refractive index in order to provide the patient with a light as possible lens.  
Clear optical class transmits what percentage of the incident light?   92%  
On the front and back surface of a glass lens __% transmission is lost to reflection.   4  
High index glass transmits ____ than 92%, because of increased surface reflections   Less  
What are advantages of AR coating?   Night time driving; less glare (less eye strain, computers, etc.); looks better aesthetically; see better (transmits more light)  
Negatives of AR coating?   cost; scratch and smudge easier  
Glass with an ABBe value greater than 50 are called....?   Crown  
Glass with an ABBE value less than 50 are called....?   Flint  
Where was the name of crown glass originally derived from?   The sheets blown for window glass  
Composition of Crown Glass   70% sand (silica); 14-16% Sodium Oxide; 11-13% Calcium Oxide; and small percentages of potassium, borax, antimony, and arsenic  
What is ophthalmic crown glass used for primarily?   Single vision lenses and also for the distance portion of most glass bifocal and trifocal lenses  
What is the refractive index of Crown glass?   1.523  
What is the ABBE value for crown glass?   59  
What is the specific gravity for crown glass?   2.54  
What is the composition of flint glass?   45-65% Lead Oxide; 25-45% Silica; 10% Soda and Potassium Oxide (mixture)  
What is ophthalmic flint glass used for primarily?   Bifocal segment of fused bifocal in which the segment must have a higher refractive index than does that of the major lens.  
What is the refractive index of light flint?   1.580  
What is the refractive index of dense flint?   1.690  
What is the ABBE value of flint glass?   30-40  
Wat is the specific gravity of flint glass?   4.00 (based on average flint glass light/dense)  
What is the composition of barium crown?   Barium Oxide 25-40%  
Barium oxide _________ the refractive index like lead oxide, but does not raise the chromatic dispersion like lead.   increases  
What is the main use for Barium Crown?   segments of Nokrome's series of fused bifocals; also used for the intermediate and near segments of some bifocals and trifocals  
What is the refractive index of Barium Crown?   1.59 (avg)  
What is the ABBE value of Barium Crown?   56.25 (avg)  
What is the specific gravity of barium crown?   3.28 (avg)  
What are absorptive lenses?   adding different metallic oxides to the raw materials in the batch will create different colored lenses.  
What color will cobalt make a lens?   Blue  
What color will chromium oxide make a lens?   green  
What color will magnesium make a lens?   Violet  
What color will uranium make a lens?   Yellow  
What does cerium oxide do to a lens?   Absorbs UV radiation  
What does iron oxide do to a lens?   Absorbs infrared radiation  
What does photochromatic glass contain?   silver halide crystals  
How does a phtochromatic lens work?   Silver halide crystals, under the influence of UV, separate into silver and halide ions; the ions then cluster together; as the cluster becomes larger it turns opaque, and thus darkens the lens. The reverse occurs in the absence of UV rays.  
Pros of glass lenses?   Best optics and clarity; most scratch resistant; can be made in a very high index  
Cons of glass lenses?   Very heavy; breakable (can cause injury); poor impact resistance  
Defenition of a plastic material?   A polymeric material (usually organic) of a large molecular weight which can be shaped by flow.  
What are other names for plastic lenses?   organic lenses; resin; hard resin  
What are the two categories of plastic lenses?   Thermoplastic and Thermosetting  
What is the difference between thermoplastic and thermosetting lens materials?   Thermoplastic can be softened when heated and can be remolded; thermosetting can not be softened once it is hardened  
What are some examples of thermoplastic material?   plexiglas; PMMA (polymethyl methacrylate); cellulose acetate; cellulose nitrate; polycarbonate; nylon; vinyl  
How is a thermosetting plastic lens made?   Usually supplied in liquid monomer form and cast into molds; a catalyst is added to the basic chemical compound polymerization occurs.  
What is the curing cycle?   The relationship between time and temperature required in the hardening cycle.  
Process turns liquid monomer into a ______, _____, and then a _____.   syrup, gel, and then a solid  
Examples of thermosetting plastic materials....   CR-39 (Columbia Resin 39); Epoxies; Phenolics  
What is another name for CR-39 plastic?   Allyl Dalycol Carbonate  
What sparked the development of plastic lenses in the 1970s?   FDA increased impact requirements on lenses and there was increased popularity of larger lens sizes. Also needed a material that would tint easier.  
Which war sparked the need for plastic material?   World War II  
What was PMMA originally developed for, and what other material was used for military aircrafts?   Windshields and aircrafts and CR-39 plastic  
Who developed CR-39 plastic and in what year?   PPG (Pittsburg Plate Glass) in 1939; A series of 170 clear allylic resin were compounded and tested, 39th compound was selected for its optimal properties for use in military aircraft  
How is CR-39 manufactured?   starts with an allyl diglycol carbonate monomer; catalysts and other ingredients are added; liquid resin is poured into mold; entire mold placed in oven and subjected to the curing cycle; the mold are removed from the oven, dismantled, and separated  
What type of mold is used to manufacture CR-39?   A mold with two glass surfaces; one is convex and one is concave.  
What is the refractive index of CR-39?   1.498  
What is the ABBE value of CR-39?   58  
What is the specific gravity of CR-39?   1.32  
What are desirable characteristics of CR-39 plastic?   lightness (low specific gravity); impact resistance; chemical inertness and resistance to virtual all solvents; resistance to pitting; resistance to fogging; tintability  
Does CR-39 need any treatments to pass the FDA impract resistance test?   No it is inertly impact resistant  
What are undesirable characteristics of CR-39?   surface abrasions (great need for scratch resistant coating; warpage upon glazing; increased thickness; inferior photochromic properties; index variability w/ temperature  
How are polycarbonate lenses manufactured?   Begins as a solid; melted down and injected into molds at 320 deg Celcius; a device squeezes each lens for 30-90 seconds in the lens forming cycle; lenses are removed from molds; processed through a coating machine; processed through a heat-curing process  
Why does each polycarbonate lens receive a hard coating?   Because the sufrace hardness is much softer than CR-39; increases scratch resistance and chemical protection  
Which is superior in impact resistance....glass, CR-39, or polycarbonate.   Polycarbonate  
What is the refractive index of polycarbonate?   1.586  
What is the ABBE value of polycarbonate?   30; very poor optical quality  
What type of molds are used for polycarbonate?   highly polished metal molds  
What is the specific gravity of polycarbonate?   1.20  
All polycarbonate lenses absorb UV radiation up to _____ nm without any andditional treatment.   380nm  
Do polycarbonate lenses accept tint?   No  
Because polycarbonate has a higher index of refraction than that of glass or CR-39, they have more....?   surface reflections; AR coating highly recommended  
What are disadvantages of polycarbonate?   Low ABBE number (undesirable chromatic abberration; surfaces are difficult to mold w/o waves or blemishes; material is difficult to work with  
Why are high index plastic lenses good?   a thinner, more cosmetically acceptable lens  
How does the ABBE value of high index lenses compare to other materials?   Lower than crown glass or CR-39, but higher than polycarbonate  
What is the highest refractive index available today in a plastic material?   1.74  
Higher index lenses have more or less surface glare?   more  
What is the refractive index of Trivex?   1.53 (slightly thicker lenses than PC)  
What is the specific gravity of Trivex?   1.11 (lightest of any material)  
What is the ABBE value of Trivex?   45 (higher than polycarbonate)  
Trivex has the ______ _________ and inherent __ protection of polycarbonate.   impact resistance and UV protection  
What are the best lenses to use in drill mount frames and why?   Trivex, due to it's tensile strength  
Which lenses are inherently impact resistant?   Trivex, CR-39, and Polycarbonate  
What can be done to glass lenses to increase their impact resistance?   Tempering  
What is thermal tempering?   Toughening glass by heating and cooling  
What is chemical tempering?   A 16 hour ion exchange process where lens is placed in molten salt bath  
What is more impact resistant, CR-39 or tempered glass?   CR-39  
Which is more impact resistant, chemical or air tempered glass?   There is only a slight difference  
Which is safer when they shatter plastic or glass lenses?   Plastic lenses....they break into a smaller # of pieces and they the edges are not as sharp.  
Surface scratches and pitting of all lenses will ______ the impact resistance?   reduce  
Do surface abrasions effect glass or plastic lenses more?   glass  
Why is polycarbonate so impact resistant?   has extremely long chains of atoms that can slide back and forth on each other and the energy of an impact will flex or deform the lens instead of break it.  
Polycarbonate lenses have about ___ times the impact resistance of CR-39.   21  
While Trivex and Polycarbonate are comparable in impact resistance, Trivex is _______ and has ______ optical quality.   Trivex is lighter (lower specific gravity) and higher optical quality (higher ABBE value)  
Which lenses are thinner polycarbonate or Trivex?   Polycarbonate  
Who started the duty to warn campaign?   Optical Laboratories Association (OLA)  
What is duty to warn?   ECP are advised to warn every patient ordering eyewear that polycarbonate lenses exceed all other lenses in impact resistance; Trivex may be offered as an alternative  
What is ANSI Z80.1 standards for?   Dress eyewear  
What is ANSI Z87.1 standards for?   occupational or safety glasses  
What is the drop ball test for dress eyewear?   uses 5/8" steel ball, weighing 0.56 ounces, dropped from 50 ft. The ball must strike within 5/8" diameter of the GC of the lens, and lens must not fracture.  
What is a fracture?   for a lens to crack through it's entire thickness and across an entire diameter, into two or more pieces OR lens material visible to the naked eye becomes detached from the ocular surface  
What is different for a drop ball test for occupational eyewear?   1 in steel ball, weighing 2.4 ounces, dropped from 50 inches  
What is minimum thickness for safety eyewire?   3 mm; plus lenses over 3.00 D can have a minimum thickness of 2.5 mm  
How many Americans injure their eyes at work?   700,000  
How many Americans injure their eyes at home?   125,000  
How many Americans injure their eyes from sports?   40,000  


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