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ABO
STudy Materials
| Term | Definition |
|---|---|
| How many millimeters in an inch? | 24.5mm = 1 inch |
| What is the spectrum of visible light, in nm? | from ~400nm to ~700nm |
| List the colors of the spectrum, from longest wavelength to shortest. | Red, Orange, Yellow, Green, Blue, Indigo, Violet |
| What is the spectrum of UV light, in nm? | from ~200nm to ~400nm |
| How are the bands in the UV spectrum categorized? | A, B, & C (with C being blocked by the Earth's atmosphere) |
| What distance is considered at infinity? Why? | 20 feet- because after light leaves its source, it diverges and becomes parallel at 20 feet |
| What is the speed of light? | 186,000 miles/second |
| What happens when light enters a transparent medium at 90 degrees? | It slows down and continues onward. |
| What happens when light enters a transparent medium at an oblique angle? | Refraction- it changes the angle of light |
| Each time light encounters a different material, (blank) occurs. | Refraction |
| Define "Index of Refraction" | the degree that light is bent in a given material based on that material's density |
| How is Index of Refraction determined? | speed of light/speed through the material |
| What is the index of refraction of polycarbonate? | 1.589 |
| What is the index of refraction of CR39? | 1.49 |
| Is material more or less dense with a higher index of refraction? | more |
| What is the index of refraction of glass? | 1.523 |
| What is the index of refraction of flint glass? | 1.70 |
| What is the standard index of refraction in the optical industry, which all instruments are calibrated to? | 1.53 |
| Define Dispersion | when light is broken down into basic colors causing a rainbow effect |
| What is abbe value (or Nu value)? | measure of how easily dispersion happens |
| Abbe values are between what numbers? | 29 and 60 |
| Do higher or lower abbe values result in the least dispersion and best vision? | higher |
| What is the abbe value for plastic? | 59-60 |
| What is the abbe value for glass? | 60 |
| What is the abbe value for high index resin? | 43 |
| What is the abbe value for polycarbonate? | 29-31 |
| What is the abbe value for flint glass? | 38-40 |
| What 3 things happen as light hits a medium? | Refraction Absorption Reflection |
| As light enters and exits any medium, what percent of light is lost (assuming medium is non-coated)? | ~8% loss of light (~4% on each surface) |
| What is the normal? | a line drawn at right angles to the material's surface at the ray's point of entry |
| What is the incident ray? | the ray coming into the material (before it enters) |
| When does the incident ray become the refracted ray? | As it enters and leaves the material |
| More curve induces (more or less?) refraction | more |
| What is a diopter? | the measure of the power of a lens to refract light |
| A lens that can focus parallel light to a point at one meter has what power? | 1 diopter |
| What is the formula for determining diopter power? D=? | D = 1/f where f is the focal length of the lens in meters |
| How do you determine focal point, given the power of a lens in diopters? | f = 1/D |
| How do you convert magnification to diopter power? | multiply the magnification by 4 (2x is 8 diopters) |
| What are the 3 types of plus lens configurations? | Bi-convex () plano convex [) meniscus )) |
| What type of lens magnifies an image? | plus lens |
| What type of lens is thicker in the center and thinner at the edges? | plus lens |
| What type of lens demonstrates "against" motion (if the lens is moved side to side, the image seems to go the opposite direction)? | plus lens |
| What are the three types of refractive errors that plus lenses correct for? | hyperopia presbyopia aphakia |
| What are the 3 types of minus lens configurations? | Bi-concave )( plano concave )] meniscus )) |
| What type of lens minifies an image? | minus lens |
| What type of lens is thicker at the edges and thinner in the middle? | minus lens |
| What type of lens demonstrates "with" motion (if the lens is moved side to side, the image seems to go the same direction)? | minus lens |
| What type of refractive error do minus lenses correct for? | myopia |
| How do you determine power in diopters using a lens clock? | take the front side measurement (usually +) and combine algebraically with the backside measurement (usually -). F + B = P where F= front curve, B= back curve, and P = power |
| If the front curve of a lens is clocked at +6 and the back curve is clocked at -5, what is the power of the lens? | +1 |
| How do you transpose an rx? | algebraically add the sphere power and the cylinder power to get the new sphere power, use the same cylinder power but switch the sign, and add 90 degrees to the axis |
| When prism changes the direction of light, this is known as _________ or _______. | diffraction or deviation |
| If a prism is strong enough, light can be separated into different colors. This is called ________. | dispersion |
| Which direction does light entering a prism bend? | toward the normal |
| The image appears toward the _________ of a prism when looking in while it is actually toward the __________. | apex base |
| What measurement is used to indicate the amount of deviation of a prism? | diopter |
| The ________ a prism, the more light it will bend. | larger |
| T or F: A prism focuses light. | False. A prism only deviates/shifts light, not focuses it |
| T or F: A prism does not magnify or minify an image. | True. it only shifts/deviates |
| Does a steeper angle on a prism result in more or less deviation? | more (greater angle of deviation) |
| If light passes through the center of a lens, it will neither converge nor diverge. This represents the _________ or the ___________. | optical center axial ray |
| What type of lens converges light? | plus lens |
| What type of lens diverges light? | minus lens |
| In a minus lens, if the pupil is below the optical center of the lens, what type of prism is induced? | Base down prism |
| In a minus lens, if the pupil is above the optical center of the lens, what type of prism is induced? | Base up prism |
| In a minus lens, if the optical center of the lens is moved toward the nasal, this induces what type of prism? | Base out prism |
| In a minus lens, if the optical center of the lens is moved toward the temple, this induces what type of prism? | Base in prism |
| In a plus lens, if the pupil is below the optical center of the lens, what type of prism is induced? | Base up prism |
| In a plus lens, if the pupil is above the optical center of the lens, what type of prism is induced? | Base down prism |
| In a plus lens, if the optical center of the lens is moved toward the nasal, this induces what type of prism? | Base in prism |
| In a plus lens, if the optical center of the lens is moved toward the temple, this induces what type of prism? | Base out prism |
| What formula determines the prism in a lens at a given point, or how much to decenter a lens to induce prism? | Prentice's Formula |
| Prentice's Formula | P = (h x D)/10 P = power in prism diopters h = OC distance in mm D = dioptric power of lens |
| How much prism is induced when a 3D lens is decentered by 5 mm? | 5x3/10 = 1.5 diopters of prism |
| How much prism is induced in a -4.25 lens decentered by 4 mm? | 4 x -4.25/10 = 1.7 diopters of prism |
| How much prism is a person inducing if he looks 6 mm below the OC in a lens -3.00 -3.25 x 180? | 3.75 diopters of prism *use power at 90 degrees since shifting down, which is -6.25 |
| How far is a lens moved to have 5D of prism in a 2D power lens? | (5*10)/2=h 25 mm = h |
| Given a +2.00 -5.00 x 180, how far must the lens be decentered to get 1 BU prism and in what direction? | (1*10)/-3.00 = 3.3 mm *power at 90 degrees is -3.00. for minus lens, BU prism is induced by moving OC down |
| A pair of glasses has been found to be 4 mm off in the PD. OD: -2.00 +2.00 x 90 OS: -0.75 sph What is the prism induced? | OD: P= (2x0)/10 = 0 diopters OS: P= (2 x -0.75)/10 = .15 diopters prism *assume off by 2 mm in each eye *OD at 180 degrees is plano |
| How many diopters of difference between eyes will induce double vision when looking away from the optical center? | 2+ diopters of difference |
| What is another name for slab off prism? | bicentric grinding |
| When 2 separate optical centers are ground into one lens, what is it called? | slab off prism or bicentric grinding |
| How do you determine which eye slab off prism should be done in? | the most minus (or least plus) lens |
| If a patient has an RX of OD +2.00 sph and OS +0.25 -4.00 x 135, which eye should have slab off prism? | the left eye (it is the most minus) |
| OD: +2.00 OS: +5.00 What amount of prism will a patient experience with a reading gaze 10 mm below the OC with this RX? | OD: 2x10/10= 2D BU prism OS: 5x10/10= 5D BU prism |
| What is diplopia? | double vision |
| Why don't you need to use slab off prism with single vision lenses? | patient can move head to look through OC |
| -2.00 -2.00 x 135 +2.50 +2.00 -0.50 x 45 +2.50 FT28 in a 56/17 PD=68. Seg ht is 15. OC dif betw. distance & near is 12 mm. total prism needed for a slab off? Which side will the slab off be on? Reverse slab? | *power at 90 degrees for each lens is OD: -3.00, OS: +1.75 *difference in power is 4.75D 1. using Prentice's rule (h=12, D=4.75) 12x4.75/10=5.7 diopters prism 2. OD 3. OS |
| What is the RX for the split prism version of this RX? OD: -0.50 3 BD OS: -1.00 | OD: -0.50 1.5 BD OS: -1.00 1.5 BU |
| What is the RX for the split prism version of this RX? OD: -1.00 -1.00 x 90 5 BU OS: -1.25 -2.50 x 90 3 BO | OD: -1.00 -1.00 x 90 2.5 BU 1.5 BO OS: -1.25 -2.50 x 90 2.5 BD 1.5 BO |
| When splitting prism, base out and base in are split in what direction? | the same direction (both eyes BO or both eyes BI) |
| When splitting prism, base up and base down are split in what direction? | opposite directions (one eye of each) |
| How many tunics (layers) make up the outer part of the eye? What are they (listed from outermost to innermost)? | 3- fibrous tunic, vascular tunic, and nervous tunic |
| What is the name of the outer tunic of the eye that is mostly composed of dense connective tissue? | The fibrous tunic |
| The fibrous tunic is divided into the __________ and the ____________. | sclera, cornea |
| How much refractive power (in diopters) does the cornea have? | 43 diopters |
| Which outer tunic contains the blood vessels that supply the retina with nutrients? | choroid tunic |
| What is the name for the white part of the eye? | sclera |
| What part of the eye is translucent and serves as a passageway for light to enter the eye? | cornea |
| Which tunic is also known as the choroid tunic? | vascular tunic |
| The ___________ helps to refract light and to focus on the retina. | cornea |
| What layer contains a high concentration of dark pigments that help absorb excess light? | choroid |
| The _________ is primarily composed of nerve tissue and forms a thin internal membrane of the eye. | retina |
| The vascular tunic consists of...? | iris, choroid, & ciliary body |
| The inner layer of photoreceptors and neurons (the nervous tunic) consists of...? | the retina |
| How many segments/chambers are contained within the inner part of the eye? What are they? | 3- anterior chamber, posterior chamber, & vitreous chamber |
| The fluid filled area between the cornea and the iris is the ________. | anterior chamber |
| The fluid filled area between the iris and the lens is know as the __________. | posterior chamber |
| The anterior and posterior chambers contain a fluid known as _____________. | aqueous humour |
| Define aqueous humour. | watery fluid produced in ciliary body which fills the anterior and posterior chambers that maintains pressure in the eye and provides nutrients to the lens and cornea |
| Aqueous humor is continually drained from the eye through the ____________. | Canal of Schlemm |
| The greatest volume of the inner chambers, forming about four-fifths of the eye, is the ________________. | vitreous chamber |
| The vitreous chamber is found between the _________ and the __________. | retina, lens |
| What is vitreous humour? | a thicker gel-like substance contained within the vitreous chamber which maintains the shape of the eye |
| What does vitreous humour do? | maintains the shape of the eye |
| What does aqueous humour do? | maintains IOP, provides nutrients to the lens and cornea |
| The cornea consists of how many distinct layers? | 5 epithelium Bowman's Membrane Stroma Descemet's Membrane Endothelium |
| The outermost layer of the cornea is called the ___________. | epithelium |
| The outermost layer of the cornea, the epithelium, rests on ______________. | Bowman's Membrane |
| The _________ and the ____________ serve to protect the cornea from injury. | epithelium, Bowman's Membrane |
| The ____________ protects the cornea through its ability to quickly regenerate. | epithelium |
| ___________ protects the cornea by providing a tough, difficult to penetrate barrier | Bowman's Membrane |
| The innermost layer of the cornea which rests on Descemet's Membrane is called the ___________________. | endothelium |
| What is the function of the endothelium? | It removes water from cornea, helping to keep the cornea clear. |
| What layer of the cornea accounts for 90% of its thickness? | the stroma |
| The __________ controls how much light enters the pupil. | iris |
| How many muscles are found in the iris? What are their names? | 2- dilator muscle and sphincter muscle |
| The ________ muscle opens the pupil, allowing more light into the eye. | dilator |
| The ___________ muscle closes the pupil, restricting light into the eye | sphincter |
| The ___________ is just behind the pupil. Its purpose is to focus light on the retina. | crystalline lens |
| Define accommodation. | the process of focusing on objects based on their distance |
| The __________ surround the lens, helping it achieve accomodation. | Ciliary Body |
| What are zonules? | fibrous strands that attach the ciliary body to the lens |
| Describe the process of how the lens allows the eye to focus up close. | the ciliary body contracts and the zonules relax, allowing the lens to thicken and add power |
| Describe the process of how the lens allows the eye to focus at distance. | Describe the process of how the lens allows the eye to focus at distance. |
| The retina consists of photoreceptor cells called _______ and ________. | rods cones |
| Which photoreceptor cells are the more numerous ones (apprx. 120 million)? | rods |
| Which photoreceptor cells are contained mostly at the periphery? | rods |
| Which photoreceptor cells are suited to night vision and peripheral vision? | rods |
| Which photoreceptor cells have the primary function of detail and color detection? | cones |
| Which photoreceptor cells are the less numerous ones (apprx. 6 million)? | cones |
| Which photoreceptor cells are largely concentrated in the fovea? | cones |
| How many types of cones are there? Describe them. | 3- Each type receives only a narrow band of light corresponding largely to a single color. |
| What colors do the three types of cones correspond to? | red, green, or blue |
| The signals received by the cones are sent via the ________ to the brain where they are interpreted as color | optic nerve |
| _________ refers to an eye free of refractive errors. | emmetropia |
| Define myopia. When does it occur? | near-sightedness It occurs if the eye is longer than normal or the curve of the cornea is too steep, causing light rays focus in front of the retina. |
| What type of refractive error causes light rays to focus in front of the retina? | myopia |
| Define hyperopia. When does it occur? | far-sightedness It occurs if the eye is too short or the curve of the cornea is too flat, causing light rays to focus behind the retina. |
| What type of refractive error causes light rays to focus behind the retina? | hyperopia |
| When does astigmatism occur? | It occurs when the cornea has an oblong, football-like shape in one or more directions (or axes) causing light rays to focus on more than one point on the retina. |
| What type of refractive error causes light rays to focus on more than one point on the retina? | astigmatism |
| When does presbyopia occur? | When, because of aging, the crystalline lens begins to lose elasticity and is no longer able to accomodate to focus at near points. |
| Around what age does presbyopia occur? | begins at 40 |
| Around what age is the loss of elasticity of the crystalline lens complete? | 65 |
| What is the term for the condition in which two eyes have unequal refractive power. | Anisometropia |
| _________is an extreme case of anisometropia where one eye is myopic and the other is hyperopic. | Antimetropia |
| The unequal refractive states in antimetropia can lead to __________ and ___________. | diplopia (double vision) asthenopia (eye strain) |
| If there is a large difference between the two eyes, the brain will often suppress the vision of the blurrier eye. What is this condition called? | amblyopia |
| What is the name of the group of muscles responsible for the stabilization of eye movement? | extraocular muscles |
| How many extraocular muscles are there? Name them. | 6: Lateral rectus Medial rectus Superior rectus Inferior rectus Superior oblique Inferior oblique |
| What is the name of the muscle that moves the eye outward, away from the nose? | Lateral rectus |
| What is the name of the muscle that moves the eye inward, toward the nose? | Medial rectus |
| What is the name of the muscle that moves the eye upward and slightly outward? | Superior rectus |
| What is the name of the muscle that moves the eye downward and slightly inward? | Inferior rectus |
| What is the name of the muscle that moves the eye outward and downward? | Superior oblique |
| What is the name of the muscle that moves the eye outward and upward? | Inferior oblique |
| What is diplopia? | double vision |
| Difficulty with fusion can cause double vision (diplopia), or it can cause the brain to "turn off" one image in an effort to eliminate diplopia. This condition is called ____________. | suppression |
| What is suppression? | When one of more of the extraocular muscles are either too strong or too weak, causing difficulty with image fusion. Suppression results when the brain "turns off" one image in order to eliminate double vision. |
| Define phoria. | when the eye has a tendency to turn from its normal position (such as when the patient is tired |
| Define tropia. | when the eye has a definite or obvious turning from its normal position |
| What is it called when the eye has a tendency to turn downward? | hypophoria |
| What is it called when the eye has a tendency to turn inward? | esophoria |
| What is it called when the eye has an obvious upward deviation? | hypertropia |
| What is it called when the eye has an obvious outward deviation? | exotropia |
| What is the name of electromagnetic radiation that lies just above the visible spectrum (750nm-1mm)? | infrared |
| What is the name of electromagnetic radiation that lies just below the visible spectrum (1nm-400nm)? | ultraviolet |
| List the types of electromagnetic radiation in order of increasing wavelength. | gamma rays x rays UV Visible spectrum (VIB G YOR) Infrared Microwave Radio waves (FM-AM) Long radio waves |
| What notation is used to indicate index of refraction? | n |
| What is the index of refraction (n) of air? | 1 |
| What is the index of refraction (n) of vitreous humour? | 1.33 |
| What is the index of refraction (n) of aqueous humour? | 1.33 |
| What is the index of refraction (n) of the cornea? | 1.37 |
| What is the index of refraction (n) of the crystaline lens? | 1.42 |
| What is the index of refraction (n) of CR-39? | 1.49 |
| What is the index of refraction (n) of crown glass? | 1.52 |
| What is the index of refraction (n) of polycarbonate? | 1.58 |
| Light rays pass through a lens and converge 0.50 m from the lens. What is the power of the lens? | P=1/f = 1/.5 = 2D |
| What is the ideal base curve for a mild rx (-1.75 to +2.00)? | 6 |
| For plus powers, base curves should ideally be __________ than 6. | more |
| For minus powers, base curves should ideally be __________ than 6. | less |
| What would be the ideal base curve for moderate plus powers (+2.25 to +5.25)? | 8 |
| What would be the ideal base curve for high plus powers (+5.50 to +8.75)? | 10 |
| What would be the ideal base curve for very high plus powers (+9.00 to +10.50)? | 12 |
| What would be the ideal base curve for super high plus powers (+10.75 to +12.25)? | 14 |
| What would be the ideal base curve for extremely high plus powers (+12.25 and up)? | 16 |
| What would be the ideal base curve for moderate minus powers (-2.00 to -4.50)? | 4 |
| What would be the ideal base curve for high minus powers (-4.75 to -8.00)? | 2 |
| What would be the ideal base curve for very high minus powers (-8.00 to -9.00)? | 0.5 |
| What would be the ideal base curve for super high minus powers (-9.00 and up)? | plano or minus |
| A lens clock is calibrated to read materials with a refractive index of __________. | 1.53 |
| Hi-index materials will have true power _________ than the measurement given by a lens clock. Why is this? | greater than The lens clock is calibrated to read materials with a refractive index of 1.53 |
| Using a lens clock, if we find the curve on the front surface of the lens to be +6.00 D and determine the back surface to be toric with a measurement of -8.00 D in the 90º meridian and -5.00 in the 180º meridian, what is the true power of the lens? | +1.00 -2.00 x180 |
| How many types of refractive errors are there? What are they? | 7 Hyperopia Myopia Simple hyperopic astigmatism Simple myopic astigmatism Compound hyperopic astigmatism compound myopic astigmatism mixed astigmatism |
| _____________ has a plano meridian with a plus meridian. | simple hyperopic astigmatism |
| _____________ has a plano meridian with a minus meridian. | simple myopic astigmatism |
| _____________ has two plus meridians. | compound hyperopic astigmatism |
| _____________ has two minus meridians. | compound myopic astigmatism |
| _____________ has one plus and one minus meridian. | mixed astigmatism |
| What type of refractive error is the following rx? -1.00 -1.50 x079 | compound myopic astigmatism |
| What type of refractive error is the following rx? +1.00 -1.00 x126 | simple hyperopic astigmatism |
| What type of refractive error is the following rx? pl -2.50 x008 | simple myopic astigmatism |
| What type of refractive error is the following rx? +5.25 -2.50 x169 | compound hyperopic astigmatism |
| What type of refractive error is the following rx? +0.75 -2.50 x098 | mixed astigmatism |
| What type of refractive error is the following rx? +3.00 sph | hyperopia |
| What type of refractive error is the following rx? -2.50 sph | myopia |
| What material accepts tint best? | CR-39 |
| ____________ is twice as heavy as CR-39 but very durable. | crown glass |
| The tints available with glass are...? | grays, photochromics, pinks, some browns and greens |
| Why is UV coating not applied to glass? | gives it a yellow tint |
| ____________ is the recommended material for safety glasses. | polycarbonate |
| Plus cylinder- when the cylinder is surfaced on the front of the lens rather than the back- is mainly used in what circumstance? | diving masks when power calls for a plus lens with cylinder |
| _________ are added to glass to create basic tints. | metallic oxides |
| __________ are added to glass to create photochromic effects. | silver halides |
| _________ is the original material used to make AR coatings. | magnesium flouride |
| AR coatings work through ______________. | destructive interference |
| A plano base curve should be used on what powers? | -6.00 and up |
| A 2.00 base curve should be used on what powers? | -4.00 to -6.00 |
| A 4.00 base curve should be used on what powers? | -2.00 to -4.00 |
| A 6.00 base curve should be used on what powers? | -2.00 to +2.00 |
| A 8.00 base curve should be used on what powers? | +2.00 to +4.00 |
| A 10.00 base curve should be used on what powers? | +5.00 to +8.00 |
| A 12.00 base curve should be used on what powers? | +8.00 and up |
| Specific gravity describes what? | the density of a material by comparing its density to the density of water |
| The higher the specific gravity of a lens material, the _________ the density, and consequently, the ________ a lens of that material will be for a given power and size. | higher heavier |
| What is the standard fitting distance for a reading power? | 16 inches or 40 cm |
| The standard fitting distance for the intermediate distance in a trifocal is generally _________. | 30 inches |
| The intermediate distance in a trifocal is ________ % of the add power. | 50% |
| Where is the fitting height measured for a bifocal? | lower lid line |
| Where is the fitting height measured for a trifocal? | bottom of the pupil |
| What type of bifocal is the heaviest? | executive |
| Is image jump greater in round top or flat top bifocals? | round tops |
| What is the power through the intermediate segment of a trifocal lens with the following rx? +0.50 +1.00 x043 Add: 2.50 | +1.75 +1.00 x043 |
| Image jump will be more dramatic in a bifocal when ____________. | The OC of the distance area and the OC of the reading portion are far apart |
| Progressives are fit at what point? | pupil center |
| If a patient has cataract surgery and the crystalline lens is removed and not replaced with an implant, the patient will need what type of lens to compensate? | full field and aspheric lenticular |
| What power range is usually used for lenses for a cataract patient after surgical removal of the crystalline lens? | +10.00 to +16.00 or more |
| As a lens moves away from the eye, it becomes more _________. | plus |
| As a lens moves closer to the eye, it becomes more ________. | minus |
| Modification to a power should be made to accommodate for change in effective power at what point? | 7.00 diopters or more |
| If a lens sits closer to a patient's eye than the distance at which he was refracted, the power should be altered in what way? | more plus |
| What is the standard instrument vertex? | 14mm |
| For a +10 power lens, every millimeter closer vertex distance should result in approximately ______ diopters of ________ power. | .1 additional |
| When light enters a prism, shorter wavelengths are bent to a __________ degree than longer wavelengths. | greater |
| Define chromatic aberration. | the tendency of a material to separate/disperse light that enters it into its various colors |
| A material's level of chromatic aberration can be gauged by its ___________. | abbe value |
| The Abbe value of a material is __________ proportional to the chromatic aberration induced as light passes through it. | inversely |
| Generally speaking, the _______ the index of a lens material, the _______the chromatic aberration, and the _________ the Abbe value. | higher higher lower |
| The __________ of the material describes the percentage of incident light reflected from a highly polished surface of that material and is calculated from the refractive index of a material | reflectance |
| What is the formula used to determine reflectance: When light hits a lens surface in air normally, the percentage of light reflected at each surface is calculated using this formula. | R = (n - 1)^2/(n + 1)^2 * 100% where R = reflectance and n = refractive index of material |
| Calculate the reflectance of polycarbonate | R = (1.58-1)^2/(1.58+1)^2 R = .3364/6.6564 R = .05054 = 5.054% per surface |
| The ____________ of a lens material describes the amount of light (usually specified for a given waveband) that will pass through that material. | transmittance |
| ________is most stable, scratch-resistant, and provides the best optical quality of all lens materials. | glass |
| What type of aberration is more pronounced in high-index glass than regular glass? | chromatic aberration |
| Name 3 positive aspects of glass lenses. | best optics stable material scratch resistant |
| Name 3 negative aspects of glass lenses. | difficult to tint lacks impact resistance (too brittle) heaviness |
| Glass is ____x heavier than plastic. | 2 |
| Name 3 positive aspects of CR-39 lenses. | easily tintable lighter than glass less likely to fog |
| Name 2 negative aspects of CR-39 lenses. | more susceptible to scratching lower index of refraction than glass |
| Name 3 positive aspects of polycarbonate lenses. | impact resistant thinner & lighter than plastic inherent UV protection |
| Name 4 negative aspects of polycarbonate lenses. | not easily tinted soft (scratches easily) poor optics susceptible to stress fractures in drill mounts |
| What are the benefits of aspheric lenses? | reduced spatial distortion reduced magnification or minification maintains a thin and flat lens profile |
| Patients who are sensitive to chromatic aberration may have issues with which types of lens materials? | polycarbonate high-index possibly trivex |
| What is the abbe value of trivex? | 43-45 |
| What is the index of refraction of trivex? | 1.53 |
| What materials have natural UV protection? | trivex & polycarbonate |
| What is the lightest lens material available? | Trivex (specific gravity of 1.11) |
| What are the positive aspects of trivex? | Impact resistance of polycarbonate Better optics than polycarbonate Tintable Lightest material on the market Inherent UV protection High tensile strength (ideal for drill mounts) |
| Internal reflections viewed as multiple rings inside the lens are most common with what type of lens? | high minus |
| Blurred or ghost images due to light reflection within a lens can significantly impair vision, especially during what situation? | at night around bright lights common in dusk or night time driving conditions |
| Bright sun light hitting the back surface of a sun lens that is not AR coated, depending on the angle, can either ________________ or can ________________. | be reflected directly back into the eye "fill" the lens with reflected light |
| _____________ allow more light to reach the eye increasing contrast and clarity as well as help to reduce eye strain. | AR coatings |
| Polarized lenses significantly increase driving safety, especially at what times? | morning and late afternoon |
| What is a good analogy to use to describe how polarized lenses work? | venetian blinds- they force the light to enter only at a certain angle |
| Why is polarization better than tinting? | Tinting reduces brightness. It doesn't reduce glare like polarization. Tinting can lead to increased UV damage since eyes dilate behind the dark tint. Polarization blocks harmful UV |
| These type of coatings are produced relatively cheaply. They are applied by dipping the lens into a chemical solution and curing the lens in an oven. | Dip-Coating |
| These coatings are applied to a spinning lens. The lenses must be cured after the hardcoat chemicals are applied. The curing can be done in an oven or by ultraviolet light. | Spin-Coating |
| Coatings made by adding hardcoat chemical at time that the lenses are being formed. Most manufacturers are using this type of process. Generally used by semi-finished lenses only. Non tintable | In-mould Coating |
| Coating cost much higher than other methods. Type of hard coat is produced at the same time and in conjunction w/ anti relflective coating | Vacuum Coatings |
| These are highly reflective coatings and are used to reduce the light transmission through a lens. They are produced in a vacuum process. | Mirror Coatings |
| What is the desired vertex distance when fitting a pair of glasses? | 10-14mm |
| What is the best range for degree of face form when fitting a pair of glasses? | 5-10 degrees |
| What is the desired range for pantoscopic tilt when fitting a pair of glasses? | 10-15 degrees |
| When the lenses are ground to specification, the lab will often incorporate prism. What is this process called? | equi-thinning, prism thinning, or yolk prism |
| What is the focal distance used for a lens meant for computer use? | 20-24 inches (50-60 cm) |
| What is the focal distance used for a lens meant for reading? | 14-18 inches (35-45 cm) |
| What is the minimum center thickness for a basic impact safety lens? | 3 mm |
| What is the minimum edge thickness for a basic impact safety lens? | 2.5 mm (if it is +3.00 diopters or higher) |
| A basic impact safety lens must pass what type of test? | a drop ball test of a 1 inch diameter steel ball dropped 50 inches |
| What is the minimum center thickness for a high impact safety lens? | 2.5 mm |
| A high impact safety lens must pass what type of test? | a high velocity test in which a ¼ inch steel ball is shot at a lens at 150 ft/second |
| What are the material options for a high impact safety lens? | polycarbonate, Trivex, or SR-91 |
| How can you tell if a frame is impact resistant enough to be considered a safety frame? | marked with "Z87+" on the frame front and temple. |
| What is the chassis? | The frame front |
| What are guard arms? | the wires that attach the nosepads to the frame |
| What is the endpiece? | the area of the chassis that attaches to the temple |
| What is the ideal amount of pantoscopic tilt for most frames? | 8-10 degrees |
| What is orthoscopic tilt? | when a frame has 0 degrees of pantoscopic tilt (no tilt) |
| What is retroscopic tilt? | when a frame is farther from the patient's face at the bottom of the lens than it is at the top |
| What are the 4 basic adjustments that can be made to nosepads? | width splay angle frontal angle vertical angle |
| What is splay angle? | the angle of the pads when viewed from the top of the frame- the front edges of the pads should be closer together than the back edges |
| What is frontal angle? | the angle of the pads when viewed from the front of the frame- the tops of the pads should be slightly closer together than the bottoms of the pad following the contours of the nose as it gets wider from top to bottom |
| What is vertical angle? | the angle of the pads when viewed from the side of the frame. Since most frames will have some amount of pantoscopic tilt, the bottoms of the pads should be slightly closer to the frame front than the tops |
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mgm9480
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