Exam 1 Flashcards
How many curves are generally used for base curves?
4 to 6
What is the mold consisted of in a traditional lens?
Base Curve and Add Power
What part of the lens is grinded by the lab?
Back surface
What are traditional PROGRESSIVE lenses designed for?
Lensometer
Not the patient
What happens when you decrease the corridor?
Astigmatism will increase
Which age group look for fashion more than visibility?
18 to 34 yrs
Which frame has crystals in it?
Carbon fiber
Which type of frame do you insert cold lenses into?
Carbon fiber
What is Nylon made of?
Cellulose Acetate Propionate
Which frames are Hypoallergenic?
Nylon Stainless Steel Titanium Optyl Polyamide
What frame do you use hot water for?
Nylon
Which do you not leave on your car dash? Why do you not leave it there?
Optyl
It will go to the default position
What specific safety frame do you not use acetone for?
Polycarbonate
It will turn into milky texture
What is a major metal allergy?
Nickel
Trical
If a pt. has excessive perspiration, what type of frames should you give to your pt.?
Corrosion resistant Aluminum Titanium Metal Alloys (Often) Stainless Steel
What type screw mounting do you not do on Glass lenses?
Conventional screw mounting
What happens to the light speed when you go from Polycarbonate to Trivex?
Go faster
If you have a pt. with 3 BU. How many degrees will the lens be at?
1.71
3 x .57 = 1.71
What are 2 things prism should be displayed?
Quantify ANGLE OF DEVIATION IN DEGREES and
the AMOUNT OF DISPLACEMENT
How much prism power does a prism have if it displaces a ray 5cm. from a distance of .5m from the prism?
10 PD
5cm/.5m
A 20Δ BU will deviate a ray how far at 5m and in what direction?
100 cm
20 x 5
A prism deviates light 4 degrees. What is the power of the prism?
4 x 1.75 = 7.00
Your patient has the following Rx: OD Plano - 7.00 x 090 OS -3.00 DS. 3ΔBO On which lens should the prism be Rxed?
OS
OD = -7.00 +7.00 x 180 OS = -3.00 DS (not as thick)
Your patient has suffered a stroke and has a OD blind spot superior temporal. You want to prescribe prism to bring the non-seeing area into his seeing area by 30 degrees.
52.5 BU
30 x 1.75 = 52.5
What part of a minus lens is fixed?
Center
This is why the edges are variable
What happens when you decrease the refractive index?
Increased edge (minus) and center (plus)
Calculate the Lens Thickness
Given: F1 = +2.00, F2 = -8.00 and n = 1.5
lens dia. = 80 mm, and tC = 2 mm.
T = 11.6 mm
S = (Fh^2) ÷ 2000(n - 1)
s1 = -6(40^2) ÷ 2000(1.5 - 1) = 9.6mm + 2mm =
11.6 mm
Note: Every minus lens has a minimum thickness where you must add 2mm
When looking for Thickness of lens, what would occur if you increased the diameter?
The lens would get THICKER
What is the formula for Total Light Transmission?
TIT = L - (reflection + absorption)
What is the formula for Reflection?
R = (n’ - n)^2 ÷
(n’ + n)^2
How much light is transmitted by CR-39, assuming no absorption?
92.2%
R = (1.498-1)^2 ÷
(1.498 + 1)^2 = 4.0% (F1)
Then 96 would be the I, and since 4% was the reflected, it would be 96% x 4% = 3.8% reflected. 96% - 3.8% = 92.2%
In regards to absorption, if you add more layers, what would that do to the absorption?
Absorption will increase and light being refracted will decrease
What does Lambert’s law state?
Given an absorptive material such as a tinted lens, layers of equal thickness absorb equal % of light regardless of the intensity…
What is the formula for light transmitted after absorption?
Ix = Io x q^x
Find T of crown glass, n =1.523,assuming the q value = 0.8 / 2 mm and tc = 6mm
46.9%
F surface = 4.3%
95.7 (0.8^3) = 0.49
B surface =0.49 x 0.043 = 0.021
T= Ix - Rb
0.49 -0.021 = 0.469 or 46.9%
What is the short cut formula for Total light transmitted?
T = Io x (q^x)
What is the order of transmission for UV protection?
CR39 (dyed) > Polycarb > CR39 (not tinted) > Crown Glass
What part of the lens will have the least amount of tint?
Thickest part of the lens
Oxidates will go to the thickest part of the lens
How do you calculate what AR you need to use?
Square root the n of the lens you have.
True or False. Single layer AR will allow more wavelengths to pass through?
False. Only one wavelength will pass through
What makes the AR coating tough now a days?
Silica
What is unique about teflon coating?
The increase the degree of water angle, thus beading off.
What is a major drawback of modern AR coatings?
No UV protection
Use AR with transition (photochromics)
Your pt. wants to know how much thinner will be the lenses, what do you do?
Current lens / New lens
Ex. 498/586 = 0.85 thick
15% thinner
What is the benefit of Aspheric lens in plus lens?
Flatten at the EDGE
Note: Only on side
What is the benefit of minus aspheric lenses?
Steepen at the edge of the lens
Note: Only on one side
Which pts. have better optics plus or minus?
Plus
What is the main feature of an Aspheric Lens?
Minimize off-center errors
What are 3 things that ashperic lenses reduc
SAD
Spherical Abberation, Astigmatism and Distortion
What are three main features of having flatter back curves?
Reduces corneal vertex distance, Minimize mag, and pts. eye appear normal in size
When you need a base curve for a minus lens, what do you do?
Spherical equivilent and add 8
Note: For Plus lens you add 4 to the Rx.
Where are Aspherics traditionally placed and where are Atorics placed now?
Aspherics = Front
Atorics = Back
If your pt. shows high amount of cylinder, which type of lens would you fit them in?
Atoric
Note: They are limited both material and index
How are Aspheric lenses denoted?
With Laser engraving. Marks are usually 17 mm from the center
What is a good method for reducing edge thickness?
Roll and Polish
What is one main disadvantage of an Aspheric lens?
Cannot compensate for all cylinder power
What are 2 terms you don’t use when giving pts. recommendations of glasses?
Unbreakable and Shatter Proof
Note: There is no law where we must put pt.s in POLY or TRIVEX
True or False. Edging, Coating and tinting a lens becomes a manufacturer is liable for the impact resistance of the lens?
True. IMPACT RESISTANCE ONLY
What requirements must a dress lens pass?
Performance test
Which lens fogs easily?
Crown Glass
What type of tempering must be done for safety lens?
Heat tempering
What is the best plastic with high optics?
Trivex
The FDA will not approve of a lens that has a _______ in its lens?
Fracture
What type of lenses must be tempered in either heat or a chemical bath?
Glass lenses
When you put on a AR coat, how much protection does the coat provide for the lens?
14 times more
What type of lens requires just the basic impact test?
Plastic
-3.0mm at the lens thinnest point
How many lens are tested for each type of lens?
3 lenses are tested
If you see a “+” on the lens, what does this mean?
High impact velocity test was conducted
What will be the marker for a safety lens
z87-2
When can you have diplopia with an aphakic eye?
The image will be 25% with the Normal eye
What does Mp stand for?
Power Magnification
MP = 1 ÷ [1 - h(FV)]
What does Ms stand for?
Shape factor
MS = 1 ÷ [1 - F1(tc ÷ n)]
How do you calculate Mspec?
Mp x Ms = Mspec
How do you calculate %Mspec?
(Mpsec - 1)x 100 = %
What are the possible ways to alter Mspec for a plus lens?
Increase the index
Decrease Vertex, BC or Tc
What are the possible ways you can decrease minification?
Increase BC or Tc
Decrease Index or vertex
When you have a high plus or high minus, which way do you move the bevel?
Towards the front of the lens
If you have a pt. with anisometropia, where do you move the bevel to?
The front
When there axial ametropia, what does this mean?
The retinal image cannot fully fuse at the fovea due to length of the eye
What would happen to the lens if there was no panto?
Sphere power would increase
How do you find the tilt angle?
Measure in mm between 1 and 2, then double it to find the tilt angle
How do you find the wrap angle?
measure in mm between 1 and 3 and double it to find the wrap angle in degrees
What formula do yu use for calculating the new sphere?
S= S {1+(sino^2)/2n}
How do you calculate the induced cylinder?
C = S’(tan a^2)
When do you cancel vertical prisms?
When they are the same vertical base
When do you cancel horizontal prism?
BI and BO
What are the 2 factors in regards to prismatic effect when OC is decentered?
Cylinder power and Axis Orientation
What is the formula for when you would like to Focal Power of a meridian in question?
F = Fs + Fc (sin0^2)
You are checking your own glasses in the lensometer and find your right lens has unwanted prism. The prism reads 2.00 base 30. What vertical and horizontal values does this prism have?
OD 2 Base 30
V = P sin 0
H = P cos 0
V = 2 sin 30
=1 PD
H = 2 cos 30
= 1.73 PD
What is the formula when you have base thickness of a prism?
P = 100g ( n - 1 ) / d
What is the best setup for a pt. who requires little movement from distance to near?
Raising segment height
When should you NOT preform slab off?
If the combined Prismatic effect is less than 1.5 PD
Where do you normally grind on a slab off on a pair of lenses?
F1
Which lens do you use a FT for?
Most minus
OD -7.00 DS OS -4.50 DS Add +2.00
seg lines 5 mm below distance OCs (seg drop)
reading level 10 mm below distance OCs
Correct the vertical imbalance using dissimilar segment styles for each lens
Give the pt. OD FT and OS RS Ultex E
OD = 7 x 1 = 7BD
OS 4.5 x 1 = 4.5 BD
Vert. Imbalance = 2.5 BD OD
Dist. needed btwn seg OCs: 2.5 = 2.00 x 1.25 cm
Seg OC must be 12.5 mm + 5 below seg top
What BF segment OC = 17.5 mm below the top of the segment line?
Distance Rx -2.00D OU with +2.00 add Frame size 50 mm with 18 mm DBL Patient’s PD 64/60 Segment inset = ? Total inset = ?
1) Calculate decentration (inset) Frame PD - Pt. PD = 68-64 or 4 mm. Move OC’s 2 mm in, each lens
2) Seg Inset Dist. PD - Near PD = 64-60/2 or… 2 mm in, each lens from dist. OC’s
3) Total Inset Frame PD - Near PD 68-60/2
4 mm in, each lens from GC
What is the formula for Linear FOV?
FOV = [(d + 2.7) x (2h)] ÷ 2.7
Calculate image jump
If FA = +2.50 D with a RD 22 segment…
1.1 x 2.50 = 2.75 BD
Calculate the Displacement.
If the segment were a +2.00 Add Round 22 bifocal, calculate ΔDD
ΔDD = 11-5 = 6 = .6 x 2.00 =
1.2 BD
What does Total Image displacement tell you?
The sum off both Prism in add and distance
ΔTD) =(dC x FC) + (dS x FS
In regards to lens power effectivitty, if the vertex increase what happens to the plus lens?
More effective plus lens
What is the formula for lens power effectivity?
Fn = Fo ÷ (1-dFo)
Fo = BVP needed at the origina vertex distance
