Exam 2 Flashcards
What is a prism?
A prism is formed by two flat surfaces inclined at an angle to one another and the image is distorted towards the apex. rays going through a prism bend towards the base but the image forms at the apex
What is the focal power of a prism?
prisms have no focal power. they deviate light but does not change its vergence
what is the angle of deviation formula?
d=a(n-1) deviating angle= apical angle(index of refraction-1)
the prism diopter formula
P= displacement in cm/ distance in meters
how does the prism diopter relate to the angle of deviation?
P=100tan d
OR
P= 100 tan [ a(n-1) ]
how does a plus ophthalmic lens behave?
like stacked prisms with their bases toward the center of the lens. so BD when you look above the midline and BU when you look below the midline.
how do minus ophthalmic lenses behave?
stacked prisms with their apex toward the center. BU when you look above the midline and BD when you look below
What prismatic effect is at the center of prism ophthalmic lenses?
there is no prismatic effect at the center
What is the prentice rule?
P=cD
P= Prism diopters
c= distance from OC in cm
D= the dioptric power of the lens in the meridian
the amount of prismatic effect an observer experiences through a particular point on a lens depends on how far the point is from the OC of the lens.
What are the possible outcomes of a prismatic effect?
When a patient is looking through any point other than the optical centers of the lens, the induced prism may have two possible effects:a. both eyes rotating the same amount and the same direction, may cause distortion.b. eyes rotates different amounts in different directions, will cause the pt discomfort (aka. Asthenopia)
What happens to the visual system when the eyes move in opposite or same direction in different amounts?
the visual system has to perform fusional vergence so they dont see double and this causes eyestrain or asthenopia.
what is a cancelling effect of prisms?
both eyes rotate the same amount in the same direction
ex: both lenses cause 3 BU so both eyes rotate down. There can be partial cancelling too. There is no fusional vergence in cancelling. Subtract the smaller prism from the larger prism, and assign the prism base direction to the
lens originally having the larger prism amount
what is the compounding effect?
the eyes rotate different amounts in different directions (leading to asthenopia). Add the two prism amounts together
What prism combinations do cancelling occur with?
BU and BU
BD and BU
BI and BO
What prism combinations do compounding occur with?
BU & BD
BI & BI
BO & BO
What are the ANSI standards?
American national standards institution sets the standards and tolerances for error in spectacles.
Vertical = 1/3 or 0.33
horizontal = 2/3 or 0.66
What is the visual perception of a unwanted BD prism?
Floor seems concave, people and objects seem taller/ skinnier and floor seems to slant uphill
What is the visual perception of a unwanted BU prism?
Floorseemsconvex
Peopleandobjectsseemshorter/fatter Floor seems to slant downhill
What is the visual perception of a unwanted BI or BO prism?
Horizontal expanses are sloped walls/ vertical expanses curve in or out
Where on the lens is Prisms prescribed?
on the PRP or MRP
For a single unmounted lens where should the prescribed prism be?
with in 1mm of the intended position or the prismatic effect must not be more than 1/3
For a mounted pair of lenses the distance between the PRPs must be what?
2.5 mm of the requested PDs. or the horizontal prisms should not total more than 2/3 prism diopters
How is the resultant prism calculated?
solve for hypotenuse of the triangleby using pythagorean theorem. P= sq(H^2 + V^2) H = BI or BO
V= BU or BD
Tan a= Opposite / adjacent
Where is angle a formed?
with the longest side of the triangle and the hypotenuse
How do we resolve a prism?
resolving prism is to put everything back into vertical and horizontal components.
V= P (sin a)
H = P (cos a)
How is obliquely crossed prisms calculated?
A = smaller base meridian
B = larger base meridian
0 = angle between base directions
tan y= B sin 0 / A+B cos 0
What is the difference between a prism diopter and a prism centrad?
a ray that is displaced 1 cm at a distance of 1 m from a prism on the arc of a circle with a 1 m radius (1 cm arc displacement) for small angles of deviation, prism centrads and prism diopters and almost equal.
Splitting horizontal prisms
both prisms should be BI or both should be BO.
ex: OD: 3 BI/ OS: 3BI = OD: 2 BI/ OS: 4BI
net effect is the same
Why would you have an uneven split between prisms?
could be because of eye dominance, improve cosmetic appearance, or to equalize lens thickness. IMPORTANT EX: magine a patient has the OD with higher minus (-5.00) and the OS with less minus (- 2.00). You might want to prescribe less prism for the OD which already has a larger edge thickness due to the prescription, and you might want to prescribe more prism for the OS which has a thinner edge thickness due to having less minus power. So you could split the prism between the two eyes so OD has less prism and OS has more prism in order to balance the thickness of the lenses
how should vertical prisms be split?
they should have opposite base directions ex: 3 BU and 3 BD = 2 BU and 4 BD
What is the risley’s prism or rotary prism?
combination of 2 prisms one on top of the other. allows for oblique crossing of prisms.
What is the clinical application of the risley’s prism?
to measure ductions and versions with the phoropter and measure high amounts of prisms in spectacles with lensmeter.
in a Risley’s prism what happens when bases and together?
combined full effect
In a Risley’s prism, what happens when bases are opposite each other with apex to base?
cancel each other out.
How does Risley’s prism work?
when 0 is at 90 meridian, you add BI or BO and when 0 is at 180, you add BU or BD
decentering
never decenter the pupil. only decenter the OC in relation to the pupil. so if OC is moved 5 mm temporally then pupil will appear to be more nasal
oblique decentration
calculate approximate power in the oblique meridian
Foblq = F sph + F cyl sino
o = meridian of interest minus axis f cyl
How much prism is the eye experiencing is the patient’s PD is equal to frame PD?
There is no decentration because pupils are looking through the OC.
How is lens thickness calculated?
P = 100g(n-1)/ d P = amount of prism g = difference in thickness between apex and base n= refractive index d = distance between apex and base.
Why is spectacle correction for anisometropia and presbyopia a problem?
it causes diplopia because the patient is no longer looking through distance OC and they have anisometropia. there will be a vertical imbalance prismatic effect while doing near work.
How do anisometropic patients adjust?
usually adapt by moving their head when looking at different distances to keep view through the OC to avoid diplopia. but when using multifocals, he cant look through the OC to read forcing him to look through neat OC.
what is Bicentric grinding or slab- off and reverse slab-off
The process used to change the amount of prism in the reading portion of the lens w/o defecting prism in the distance portion.
What is special about reverse slab off?
reverse slab-off is molded on the convex (plus side) of plastic lenses and is the exact opposite of regular slab-off. this results in added BD in the reading area. (reduces BU)
What is bicentric grinding
removing BD prism at reading level (adding BU)
What is the easiest way to treat anisometropia and presbyopia ?
using CL and reading glasses or multifocal CLs but this is contraindicated in elderly
how far below OC is a round 22?
11 mm below segment line
Ft-28 and FT- 25/ FT -38
5 mm near OC below segment line
Executive / PAL / Franklin
at the segment line
If a prism was put in the path of light with Base to the left where would you see the image?
to the left.
Prentice rule for multifocals
d= P/D x10 P = imbalance from the distance Rx at reading level D= add power of Rx d= distance needed between the segment OC s to neutralize distance induced imbalance
what is the standard test for determining impact resistance?
drop ball test
drop ball test
5/8 inch steel ball 0.56 ounce must be dropped 50 inches from floor on lens. glass lenses after edged and hardened. plastic tested uncut finished stage.
what type of glass lenses are exempted from testing after hardened?
raised multifocal prism segment multifocals slab off lenses lenticular lenses isekonic lenses
batch testing
done on plastic lenses.
what are the effects of lens coating on impact resistance
when a plastic lens is either scratch resistance coated or AR coated, the impact resistance of the lens decrease.
what are the effects of surface scratching on impact resistance
scratches reduce impact by introducing weak spot on lens. scratches on back reduce impact resistance more
Who has the responsibility making sure the lens passes the impact resistance?
no real answer, but mainly manufacturer.
how long does FDA require records be kept?
3 years after purchase of eye glasses
FTC or federal trade commission
Established to prevent unfair business practices (deceptive advertisement, monopoly)Eyeglass I Rule (1978) Requires that patients receive a copy of their spectacle lens prescription so that they may fill that prescription wherever they want. (Whether or not the patient asks for the prescription)Minimal information requiredSphere powerCylinder power and axisPrismSignature of the prescribing optometrist
what is the eye glass II rule?
1989, No longer list minimal information needed for the spectacle prescription.•Like Eyeglass I- prohibit disclaimers written on the prescription. “Not responsible for accuracy of ophthalmic prescription materials obtained from third party dispensers.
what is OSHA in charge of?
Occupational Safety and Health Administration in charge of regulating safety practices in workplace and in educational setting
what did ANSI do?
set the established eyewear standards. latest version is the 2010 version and is the Z87.1
what is the 2 categories for safety eyewear?
basic and high impact
basic impact safety
minimum central thickness of 3 mm, drop ball test 1 in ball dropped from 50 inches. lens markings of Z87.1 and manufacturer logo. warning label: notice for the wearer declaring it meets basic impact requirements, but has no high impact properties
high impact safety eyewear
Minimum center thickness of 2 mm•High speed ¼ inch steel ball travelling at 150 feet/sec•Lens markings requirements: Manufacturer logo and identifying Z87.1 AND a + sign
+ means high impact on frame and lens.
Portion of spectacles that holds the lenses in front of the eyes
frames
what part of the frame contains the lenses?
front
what attaches to the front and helps hold the spectacles in place?
temples
what are the exceptions of frames that do not have temples?
Pince-nez
clip ons
lorgnettes
Who are half eye glasses usually used for?
presbyopes (single vision)
What is the name of the end piece when it meets at a 45 degree angle?
mitre
What is the name of the end piece when it meets at a 90 degree angle?
butt
What is a numont mounting
the lenses are held at only the nasal edge
What is the difference between a riding bow and a comfort cable?
A comfort cable is metal and riding bow is plastic. both good for kids and sports
What is the most common plastic? what are it’s properties?
Cellulose acetate, it is thermo plastic, but does not have a memory, becomes brittle with age and can be allergic if not coated
What is 25% more lighter than the most common plastic? What are its properties?
propionate it is better for injection molding but not good for color stability
what is 30% lighter than the most common plastic and what are the properties?
Optyl and it is thermoelastic
what is an option for patients allergic to plastics?
optyl
What are nylon’s properties?
it is very flexible but loses its flexibility if not soaked over night. it is added with other materials to add strength and stability.
what are the properties of polyamide/copolyamide?
it is 72% lighter than cellulose acetate and is nylon based so good for sports and is hypoallergic.
what plastic is the most flexible?
grilamide
what are the properties of carbon fiber?
black, strong but stiff, and 40% lighter than cellulose acetate.
What is a plastic that is impact resistant?
polycarbonate this does not tint well
what is kevlar used for?
safety eyewear. it is strong and light weight but difficult to adjust.
what is the most allergenic metal?
nickel, 10% of population allergic
what is another name for nickel silver and what is it made of?
german silver, copper, nickel and zinc
what is monel metal made of?
nickel, copper and iron
what are the pros and cons of stainless steel/
pros: hypoallergic, does not corrode, strong
cons: heavy
how is titanium certified by the vision council of america?
it cannot have any nickel in it. it is either 100-90% or beta titanium with 70%
what is combination titanium?
40-50% titanium and rest in nickel, it loses its hypoallergenic properties.
what is bronze made from?
copper and tin
what is FX9 made of?
copper, magnesium, tin, aluminum
what is genium made of?
chrome, steel, carbon, manganese, silicone
What is the old system of measuring lenses?
the datum system ( 2 lines crossing with the datum center )
What is the new system of lens measurement called?
boxing system this includes the bevel (0.5mm)
What is A in boxing measurements?
the horizontal measurement in mm frame/ lens size (remember to add 0.5 on each side to account for bevel)
what is B in boxing measurements?
vertical lens size (remember to add 0.5 on each side to account for bevel)
what is DBL?
distance between lenses (bevel to bevel ) also termed bridge size
What is the GC?
geometrical center, the center of the box.
What is inset?
how much the optical center is de-centered from the geometrical center. usually nasal, but can be temporal.
What is DBL and how is it calculated?
distance between lenses or the FRAME PD. A + DBL
what is MBS?
minimum blank size
what is ED, how is it calculated?
ED= effective diameter. twice the longest distance from the GC to the edge of the lens.
What do frame markings mean?
VO2312= brand and model
1234= color
50[] 16= 50= A and 16= DBL
140 = temple length
how is temple length measured?
from the center barrel screw hole to the end of the ear piece
how are safety frames marked?
Z-87 or Z87-2 and name and logo of manufacturer on the frame front and the temples.
what does ANSI stand for?
American National Standards Institute
What is the MRP?
Major reference point, where the pupils will look through (not OC, but we want MRP at the OC)
what is PRP?
Prism reference point. where the prescribed prism will be located. if there is no prism, PRP will be at the OC.
how is inset calculated?
inset = (frame PD- patient PD) /2
positive indicates nasally and - indicates temporally
How is monocular distance PD calculated?
the center of the pupil to the center of the nose. (get OD first and the binocular PD - OD PD = OD PD)
Why is monocular PD measured?
used for progressives and aspherics
what frame shape should be given to a patient with a long face?
long vertical depth and lower temples
what frames should be recommended for shorter faces?
shorter vertical depth and higher temples
frame for base down triangle shape?
dark glasses, frame same width as lower face.
base up triangle face frame?
light colored, unobtrusive frame, preferably round.
what frames should progressive lenses use?
short vertex distance, minimal nasal cut, vertical depth
frames for high minus lenses
smaller eye size, rounded corners, high index, flatter or hidden bevels.
frames for high pluses
smaller eye size, rounded corners, shapes with small frame differences, aspheric or atoric designs
how is frame difference calculated?
A-B
What is the standard index of lens clock?
1.53
what are the sagitals like on a plano surface?
S1 = S2= 0
how are the lenses on a minus lens?
fixed Center thickness, variable edge thickness
how are the lenses on a plus lens?
fixed edge thickness and variable center thickness
What is the AC/A ratio
accommodative convergence and accommodation ratio. how many prism diopters the globe converges for every diopter of accommodation.
what is the normal AC/A ratio?
3-5 prism diopters for each diopter of accomodation
how is intermediate power calculated?
1/2 of ADD
how is range of vision related to ADD?
lower ADD = higher range of vision and vice versa
What is the OC for flat tops?
5mm below seg line
what is the OC of E lines, Executives, FT 40 or any progressives?
on the seg line = 0
what is the OC of round?
half the diameter
What influences unwanted astigmatic power in PALs?
add power, higher add power = higher astigmatic error and length of the corridor, width of distance and near zones ( the wider means smaller but harsher astigmatic regions)
What is a hard progressive design consist of
wider distance portion, wider near portion, smaller corridor and harsher astigmatism
what is a soft progressive design consist of?
narrower distance portion, narrower near portion, wider corridor, softer astigmatic errors.
how does Pt.’s PD relate to face form tilt?
if PD = frame PD –> no face form
PD < frame PD –> positive face form
PD> Frame PD –> negative face form ( but this is impractical )
how does pantascopic tilt relate to patient’s eyes?
if eyes @ OC = no pantascopic tilt
eyes above OC = pantascopic
eyes below OC = theoretical retroscopic
How is pantascopic tilt degrees calculated?
for each 1 mm above or below OC, 2 degree tilt is required
How is a vertex distance measured?
Distometer
How are segment heights calculated?
FT= lower lid margin
round= lower lid + 1mm
trifocal = measured from inferior pupil border to lowest part of eye frame MINUS 1mm
executive = lower lid MINUS 1-2mm
progressive= pupillary reflex to lowest part of eye frame rim
double D = lower lid
Blended = lower lid + 3mm
how are children’s bifocals made?
seg line should bisect pupil and in progressive additional lens fitting cross should be 4 mm higher than normal.
