Lens II Review Flashcards
Vertical Imbalance Steps.
- find power @ 90
- find optical difference. same signs subtract, opp add.
- use formula Dd/10=^
Segments
Rd 22
Location?
11mm
Segments
Rd 25
Location?
12.5mm
Segments
Rd28
Location?
14mm
Segments
Ft25, Ft28, Ft35
Location?
all 5mm.
Segments
Ultex
Location
19mm
Segments
Exec
Location
0mm
Calculating IJ
ex. 22Rd w/ +2.00ADD
To solve, use Dd/10=^
where D = near oc, d=ADD, and 10 is constant.
(11)(22)/10=22/10=2.2^IJ
Power at 90
if the axis is 0 away from 90, how much cyl is felt?
0%
Power at 90
if the axis is 30 away from 90, how much cyl is felt?
25% felt
Power at 90
if the axis is 45 away from 90, how much cyl is felt?
50% felt
Power at 90
if the axis is 60 away from 90, how much cyl is felt?
75% felt
Power at 90
if the axis is 90 away from 90, how much cyl is felt?
100% felt
Calc. power at 90
ex. +1.00-4.00x060
axis is 30 away from 90, so 25% of cyl felt. (-4.00x25%=-1.00)
take 1.00 and combine with sph. (-1.00 + 1.00 = PL)
PLANO=power at 90
t/f
A FT BF has its near optical center 5mm below the segment line regardless of the width and regardless whether its a BF or TF.
True,
The wearer can opt for two pairs of glasses to correct for VI at near.
true
Fused MF construction methods are only used with glass?
t/f
true
Reverse Slab Off
reverse is ALWAYS BD
find power at 90
find optical difference, same sub, opp add
calc VI with ^= Dd/10
^ goes in lens with LEAST - or MOST +
Reverse Slab off goes in the lens with the least minus, most plus.
t/f
true
Slab off is always ground BU on the lens with the MOST minus, LEAST plus.
t/f
True
Calc. Slab Off
find power at 90
find optical diff. (same signs sub, diff add)
Calc VI using ^= Dd/10
BU on lens w/ MOST minus & Least plus
Cosmetic appearance is a factor when using dissimilar segments to correct for VI @ near.
t/f
true
Prism segments are widely used to correct VI
true/false
false
Vertical imbalance of more than 2D should always be considered.
t/f
true
Reverse slab off is available in fused FT-28 BF
t/f
false.
reverse= front molding
Reverse slab off is placed in the most _____ lens.
minus
plus
plus
Conventional slab off is available in PAL
t/f
true
A client has VI of 2.5D BD OD, what should we do to compensate?
Slab off 2.5 BU OD.
If VI is BD, give BU to neutralize.
BU= slab off BUMM
Reverse slab off is molded not ground.
t/f
true
reverse= molded on front lens
slab off= ground on back lens
A client has VI 2.00BU OD, what should we do to compensate?
REVERSE slab off 2.00BD OD
give BD to neutralize.
BD = Reverse slab off. BDMP
VI compensation at near is required when there is a VI of 1.00D or less between two eyes.
t/f
false. 1.5VI or more.
Conventional slab off is available in a one-piece FT-28 BF
t/f
true
Conventional slab off is placed in the most _____ lens.
minus
plus
minus
Reverse slab off is base _____ prism.
up
down
down
Conventional slab off is base ____ prism.
up
down
up, BUMM
AR coatings help eliminate specular reflections
t/f
true, higher the index of refraction, more light reflected.
The pupil of the wearer reduces the effect of spherical abberation.
t/f
true, not a problem in lower power lenses because of the small limiting size of the pupil.
In curvature of field, the image is curved.
t/f
true
Reverse slab off is available in half diopter steps.
t/f
true
Chromatic aberration is a result of the nature of the material of the lens.
t/f
true
The higher the index of refraction of the lens, the greater amount of specular reflection will occur.
t/f
true
An aspheric front surface of a plus lens steepens towards the periphery
t/f.
false, flattens is periphery.
Most ophthalmic lenses have no aberrations.
t/f
false
1 surface of percent of intensity
- 3%
- 94%
4.3%
2 surfaces for percent of intensity.
- 3%
- 94%
3.94%
In coma, the abberation is a result of the rays of light oblique to the lens surface.
t/f
true
Spherical aberration results from the lens having different powers in different areas.
t/f
true
Conventional slab off is available in quarter diopter steps
t/f
true
In coma, the aberration is a result of rays of light parallel to the lens surface.
t/f
false, coma is oblique.
In distortion, the object will appear to have a pincushion look when viewed through a minus lens.
t/f
false. Pincushion is plus, Barrel is minus
In coma, the pupil of the wearer has no effect on the aberration.
t/f
false. The pupil acts as a small aperture which limits the light rays at periphery of lens.
The refracting power of a lens decreases toward the periphery
t/f
false, it increases, decreases at near OC
All lenses moved farther away from the eye will gain in plus power, we compensate for this by adding minus power.
t/f
true
The law of reflection states that the angle of incidence is always greater than the angle of reflection.
t/f
false, its equal to.
A high plus lens will show _____ distortion
pin
barrel
radio wave
pin. PLUS
A convex surface has ____ dioptric power
plus
minus
plano
plus = convex, minus = concave
As a ray of light passes from a more dense to a less dense material at an oblique angle, the refracted ray will be deviated _____ the normal.
away
toward
parallel
away, more to less dense.
Vertex distance compensation should be considered when the total power of the lens, in any meridian, is +/-7.00 or greater.
t/f
true
A high minus lens will show ____ distortion.
pin
barrel
radio
Barrel
A surface with a flatter curve will have less dioptric power.
t/f
true.
In the surface power formula, we must be sure that “r” (radius) is in meters, not mm, or cm.
t/f
true
When light strikes an object, 3 things occur. They include: refraction, absorption, and reflection.
t/f
true
As light strikes a lens with a spherical front surface, it refracts more at the periphery than near the center.
t/f
true
A surface with a shorter radius will have a flatter curve
t/f
false, steeper=shorter, flatter=longer.
As a ray of light passes through from a LESS dense to a MORE dense material at an oblique angle, the ray will be more deviated ___ the normal.
toward
away
less->more dense, toward the normal.
In coma, the aberration is a result of rays of light parallel to the lens surface.
t/f
false, coma=oblique
A plano surface has____ power
zero
minus
plus
zero
A concave surface has ____ power
minus
plus
plano
minus, convex = plus
In curvature of field, the image is curved convex towards the object.
t/f
False. Curvature=concave
Convex surfaces: t/f
covex surface bulging
converges light rays
plus power
true
Concave surfaces: t/f
minus
diverges light
concave surface is hollow
true
Radius of Curvature: t/f
Curve on a smaller circle is steeper than the curve of a larger circle.
true
Radius of curvature: t/f
the steeper the curve, the more power
true
Surface Power Formula
D=n-1/r
where D= power
nr=index of refractive material
ni=index of the incident material
r=radius of curves of meters
Vertical Imbalance of less than 2D is never noticeable to the wearer.
t/f
false, anything 1.5^ or more is noticeable.
Example of Surface Power Formula.
A concave surface with a radius of 50cm and an index of 1.6. What is the surface power? Convert 50cm to meters = .5
D=n-1/r
1.6-1=.6/.5
D=1.2
rounded to the nearest 1/8D, 1.2=1.25
Index of poly?
- 498
- 586
- 523
1.586
Index of cr-39?
- 498
- 586
- 523
1.498
Index of crown glass?
- 586
- 523
- 530
1.523
Index of trivex?
- 530
- 523
- 498
1.530
Angle of incidence is the angle formed between the refracted ray and the normal.
t/f
true
The angle of refraction is the angle formed between the refracted ray and the normal.
t/f
true
The angle of deviation is the angle formed between the refracted ray and the path the incident ray would have taken had it not been refracted.
t/f
true
Compensating for Vertex Power
CAP, FAM
t/f
True
Compensating Vertex Distance formula
power x power x distance moved/1000=amount of compensation
CAP, FAM
