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Ophthalmic Optics Block 10 > Phys And Optical Characteritisics Of Lenses 3 > Flashcards

Phys And Optical Characteritisics Of Lenses 3 Flashcards

1
Q

Find the vertical prism induced in the right eye when a patient looks 10mm below the optical center of the following lens
OD +2.00 -1.50x 135

A

Do the power cross and find where it is at 90

  1. 25BU
  2. 25x10/10=1.25
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2
Q 
A binconvex lens is clocked with the following properties:
Sag of front surface=4.5mm
Sag of back surface=3.2mm
Diameter=44mm
N=1.50
What is the power of each lens surface?
A

F=(2(n2-n1)S/(h^2)

  1. 298D
  2. 6D

Remember to change the -6.6 to a plus since it is BICONVEX

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3
Q

Using the lens clock to differentiate between progressive and single lens

A

After initial measurement, lens clock can be oriented along the vertical meridian to check for variance in power across the front surface

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4
Q

Total power of the lens when read from a lens clock

A

Sum of the front and back surface powers, however, this ignores thickness. Can cause the back vertex power of plus Rx to be underestimated when adding surface powers

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5
Q

Lens thickness

A

Describes the edge or center thickness of a spectacle lens

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6
Q

Minus lens thickness

A

Thicker in the periphery than the center

A fixed center thickness

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7
Q

Plus lens thickness

A

Thicker in the center than edges

A fixed peripheral thickness

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8
Q

Approximate sag formula

A

S=h^2/2r

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9
Q

What equation is used to estimate the sag from the surface power without calculating radius

A

S=(Ph^2)/(2(n-1))

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10
Q

Front surface sag

A

S-(PH^2)/(2000(n-1))

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11
Q

Back surface sag power

A

(PH^2)/(2000(1-n))

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12
Q

What sign are Tc and Te always

A

Positive

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13
Q

If there is a flat surface, than S1 or S2 will be

A

0

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14
Q

Change in thickness for plus lenses

A

It is positive. Center thickness is greater than edge thickness

  • h is half the lens diameter
  • BC or lens form have no impact on this formula
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15
Q 
-5.00D
N=1.6
Diameter=56mm
Center thickness=1.5mm
Calculate edge thickness
A

-5x28^2/2000(1.6-1)=-3.3mm

Must make it positive so it is 3.3mm
3.3+1.5=4.8mm

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16
Q 
Calculate maximal edge thickness 
Rx=-2.00 -2.00 X 146
Center thickness=2mm
Diameter=52mm
N=1.42
Assume optical center is at geometric center
A

Want the thickest to be at the highest power meridian so doing a lens cross, what would make -4.00 our highest meridian

(-4.00(26^2))/(2000(1.42-1))=-3.219
Make it positive 3.219

3.219+2=5.219

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17
Q 
Calculate center thickness 
Rx=+5.00 -2.00 x 49
Minimum edge thickness=2mm
Diameter=69mm
N=1.7
Assume optical center is at geometric center
A

+5.00 is our highest power meridian based on the lens cross

+5(34.5^2)/2000 (1.7-1)=4.25

4.25+2= 6.25mm

18
Q

What are the 3 factors affecting lens thickness

A

Lens power
Lens diameter
Refractive index

19
Q

Lens power and lens thickness

A

As lens power increases, thickness increases

20
Q

Lens power and RI

A

As RI increases, thickness decreases

21
Q

Eyesize and lens thickness

A

If eyesize increases, thickness increases

22
Q

Base curve and thickness

A

increased BC increases thickness

23
Q

Changing the base curve for a -8D lens will not change

A

The amount of light from the front surface

-it will change the edge lens thickness, the retinal image size, the oblique astigmatism

24
Q

Rx=-5.00 -4.00 x 45
N=1.65
Diameter=54mm
Central thickness=1mm

What is the power in the horizontal meridian?
What is the thickness at the temporal edge?

A

P=S+C(sintheta)^2
-5+(-4x0.5)=-7.00D x 180

  • 7(27^2)/2000 (1.65-1)
  • 3.9mm
    3. 9mm+1MM=4.9mm
25
Q 
Rx=1.00 -3.00 x 180
Diameter=60mm
N=1.5
What is the power at 30 degrees?
What is the edge thickness at 30 degrees
A

-1+(-3x 0.25)=-1.75D

  • 1.75(30^2)/2000(1.5-1)
  • 1.575
    1. 575+1=2.575mm
26
Q 
Rx=-3.75 -2.50 x 120
Center thickness=2mm
Diameter=50mm
N=2.6
Calculate the thickness at the temporal edge of the lens, assuming the optical center is at geometric center

Calculate the max edge thickness

A

S+C(sintheta)^2
-5.625 x 180

(-5.625(25^2))/2000(1.6-1)=2.93
2.93+2=4.93mm

-6.25 is the highest powered meridian
Plug into equation and get 3.26+2=5.26mm

27
Q

Isothickness curves

A

Curves drawn on a power cross to show the curves on which thickness is the same

  • two regions on the same curve will have the same thickness
  • lines close together=thickness is changing quickly
28
Q

What does a spherical lens look like on an iOS thickness curve?

A

Circular and centered on the optical center

29
Q

What does a cylindrical lens look like on isothickness curves

A

Straight lines parallel to the axis

30
Q

What does a spherocylindrical lens look like on isothickness curves

A

Circles, ellipses, hyperbolas, or straight lines

31
Q

Consists of small rows of equally powered prisms and yielding the same optical effect as that of a conventional prism, but less than a mm in thickness

A

Fresnel prism

32
Q

What are fresnel prisms useful for

A

Short term treatment of binocular vision problems and for evaluating whether prism can eliminate symptoms related to vertical imbalance

33
Q

Formula for combining prisms

A
  • if prisms are perpendicular, use Pythagorean theorem,

- if both base up, take the difference between the two

34
Q

What is the total prism that results from combining 2 BU and 1 BO

A

Square root of (2^2 +1^2)

2.23PD

35
Q

What is the total prism that results from combining 2 BU and 1BU

A

1 BU

36
Q

Calculating thickness of a Plano prism

A

Thickness of a prism varies with distance from the apex
The change in thickness:

(Zh)/(100(n-1))

37
Q

Using thickness difference to determine prism power

A
  • rearranging the previous formula to get the prism thickness
  • the geometric center of the concave lenson the right contributes no prism. Thus, prismpower can be determined by the difference in edge thickness using the abovefiormual
38
Q 
Calculate horizontal prism at geometric center 
Rx=-5.75Ds
Diameter=50mm
N=1.6
Temporal edge thickness=2mm
Nasal edge thickness=5mm
A

Z=(100x3x(1.6-1))/(50)

3.6 PD

39
Q

Vertical prism

A

Must take into account the amount of vertical prism induced into each eye

40
Q

If the amount of vertical prism in each eye is different

A

There is a vertical imbalance or binocular prism included

-result of having differencevertical primsatic effects in each eye at a give gaze position

41
Q

To find vertical imbalance

A

Find the difference in the amount of vertical prism in each eye

42
Q

A patient has a RX calling for OD +5.ooD and +3.00D spherical lens, how much vertical imbalance is induced when the patient looks down 10mm to read?

A

5x10/10=5BU
3x10/10=BU

Net 2 BU

Ophthalmic Optics Block 10 (4 decks)

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