Phys And Optical Characteritisics Of Lenses 3

Question 1

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

Answer 1

Do the power cross and find where it is at 90

1. 25BU
2. 25x10/10=1.25

Question 2

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?

Answer 2

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

9. 298D
10. 6D

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

Question 3

Using the lens clock to differentiate between progressive and single lens

Answer 3

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

Question 4

Total power of the lens when read from a lens clock

Answer 4

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

Question 5

Lens thickness

Answer 5

Describes the edge or center thickness of a spectacle lens

Question 6

Minus lens thickness

Answer 6

Thicker in the periphery than the center

A fixed center thickness

Question 7

Plus lens thickness

Answer 7

Thicker in the center than edges

A fixed peripheral thickness

Question 8

Approximate sag formula

Answer 8

S=h^2/2r

Question 9

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

Answer 9

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

Question 10

Front surface sag

Answer 10

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

Question 11

Back surface sag power

Answer 11

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

Question 12

What sign are Tc and Te always

Answer 12

Positive

Question 13

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

Answer 13

0

Question 14

Change in thickness for plus lenses

Answer 14

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

Question 15

-5.00D
N=1.6
Diameter=56mm
Center thickness=1.5mm
Calculate edge thickness

Answer 15

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

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

Question 16

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

Answer 16

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

Question 17

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

Answer 17

+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

Question 18

What are the 3 factors affecting lens thickness

Answer 18

Lens power
Lens diameter
Refractive index

Question 19

Lens power and lens thickness

Answer 19

As lens power increases, thickness increases

Question 20

Lens power and RI

Answer 20

As RI increases, thickness decreases

Question 21

Eyesize and lens thickness

Answer 21

If eyesize increases, thickness increases

Question 22

Base curve and thickness

Answer 22

increased BC increases thickness

Question 23

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

Answer 23

The amount of light from the front surface

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

Question 24

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?

Answer 24

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

Question 25

``` 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 ```

Answer 25

-1+(-3x 0.25)=-1.75D - 1.75(30^2)/2000(1.5-1) - 1.575 1. 575+1=2.575mm

Question 26

``` 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

Answer 26

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

Question 27

Isothickness curves

Answer 27

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

Question 28

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

Answer 28

Circular and centered on the optical center

Question 29

What does a cylindrical lens look like on isothickness curves

Answer 29

Straight lines parallel to the axis

Question 30

What does a spherocylindrical lens look like on isothickness curves

Answer 30

Circles, ellipses, hyperbolas, or straight lines

Question 31

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

Answer 31

Fresnel prism

Question 32

What are fresnel prisms useful for

Answer 32

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

Question 33

Formula for combining prisms

Answer 33

- if prisms are perpendicular, use Pythagorean theorem, | - if both base up, take the difference between the two

Question 34

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

Answer 34

Square root of (2^2 +1^2) | 2.23PD

Question 35

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

Answer 35

1 BU

Question 36

Calculating thickness of a Plano prism

Answer 36

Thickness of a prism varies with distance from the apex The change in thickness: (Zh)/(100(n-1))

Question 37

Using thickness difference to determine prism power

Answer 37

- 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

Question 38

``` Calculate horizontal prism at geometric center Rx=-5.75Ds Diameter=50mm N=1.6 Temporal edge thickness=2mm Nasal edge thickness=5mm ```

Answer 38

Z=(100x3x(1.6-1))/(50) | 3.6 PD

Question 39

Vertical prism

Answer 39

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

Question 40

If the amount of vertical prism in each eye is different

Answer 40

There is a vertical imbalance or binocular prism included | -result of having differencevertical primsatic effects in each eye at a give gaze position

Question 41

To find vertical imbalance

Answer 41

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

Question 42

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?

Answer 42

5x10/10=5BU 3x10/10=BU Net 2 BU