3. Aspherical Lenses

Question 1

Aspherical lenses meaning?

Answer 1

This are lenses that do not have the same radius of curvature over the entire surface. The lens surface changes shape.

Question 2

Explain the degree of curvature of a spherical lens?

Answer 2

Continuously uniform with a consistent radius of curvature throughout its entire surface.

Question 3

Aspherical lenses are usually based on a surface curvature that comes from?

Answer 3

Comes from a conic section

Question 4

Conic section is?

Answer 4

Is a slice through the cone

Question 5

4 basic types of conic sections?

Answer 5

1. A circle: Shape formed by a horizontal plane, or slice through an upright cone.
2. An ellipse: Shape formed by an angled plane though a cone that does not intersect the base of the cone.
3. A parabola: A curve that is formed by intersection of a cone with having one side parallel to the side of the cone.
4. A hyperbola: Shape formed when the cone is intersected by a plan that makes an angle with the base of the cone that the side of the cone makes with its base.

Question 6

The type of asphericity used on a lens surface is classified by?

Answer 6

p-values

Question 7

Knowing p value will?

Answer 7

Will help differentiate the conic sections from each other.- understand how far the surface departs from a spherical shape.

Question 8

Radius of curvature of aspherical surfaces?

Answer 8

Changing radius of curvature

Question 9

Describe the distribution of surface astigmatism on a spherical lens?

Answer 9

Spherical lenses have a varying amount of surface astigmatism everywhere expect at the centre of the lens surface.

Question 10

Why is aspherical design used?

Answer 10

Optically correct lens aberrations

Question 11

Can you have a spherical lens that is free of oblique outside of its power range?

Answer 11

No -the range is extended for higher refractive indices but only at the negative end.

Question 12

How do aspherical lenses start out in the middle?

Answer 12

In the middle aspherical lenses start out as any other spherical lens, as distance from OC increases the lens surface gradually changes its curvature at a rate calculated to offset peripheral aberrations.

Question 13

Lens made flatter by- HIGH + RX?

Answer 13

Reducing spectacle magnification and making it more attractive

Question 14

Implication of higher plus power on spherical base curve?

Answer 14

Higher plus power –> steeper base curves –> worse the lens looks

Question 15

Choosing a flatter base curve ?

Answer 15

Reduces spectacle magnification

Question 16

Steep base curve implication?

Answer 16

Stepper base curve, the easier it is to dislodge the lens especially thin metal frames.

Question 17

Rather than flattening a regular lens it is better to ?

Answer 17

Better to use an aspherical designed lens

Question 18

Describe construction of aspherical lenses using spherically based lenses?

Answer 18

For simple spheres the front and back surface is spherical.
For spherocylinders: front surface is spherical and back surface is toric.

Question 19

Describe construction of aspherics using aspherically lenses?

Answer 19

For simple spheres: Front surface is aspherical, back surface is spherical.
For spherocylinders: front surface is aspherical, back surface is spherical.

Question 20

Describe construction of aspherical lenses using atoric lenses?

Answer 20

Front surface: spherical
Back surface: typically atoric

Question 21

Can finished aspherical lenses be decentered to create prismatic effect like spherical lenses?

Answer 21

No

Question 22

What does decentration of an aspherical lens to create prism mean?

Answer 22

Means that the wearer will no longer be able to look through the midline of the aspherical zone.

Question 23

Prisms are included how in aspherical lenses?

Answer 23

Prism must be ground in prism done in surface lab so correct amount of prism will be found at the centre of the aspherical zone.

Question 24

3 ways of identifying if a lens is aspherical or atoric?

Answer 24

1. Use a lens clock
2. Use a grid pattern
3. Notice lens curvature- flatness of F1 compared to F2. BEST METHOD OUT OF THE 3.

Question 25

At the apex of aspherical lenses?

Answer 25

Apex of aspherical lenses- have the same radius of curvature. As you move away, some become less flatter or steeper- this depends on the p value.

Question 26

When is it necessary to use aspherical designs?

Answer 26

For higher plus powers

Question 27

Range of powers that are free of oblique astigmatism?

Answer 27

-22.00D t0 +7.75D
Range is extended for higher refractive indices but only on the negative end.

Question 28

Describe the center of spherical lenses and what happens as distance from OC increases?

Answer 28

In the middle- spherical lenses start out like normal spherical surfaces. As distance from OC increases, lens will gradually change curvature to offset peripheral aberrations.

Question 29

How is lens clock used- to determine if a lens is aspherical?

Answer 29

When 3 pins of a lens clock placed on the front surface of a lens- if power of front surface changed –> lens is aspherical.

Question 30

How is grid pattern used to determine if a lens is aspherical?

Answer 30

No distortion of the grid may identify some types of aspherical but not.

Question 31

When is power used to determine if a Px needs to consider a aspherical or atoric?

Answer 31

Power goes above +/- 3.00D

Question 32

Relationship between frame size and power needed in aspherical lenses?

Answer 32

As frame size increases– amount if plus power needed before recommending an aspherical design decreases.

Question 33

Nylon- cord frames need what edge thickness for grooving?

Answer 33

Nylon- cord frames need minimum edge thickness for grooving the edge.

Question 34

When are atoric lenses recommended?

Answer 34

When cyl Rx is above 2D, even when spherical component of the Rx is low.

Question 35

What type of lenses are recommended in Px’s with anisometropia?

Answer 35

Aspherical – if power difference between both eyes is greater than 2D- Significant difference is specs mag, and aspherical lenses are normally flatter, thinner and closer to the eye hence reduce magnification difference.

Question 36

2 fitting adjustments for fitting aspherical lenses?

Answer 36

1. Mono PDs
2. Height of pupil centre adjusted for pantoscopic tilt

Question 37

Full aspherical vs Non-full aspheric

Answer 37

Full aspherical: Change starts gradually and increases more rapidly as distance from the center of lenses increases.
Non-full aspheric: In the center the lens behaves like a spherically based lens.

Question 38

When is non- full aspheric used?

Answer 38

Help reduce the problems with poor fitting. But the eyes have to be centered correctly or else it is very noticeable.

Question 39

Spherical lenses reduce what aberration?

Answer 39

Reduce oblique astigmatism

Question 40

2 benefits of using aspheric lenses?

Answer 40

1. Lens is flatter at the periphery
2. Lens has lower magnification

Question 41

Relationship between curve flatness and sag

Answer 41

Flatter curve = smaller sag

Question 42

2 methods of dispensing prism ?

Answer 42

1. Through lens decentration- up/down/in/out
2. Through oblique direction

Question 43

What should be accurate before dispensing prism in aspherical lenses?

Answer 43

The lenses must have a suitable optical power or else the lens will be decentered.

Question 44

Distortion seen on graph paper with aspherical lenses- plus vs minus lenses?

Answer 44

Plus lenses: Pin-cushion distortion ->Image pushed to the edge.
Minus lenses: Barrel distortion -> Grid buldge outwards.

Question 45

Aspherical vs spherical distortion seen with graph paper?

Answer 45

Aspherical lenses produce minimal distortion, spherical lenses produce large distortion.