3. Aspherical Lenses Flashcards
Aspherical lenses meaning?
This are lenses that do not have the same radius of curvature over the entire surface. The lens surface changes shape.
Explain the degree of curvature of a spherical lens?
Continuously uniform with a consistent radius of curvature throughout its entire surface.
Aspherical lenses are usually based on a surface curvature that comes from?
Comes from a conic section
Conic section is?
Is a slice through the cone
4 basic types of conic sections?
- A circle: Shape formed by a horizontal plane, or slice through an upright cone.
- An ellipse: Shape formed by an angled plane though a cone that does not intersect the base of the cone.
- A parabola: A curve that is formed by intersection of a cone with having one side parallel to the side of the cone.
- 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.
The type of asphericity used on a lens surface is classified by?
p-values
Knowing p value will?
Will help differentiate the conic sections from each other.- understand how far the surface departs from a spherical shape.
Radius of curvature of aspherical surfaces?
Changing radius of curvature
Describe the distribution of surface astigmatism on a spherical lens?
Spherical lenses have a varying amount of surface astigmatism everywhere expect at the centre of the lens surface.
Why is aspherical design used?
Optically correct lens aberrations
Can you have a spherical lens that is free of oblique outside of its power range?
No -the range is extended for higher refractive indices but only at the negative end.
How do aspherical lenses start out in the middle?
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.
Lens made flatter by- HIGH + RX?
Reducing spectacle magnification and making it more attractive
Implication of higher plus power on spherical base curve?
Higher plus power –> steeper base curves –> worse the lens looks
Choosing a flatter base curve ?
Reduces spectacle magnification
Steep base curve implication?
Stepper base curve, the easier it is to dislodge the lens especially thin metal frames.
Rather than flattening a regular lens it is better to ?
Better to use an aspherical designed lens
Describe construction of aspherical lenses using spherically based lenses?
For simple spheres the front and back surface is spherical.
For spherocylinders: front surface is spherical and back surface is toric.
Describe construction of aspherics using aspherically lenses?
For simple spheres: Front surface is aspherical, back surface is spherical.
For spherocylinders: front surface is aspherical, back surface is spherical.
Describe construction of aspherical lenses using atoric lenses?
Front surface: spherical
Back surface: typically atoric
Can finished aspherical lenses be decentered to create prismatic effect like spherical lenses?
No
What does decentration of an aspherical lens to create prism mean?
Means that the wearer will no longer be able to look through the midline of the aspherical zone.
Prisms are included how in aspherical lenses?
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.
3 ways of identifying if a lens is aspherical or atoric?
- Use a lens clock
- Use a grid pattern
- Notice lens curvature- flatness of F1 compared to F2. BEST METHOD OUT OF THE 3.
At the apex of aspherical lenses?
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.
When is it necessary to use aspherical designs?
For higher plus powers
Range of powers that are free of oblique astigmatism?
-22.00D t0 +7.75D
Range is extended for higher refractive indices but only on the negative end.
Describe the center of spherical lenses and what happens as distance from OC increases?
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.
How is lens clock used- to determine if a lens is aspherical?
When 3 pins of a lens clock placed on the front surface of a lens- if power of front surface changed –> lens is aspherical.
How is grid pattern used to determine if a lens is aspherical?
No distortion of the grid may identify some types of aspherical but not.
When is power used to determine if a Px needs to consider a aspherical or atoric?
Power goes above +/- 3.00D
Relationship between frame size and power needed in aspherical lenses?
As frame size increases– amount if plus power needed before recommending an aspherical design decreases.
Nylon- cord frames need what edge thickness for grooving?
Nylon- cord frames need minimum edge thickness for grooving the edge.
When are atoric lenses recommended?
When cyl Rx is above 2D, even when spherical component of the Rx is low.
What type of lenses are recommended in Px’s with anisometropia?
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.
2 fitting adjustments for fitting aspherical lenses?
- Mono PDs
- Height of pupil centre adjusted for pantoscopic tilt
Full aspherical vs Non-full aspheric
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.
When is non- full aspheric used?
Help reduce the problems with poor fitting. But the eyes have to be centered correctly or else it is very noticeable.
Spherical lenses reduce what aberration?
Reduce oblique astigmatism
2 benefits of using aspheric lenses?
- Lens is flatter at the periphery
- Lens has lower magnification
Relationship between curve flatness and sag
Flatter curve = smaller sag
2 methods of dispensing prism ?
- Through lens decentration- up/down/in/out
- Through oblique direction
What should be accurate before dispensing prism in aspherical lenses?
The lenses must have a suitable optical power or else the lens will be decentered.
Distortion seen on graph paper with aspherical lenses- plus vs minus lenses?
Plus lenses: Pin-cushion distortion ->Image pushed to the edge.
Minus lenses: Barrel distortion -> Grid buldge outwards.
Aspherical vs spherical distortion seen with graph paper?
Aspherical lenses produce minimal distortion, spherical lenses produce large distortion.
