7 - Aspheric Lenses - Exam 2 Flashcards
Spherical and toric lenses both have circular curvatures. How many circular curvatures does each style of lens have?
Spherical lenses have one circular curvature
-therefore one radius of curvature
Toric lenses have two perpendicular circular curvatures - therefore two radii of curvature
Does an aspheric lens have a constant radius of curvature?
No
What are 2 advantages of aspheric lenses?
- May reduce off-axis aberrations
2. Reduce lens thickness
What is 1 disadvantage of aspheric lenses?
More difficult to manufacture than spherical lenses
What is the asphericity of a circle?
Q = 0
How does an oblate ellipse compare to a circle?
Steeper - more curvature
Q > 0
How does a prolate ellipse compare to a circle?
Flatter - less curvature
-1 < Q < 0
What is the asphericity of a parabola?
Q = -1
What is a 3D parabola called?
Paraboloid
Do paraboloids have any spherical aberrations?
No
What is the asphericity of a hyperbola?
Q < -1
What is a 3D hyperbola called?
Hyperboloid
In which meridian is the power drop off faster in an aspheric lens, sagittal or tangential?
Power drops off faster in tangential meridian
What is the difference between a full aspheric lens and a nonfull aspheric lens?
Full - Asphericity starts at the vertex of the lens
Nonfull - has spherical and aspherical components
What is the cap in a nonfull aspheric lens?
The central spherical area of the lens
Is it possible for a lens that does not have a +6.00 BC fit into a frame designed for that base curve?
Yes
Aspheric lenses may approximate that shape without changing the optics of the lens
When talking about PALs, which portion of the lens is typically spherical and which is typically aspheric?
Distance portion is usually spherical
Add portion is usually aspheric
What is an atoric lens?
Similar to aspheric, but for spherocylindrical prescriptions
What cylindrical power are atoric lenses recommended for?
> 2.00 D of astigmatism
What is a finished single vision atoric lens composed of on each surface?
Spherical front surface
Atoric back surface
What is a semi finished single vision atoric lens composed of on each surface?
Atoric front surface
Spherocylindrical back surface
Can decentration be used to achieve prism in an aspheric lens?
No
However prism can be ground into the lens
What are 4 ways of identifying aspheric lenses?
- Use a lens clock
- Use a grid pattern behind the lens
- Note the lens surface curvature
- Look for manufacturer markings
What the DRP coincide with especially in the case of aspheric lenses?
The optical center
What is the maximum amount of pantoscopic tilt for aspheric lenses?
10 degrees
counteracted by 5mm decentration
What are 4 types of patients who would benefit from aspheric lenses?
- High plus or high minus Rx
- Anisometropic patients
- Patients that want good cosmetics
- Older patients - aspheric lenses are lighter
