OP - Sphero-cylindrical Lenses - Week 8 Flashcards
What does a positive power cylinder look like?
Consider a cylinder with a given radius, ro.
A cut of this cylinder parallel to the cylinder axis is a positive power cylinder.
Define the formula for the power of cylinders.
Power = 1/ro
ro is the radius of curvature
Define the two axes of a cylindrical lens, and what level of power is associated with them. Define the angle between the two axes.
Cylindrical axis - axis parallel to the length of the cylinder.
Power axis - axis of power found 90* to the cylindrical axis, is perpendicular.
Cylindrical axis has no power.
The power axis has the maximum power.
If you bring a point of light into focus using a cylindrical lens, what does the point of light become, and what does it align with?
Focusing a point source with a cylindrical lens results in a line of light rather than a point.
This aligns with the cylindrical axis of the lens.
What does a negative power cylindrical lens look like?
Consider a cylinder that is pressed into a rectangle.
The impression left behind is the negative lens.
What is the difference between using x and @ in a cyl prescription?
x is used when specifying the cyl axis
@ is used when specifying the power axis
x for cyl axis is generally used
Consider compound astigmatism. It would be necessary to use two cyl lenses taped together. This is impractical. Explain how this can be avoided.
Give an example with the following mixed compound astigmatism:
90* = +63.00D
180* = +58.00D
+60.00D is needed to focus onto the retina.
Name the two cylindrical lens needed to correct this for each axis, and name the sphero-cylindrical lens combination that could acomplish this.
Two orthogonal cylinders can be made using a sphero-cylindrical lens - spherical lens + cylinder lens
To correct the the compound astigmatism with purely cylindrical lens, you will need:
-3.00DC x 180*
+2.00DC x 90*
However this can be done using the following spherical and cylindrical lens:
+5.00DC x 180*
-3.00DS
or
-5.00DC x 90*
+2.00DS
What is the notation used for spherocylindrical lenses?
S / C x α
S is the spherical power
C is the cylindrical power
α is the axis
When designing a spherocylindrical lens, it is possible to use a positive cyl, or negative cyl.
What is used in optometry by convention, and what is the other used for?
In optometry, negative cyl is always chosen.
Positive cyls are used in ophthalmology.
Consider a spherocylindrical lens, what is the power along axis α, and what axis does α represent? What is the power along the power axis?
Axis α is the cylindrical axis, and has no cylindrical power.
Therefore in a spherocylindrical lens, it will have S power only.
The power axis will have maximum cyl power, and full S power, as S power applies equally to all axes.
The power is therefore S + C.
Describe the steps to correct astigmatism in terms of bringing the focus point forwards or backwards with spherical power, then correcting with cylindrical.
60.00D is needed for focus onto the retina.
Pick one of the two focus points of the astigmatism, and bring it forward or backward onto the retina.
The other point is affected with equal magnitude as spherical power affects all meridians.
Correct the remaining simple astigmatism with a single cylindrical lens.
Describe transposition of cylinder lens in three steps.
- New sphere = old sphere + cylinder
- New cylinder = old cylinder with sign changed
- New axis orthogonal to old axis
What kind of curvatures would a spherocylindrical lens have, and what other kind of construction is available?
What do the curvatures of this correspond to?
Could have a spherical front surface, and cylindrical rear surface, or vice versa.
Toric lenses are available, with 2 principle radii of curvature that correspond to 2 principle powers.
True or false
All spherocylindrical lenses have at least one toric surface.
True
Name the three types of toric surfaces
Tyre formation
Barrel formation
Capstan formation
