Lecturer: Nada Aldahlawi

Email: aldahlawi.n@gmail.com

Lec 6 Course OPTO 225

(Ophthalmic Optics and Dispensing I) Astigmatic lenses Crossed Cylinders

Properties of crossed cylinders Exercise (Cylindrical lens or Sphero-cylindrical lens) Transposition -Cross-Cylindrical prescription can be written in its equivalent sphero-cylindrical form

- The process of changing from one form to another is known as transposition

Determination of the cylindrical power

1- Hold the lens close to the eye, so that the major meridians are vertical and horizontal

2- Move the lens vertically or horizontally along the power meridians

3- Observe the type and magnitude of the apparent movement

4- Select a cylindrical lens of opposite power

5- Align the axes of the 2 lenses

6- Report the transverse test

7- Increase the power of known cylindrical lens until movement in step 6 is neutralized

8- The power of the known cylinder is the same but opposite to that known cylinder lens.

Astigmatic lenses

1- The lens is cylindrical or spherical

2- determine the power(s) along the major meridians

3- Determine the axis of the cylindrical component

The Crossed- Cylinder Form

1- A crossed cylinder lens is one having a plus cylinder ground on the front surface

and a minus cylinder ground on the back surface, with the axes of the two cylinders being 90o apart

2- In optometric practice, crossed cylinders are used in a) refining the axis and power of the patient’s cylindrical correction,

b) are also used for near-point testing (e.g., to determine the power of a tentative bifocal addition)

????? Crossed Cylinders

Properties of crossed cylinders Transposition (Cylindrical lens or Sphero-cylindrical lens) In order to identify an astigmatic lens, we have to determine whether: To identify an astigmatic lens:

1- do the rotation test

2- determine the major meridians the astigmatic lens

- the astigmatic lens has 2 power meridians: the meridians of highest powers and lowest powers. Usually both meridians are 90o apart

i) hold the lens before the eye

ii) observe the target

iii) if there is an apparent break in the target, then we are not looking through the meridians of the lens

iv) Slowly rotate the lens until the line is perfectly vertical or horizontal

v) Mark the position of the line target on the lens

vi) This represents 1 of the major meridians

vii) the other meridian will be 90o away from this position

3- Do the transverse test

i) Do the transverse test as describe above

ii) If there is no power along the l of the meridians, then the astigmatic lens is a cylindrical lens.

iii) If there is power along both meridians, then the lens is sphero-cylindrical lens.

Identification of sphero-cylindrical lens

1- Do the rotation test to identify the lens as astigmatic lens

2- Mark the major meridians of the lens

3- Do the transverse test

4- If there is apparent movement along both principle meridians, then the lens must be Sphero-cylindrical

5- Do the rotation test to determine the power along the 1st principle meridian.

1- Spherical - power and radius is the same in all meridians

2- Aspheric - radius changes from the center to the outside (becomes less curved usually)

3- Cylindrical- different powers in different meridians

Surface type Rules of Transposition

- Given: cross cyl form:

+1.00DC x 90 + 4.00DC x 180

required: transpose to alternate spherocyl form

Procedures:

a) write either cross cyl as the sphere. +1.00DS

b) subtract the cylinder chosen as the sphere from other cylinder to find the cylinder power. 4 – 1 = +3.00DC

c) Axis of the cylinder is the same as the axis of x-cyl that was not chosen as the sphere. Axis : 180

d) The sphero-cyl form is :

+1.00DS +3.00DC x180 or

+4.00DS -3.00DC x 90 1- Sphere-Cyl from cross-cyl

- Given: either of the sphero-cyl form (plus or minus)

+1.00DS +3.00DC x 180

- required: transpose to x-cyl form

Procedures:

a) 1st x-cyl = sphere of the sphero-cyl Rx written as a cylinder with axis at right angle to axis of cyl in sphero-cyl form:

+1.00DC x 90

b) 2nd x-cyl = sum of sphere and cyl from sphero-cyl rx, written as a cyl with axis the same as cyl in sphero-cyl form:

+1 +3 = +4.00DC x 180

c) x-cyl form:

+1.00DC x 90 +4.00DC x 180

2- Cross Cyl from Sphero-cyl:

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# Lec 6 OPTO 225 (Ophthalmic Optics and Dispensing I)

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