L4 Transposition Flashcards Preview

Opthalmic Lenses and Dispensing > L4 Transposition > Flashcards

Flashcards in L4 Transposition Deck (10):


1) Sphere - add sphere and cyl together
2) Cyl - change the power of the cyl (change the sign)
3) Axis - change the axis of the cyl by 90


Toric Lenses

Two curved surfaces, but only one is spherical. The one which corrects for astigmatism is known as toridal. Results in lenses with different thickness throughout the lens


Bitoric Lenses

Inefficient, not great to look through


Toric Prescriptions can be written in:

1) Sphere/cyl form
2) Cross Cyl form
3) Toric Lens form
4) Optical Cross form


Cross Cyl to Sph/Cyl form

1) Take one of your cross cyl powers to represent your sphere
2) The difference between the sphere and the second cross cyl is the cylinder power
3) The axis to be used in the sph/cyl form is the one that wasnt chosen as the sphere


Optical Cross to Sph/Cyl form

Sphere = take your first value as the sphere - for minus cyl chose the most positive
Cyl = difference between the 2 values
Axis = axis direction of the first number


Toric Transposition (the curves)

Sphere curve = curve on the spherical surface (always by itself)
Base curve = lowest curve value (closest to 0) and flattest curve on the toroidal surface. Its opposite the the sphere curve, so if sphere is at the front, base will be on the back
Cross Curve = the highest curve value (furthest from 0) and steepest curve on the toroidal surface - 90 degrees to the base curve


How are toric transpositions written?

- Front surface (F1) on the top - this will be convex and so positive
- Back Surface (F2) on the bottom - this will be concave and so negative
Depending on which surface was the toric surface this would be:
- bc/cc divided by sc
- sc divided by bc/cc


Toric Transposition from Sph/Cyl form

Sphere curve = Sphere - base curve (DS)
Cross Curve = Cylinder = base curve (DC)
cc axis is axis from sph/cyl form
Base Curve = sphere - sphere curve (DC)
bc axis is perpendicular to cross curve axis


Reminder for toric transposition from sph/cyl

- when the base curve is given in the question, transpose the prescription so that the sign of the cyl is the same as the base curve
- when the sphere curve is given , transpose the prescription so the sign of the cyl is opposite to the sign on the sphere curve