Dispensing - toric transposition

What is the axis meridian?

Surface with no power

What is the power meridian?

Surface with maximum power

What are toric lenses?

• made from meniscus kenses

• One convex and one concave

• One surface is spherical

• One surface is toroidal (contains a maximum surface power and a minimum surface power)

What way do the angles on a protractor run for the eyes?

Anti-clockwise

What are the 4 different ways to write a Toric prescription?

• Sphere / Cyl form

• Cross-Cyl form

• Toric lens form

• Optical cross / Power cross

What is a sphere/cyl form?

• Most common and the way in which we issue prescription

What is crossed cyl form?

• Describes a prescription in terms of the axis of two cylinders

What is a toric lens form?

Describes the powers required on the back and front surfaces of a lens to give right correct overall prescription

What is an optical cross / power cross?

Allows you to visualise the powers along each meridian

How do we complete this optical cross?

• -1.50 is spherical so lies across the entire lens

• -1.00 is cyl at an axis on 90, and the power lies perpendicular to this so at 180

• Giving it a power of -2.50

How can you split the prescription: -1.50/-1.00 × 90 into 2 optical crosses?

How can you write the previous prescription in the cross cyl form?

-1.50 × 180/-2.50 × 90

How do you change +1.00/-2.50 × 40 into the crossed cyl form?

+1.00 × 130/ -1.50 × 40

How do you convert from crossed cyl into spherical form?

• the most positive is the sphere

• Diff between the sphere and the second cross cyl is the cylinder power

• Axis is the one that was jot chosen as the sphere

How to convert from optical cross to sph/ cyl form?

Convert this optical cross into sph/cyl form

Why do we have alternative sph/cyl forms?

• diff optoms

• Labs often glaze in plus cyl form

• Lenses are often given in a range, and you may need to transpose if a power is within a range and its cost

Explain this plus and minus cyl formatting

• Minus cyl: -2.00 all over with -3.00 along 180 making it -5.00

• Plus cyl: -5.00 all over with +3.00 along 180 making it -2.00

What three curves are in toric transposition?

• Sphere curve (SC) - the curve on the spherical surface

The two curves in the toroidal surface:

• Base curve (BC) - lowest curve value (and closes to 0) and flattest curve on the toroidal surface

• Cross curve (CC) - the highest curve value (farthest from 0) and steepest curve on the toroidal surface

How are toric transpositions written?

How is this written with the different curves?

Equation for sphere curve?

Sphere curve = sphere - base curve (DS)

Equation for cross curve?

Cross curve = cylinder + base curve (DC)

CC axis = Axis from the spherical surface/cyl form

Equation for base curve?

Base curve = sphere - sphere curve (DC)

BC axis = perpendicular to cross curve axis

When the base curve is given in the question, how should we transpose the prescription?

• So that the sign if the cyl is the same as the base curve

When the sphere curve is given, how should we transpose the prescription?

• So that the sign of the cyl is the opposite to the sphere curve

What is the phrase to remember this?

Same Base Opposite Sphere

Try this toric transposition

Try another toric transposition