Transposition
R.D.Gopinath
Supervisor – Optical Sales
Aravind Eye Hospital
Definition
 An application of changing the lens form one to
another.
 Usually it is changed from ‘+’ form to ‘ – ‘ form.
Why we do the simple transposition?
 Usually we change the lens ‘+’ form into ‘-’.
 To reduce the unwanted central thickness.
 Minimize the peripheral aberration.
 Light weight
 Easy adaptation.
Types
Simple transposition.
 Applies to convert the lens into different forms.
Toric transposition.
 Is applied only for selecting the appropriate tools
in cylindrical lens surfacing.
 Different steps followed in calculating the
surfacing tools.
Rules – Simple transposition
 Algebric sum of sphere and cylinder is a new sphere.
 Cylindrical power is a old one but
 Sign and axis of cylinder will be in opposite.
 Examples;

+2.5 D Sph / +3.0 D cyl x 150*

a) + 5.5 D Sph

b) 3.0 D cyl

c) – cyl & 60*

Final Rx : + 5.5D Sph / - 3.0D Cyl x 60*
Few examples
1) -- 1.5 D Sph / -- 4.0 D Cyl x 105*
Answer : -- 5.5 D Sph / + 4.0D Cyl x 15*.
2 ) + 2.0 D Cyl x 90*
Answer : + 2.0 D Sph / -- 2.0D cyl x 180*
3) -- 1.5 D Sph / + 4.0 D Cyl x 105*
Answer : + 2.5 D Sph / -- 4.0 D Cyl x 15*
Different methods of simple transposition
 Sphere into two cylinders.
+3.0D Sph = +3.0D cyl 90* + +3.0D cyl 180*
 Cylinder into sphero-cylinder.
+3.0D cyl 90* = +3.0D Sph / --3.0D cyl 180*
 Sphere-cylinder into

Alternate Sphero-cylinder.

Two cylinders.
Toric transposition
 Objective : To select the proper tools in cylinder lens surfacing.
Rules:
 Choose the Base curve first for proper curvature.
 Do simple transposition if sign of base curve &cylinder not same.
 To find out the spherical surface power,

Subtract the base curve from sphere.
 To find out the cylindrical surface power,

Fix the Base curve at the right angle to the axis of cylinder.

Add the base curve with cylinder
 Both spherical & cylindrical surface determines the lens power.
Toric transposition - Example
 + 1.0 D Sph / + 2.0 D Cyl x 165* (-6.0 Base curve)
 + 3.0 D Sph / - 2.0D Cyl x 75*
 + 3.0 – ( -6.0) = + 9.0 D Sph.
 - 6.0D Cyl x 165*
 -2.0 + ( -6.0) = - 8.0D Cyl x 75*

+ 9.0D Sph
-6.0D Cyl x 165*/ - 8.0D Cyl x 75*
Examples…
1) +1.0D Sph / +2.0D Cyl x 180* ( +6 Base)
- 5.0D Sph
+ 6.0D Cyl x 90* / + 8.0D Cyl x 180*
2) – 3.0D Sph / - 1.5D Cyl x 90* ( -6 Base)
+ 3.0D Sph
- 6.0D Cyl x 180 / - 7.5 D Cyl x 90*
3) + 1.5D Cyl x 90* ( -8 Base)
+9.5 D Sph
- 8.0D Cyl x 90* / - 9.5 D Cyl x 180*
Points to remember…
 Transposition applies only in optical surfacing lab.

To choose the correct tool in lens surfacing.

To bring the lens into proper curvature & thickness.