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It is a technical formula to change the lens form from one type to another. Simply transposition is used to convert one form to another, usually “+” to “-“.
Types of Lens Transposition
- Simple transposition
- Toric Transposition
3 rules of eyeglass prescription lens transposition.
- Sum: Add the sphere and cylinder powers of prescription to determine the new sphere power.
- Sign: Change the sign of the Cylinder
- Axis: Change the axis by 90 degrees (if the axis is less then or equals to 90°, add 90° to the axis, and if the axis is more than 90°, subtract 90° from the axis).
- +3.50D Sph / +2.00 Dcyl × 140°
- New Sphere power = + 5.50 D sph (Add spherical +3.50 and cylindrical power +2.00)
- New Cylinder power = – 2.00 D cyl (Sign Changed)
- New Axis = 50° (140°-90°)
- Final Transposed Power is: +5.50D Sph / -2.00 Dcyl×50°
- +3.00 D Sph / -1.50 Dcyl x30 °
- New Sphere power = + 1.50 sph (Add spherical +3.00 and cylindrical power -1.50)
- New Cylinder power = +1.50 D cyl (Sign Changed)
- New Axis = 120° (30°+90°)
- Final Transposed Power is: +1.50D Sph / +1.50 Dcyl × 120°
Toric Lens Transposition
For selecting proper cylindrical lenses toric lens transposition can be made. Toric lens prescription has 3 parts, Base Curve, Cross Curve and Sphere Curve. This type of transposition will be made at the lens production factories so that the appropriate lens were produced as per the prescription of the lens.
Uses of Lens Transposition
It is used by Eye Care Professionals and Opticians to make the prescription easy, It is also used when the exact power of lenses were not available in the inventory of eye hospitals or an eye clinics.