SIMPLE AND TORIC TRANSPOSITION

Transposition:

    It is technical formula to change the lens form from one type to another . There are two types of transpositions . Simple and Toric  Transposition.

    Simple transpositions are used to convert one form to another , usually”+” to “-”.Toric transposition is used to for meniscus and cylindrical lens manufacture.

    When a cylindrical component is to be incorporated in to a spectacle lens , it can be ground in to either the front (r ) back surface of the lens.

    Traditionally front surface cylinders have been used in single vision lenses and back surface cylinders have been used on fused bifocals and on executive style one piece bifocals.

    Cylindrical lenses used in refracting are available in either plus cylinder   ( r) minus cylinder form.

    Although ophthalmologist usually use plus cylinders for refracting , optometrist usually use minus cylinders, because they lend themselves more readily to the fogging method of refraction used by optometrist

Why we do the simple transposition?

    Usually we change the lens “+” form in to “-” to reduce the central thickness of lens . It also minimizes the peripheral aberration ,reduces weight and gives easy adaptation.

Toric transposition:

    Toric transposition is an application for selecting the proper form (Example: meniscus )of cylindrical lens so that it is easy to make a good correcting lenses.

Rules of Toric Transposition:

    1.Choose the proper Base curve first.
    2.Do simple transposition if sign of base curve and cylinder are not same.
    3.Find out the spherical power , subtract the base curve from sphere.

    4.Find out the cylindrical surface power.

        (a) Fix the base curve as a cylinder right angle to the axis of cylinder.
        (b) Add the base curve and cylindrical . This is the second cylinder.

    5.Both spherical (numerator) cylindrical surface (denominator) determines the cylindrical lens power.

Example:

+1.00 DSPH / +2.00DCYL 165   (-6.0 BASE CURVE)

Base curve                           =     -6.0

Simple transposition (Base curve and cylindrical power are not same)

                                              =+3.00DSPH /-2.00 DCYL *75 

Spherical surface power     =+3.00-(-6.00)=+9.00DSPH

Cylindrical surface power  =-600DCYL *165

                                              =-2.00+(-6.00)=-8.00DCYL*75

Final RX                               =+9.00DSPH/-6.00DCYL 165/-8.00DCYL 75

Example :

+1.00 DSPH / +2.00DCYL 180(+6 base) 

Base curve                              = +6.00 

simple transposition             = not required 

Spherical surface power      = +1-(+6)=-5 

cylindrical surface power    = +6*90

                                                 =+2+(+6)=+8 

Final Rx=-5.00DSPH / +6.00 DCYL *90 / +8.00 DCYL 180

Key points to remember :

Transposition is applied only in optical surfacing LAB.

    1.To choose the correct tool in lens surfacing.

    2.To bring the lens in to proper curvature and thickness.

1.TRANSPOSE THE FOLLOWING CROSS CYLINDER IN TO TORIC ON -6 BASE +2.00 DCYL 180 +4.00 DCYL 90.

2.TRANSPOSE THE FOLLOWING SPHERO-CYLINDER IN TO TORIC ON +6 BASE  OF +0.75 DSPH with +1.00 DCYL 135*