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.