toric transposition

Question 1

what kind of lens do you need to correct astigmatism ?

Answer 1

. lenses to correct astigmatism have two principal powers at perpendicular meridians to one another
. the simplest way of achieving two perpendicular powered meridians is on the basis of a cylinder
. theses lenses have one surface with no power which is 90deg away from a surface with maximum power

Question 2

what is the surface with no power ?

Answer 2

axis meridian

Question 3

what is the surface with maximum power ?

Answer 3

power meridian

Question 4

how many curved surfaces do meniscus lenses have ?

Answer 4

. meniscus lenses have 2 curved spherical surfaces , one concave and one convex

Question 5

how many curved surfaces do toric lenses have ?

Answer 5

. a toric lens also has two curved surfaces, but only one is spherical , the one which corrects for astigmatism is known as toroidal

Question 6

what direction is the axis ?

Answer 6

.the axis can be in any direction

.power is 90deg to axis

Question 7

what are the ways to write toric prescription in ?

Answer 7

. SPH/CYL form
. cross cyl form describes the prescription in terms of the axis of two cylinders
. toric lens form - describes the powers required on the front and back surfaces of the lens to give the correct prescription
. optical cross form - good for seeing where power lies and axis - knowing where power lies helps with frame choice , knowing where thickness of glasses lies

Question 8

how to go from cross cyl to sph/cyl form ?

Answer 8

. take one of your cross cyl powers to represent your sphere
. the difference between the sphere and the second cross cyl is the cylinder power
. the axis to be used in the sph/cyl form is the one that was not chosen as the sphere

e. g.
- 1.00 x 180
- 3.00 x 90

answer : -3.00 –1.00 = -2.00
-1.00/-2.00 x 90

Question 9

how to go from optical cross to sphere / cyl forms ?

Answer 9

sphere - take your first values as the sphere , if you want to work in minus cyl form then make sure this is the most positive

cyl - ask question - what do i need to do to get from this value to the next ?

axis- use axis direction of the first number

Question 10

what is the sphere curve is toric transposition ?

Answer 10

the curve on the spherical surface

sphere - base curve ( DS )

Question 11

what is the base curve in toric transposition ?

Answer 11

the lowest curve value ( closest to zero ) and flattest curve on the toroidal surface

sphere - sphere curve ( DC )
axis is perpendicular to cross curve axis

Question 12

what is the cross curve in toric transposition ?

Answer 12

the highest curve value ( furthest from zero ) and steepest curve on the toroidal surface

cylinder + base curve ( DC )
perpendicular to cross curve axis

Question 13

how to write toric transposition ?

Answer 13

. front surface F1 on top
this will be convex and therefore positive

. back surface F2 on the bottom
this will be concave and therefore negative
which ever is positive will be front surface

sphere curve (+)    /  base curve/cross curve (-)
base curve/cross curve (+)  / sphere curve (-)

Question 14

what is important to know when base curve is given ?

Answer 14

. transpose the prescription so that the sign of cyl is same as base curve

Question 15

what is important to know when sphere curve is given ?

Answer 15

transpose the prescription so that the sign of the cyl is opposite to the sign of the sphere curve

same base, opposite sphere