5: How does the JCC work?

what formula is used to break down a sphero cylindrical Rx into meridional powers

meridional power= S + ( C x sin²feta)

S is sphere power in D

C is cylinder power in D

when feta is 90 degrees, sine 90 = 1² = 1

so meridional power is S+C

when feta is 45: sin (45) = 0.7071²= 0.5

graphical representation of lens power… spherical lens

a -2.00 spherical lens

along every merdian it shows a power of -2.00DS which is shown as a straight line

graphical representation of lens power… cylindrical lens

plano cylinder

at 180 its power is 0 which is shown at the top right and left, this is represented as the S

at 90 degrees, in middle of graohy the power is -2.00D which is represented as the S+C

graphical representation of lens power: sph/cyl lens

along 160 the power is -0.50

along 70 the S+C is -2.25 (cyl alone is -1.75)

-3.00/-1.00 × 155

along 155 the eye is myopic by -3.00

along 65 the eye is myopic by -4.00

but need to know what is the ametropia along 90 degrees, or 120 degrees in an eye with principle meridians of 65 and 155, to know how JCC works - doe using the meridional power equation

shows ametropia at different meridians

to check accuracy of when placing the -3.00/-1.00 × 155 lens in front of the eye the dots must allign for a residual refractive error of 0 which indicates that the eye is fully corrected

using graphical rep of ametropia and lens power to understand how JCC works

-3.00/-0.75 × 170 starting point

-3.00/-0.75 × 155 end point of axis determination

residual ametropia- difference between actual ametropia in each meridian and the combined power of the jcc and trial frame lenses in each meridian

cylinder axis refinement for -3.00/-0.75 × 170

handle along axis of trial frame cylinder

in position 1 the JJ: +0.25/-0.50 × 125 : this is because the red markings are 45 degrees awar from the cyl axis so 170-45 = 125

in position 2 the JCC is +0.25/-0.50 × 35 - which is 45 degrees away from the cyl axis on the other side

combined power of the rial frame lenses and JCC

• in position 1

lens in trial frame: -3.00 /-0.75 × 170

JCC: +0.25/-0.50 × 125

• in position 2

lens in trial frame: -3.00/-0.75 × 170

JCC: +0.25/-0.50 × 35

examine whether more of ametropia is corrceted with postiion 1 or

what is the combined power of the following 2 lenses

+1.00/-1.00 × 160

+0.50 /-0.75 × 160

principal meridians are the same

so add sph and the cyls

combined power = +1.50/-1.75 × 160

also known as the resultant

add together the following 2 lenses

-3.00/-0.75 × 170

+0.25/-0.50 × 125 ( JCC in position 1)

done via astigmatic decomposition

astigmatic decomposition

based on dividing up a prescription so that it can be represented by 3 terms

1. M

2. J180

3. J45

what is M

S + C/2

J₁₈₀

-0.5 x C x cos(2A)

A being the axis

take the axis of the cylinder then double this

then take the cosine of this vale. then multiply it by minus half of the cylinder

J₄₅

-0.5 x C x sin(2A)

astigmatic decomp of +4.25/-1.50 × 20

M= +4.25 + (-1.50/2) = +3.50D

J₁₈₀ = -0.5 x ( -1.50 ) x cos (40) = + 0.575D

J₄₅ = -0.5 x (-1.50) x sin (40) = +0.482 D

a JCC with the minus axis lined up with 180 gives

M= plano

J180 = +0.25 as its axis is -0.25 so power along this is +0.25D

J45= plano

a JCC with the minus axis lined up with 90

M= plano

J180= -0.25

J45 = plano

a JCC with minus axis lined up with 45

M= plano

J180= plano

J45 = +0.25

a jcc with minus axis lined up with 135 gives

M= plano

J180 = plano

J45= -0.25

resultant values of astigmatic decomp

need to separately work out the

resultant sphere

resultant cylinder

resultant axis

resultant sphere

take the sum of all the J180 terms and square it and do the same for the J45. Add the two together and take the square root of this and add it to the sum of the M terms

resultant cylinder

take the sum of all the J180 terms and square it. do the same for the J45 and add the two together, take the square root of this and multiply it by -2

resultant axis

divide the sum of all the J45 terms by the sum of all the J180 terms; clc 0.5 the arctan of this

express the combination of the following lenses

+4.25/-1.50 × 20

-2.50/-1.00 × 10

sum of the components

M= +3.50 + - 3.00 = +0.50

J180 = +0.575 +0.470 = 1.045

J45 = 0.482 +0.171 = +0.653

converting the decomposed terms to standard clinical notation

resultant sphere

M = +0.50

J180 = +1.045

J45= +0..653

= +1.732D

converting decomposed to standard

resultant cylinder

= -2.464D

resultant axis decomp to standard

so the resultant prescription from combination of +4.25/-1.50 × 20 & -2.50/-1.00 × 10 is =

+1/73 /-2.46 × 16

steps in astigmatic decomp to determine the combined sph/cyl Rx of 2 or more sph/cyl lenses

what is the sum of

-3.00/-0.75 × 170

+0.25/-0.50 x 125 (JCC in position 1)

= 2.92 /-0.90 × 153.2

this is being offered to correct ametropia of -3.00/-1.00 × 155

powers are similar so does qell in terms of correcting the eye

residual error is still not 0

position 2:

-3.00/-0.75 × 170

+0.25/-0.50 × 35

= -2.92/-0.90 × 6.8

this is being offerred to correct ametropia of -3.00/-1.00 × 155

lost of residual error

so px more likely to prefer posotion 1 as residual error closer to 0 than position 2

what is done after woking out that position 1 was preferred

rotate the cylinder axis towards the negative axis markings of the jCC

moved by 20 degrees

so it is now -0.75 × 150

power oc cyl= rotated by certain by amount of degrees

re position the JCC handle along the trial frame cylinder axis

handle now at -0.75 × 150

at position 3: -3.00/-0.75 × 150

JCC: +0.25 /-0.50 × 105

position 4:

trial frame lenses -3.00/-0.75 × 150

JCC= +0.25/-0.50 × 105

work out astigmatic decomp again to work out which position would be better

position 3 & 4

square dots correspond to the refractive error of the eye

blue dots correspond to the refractive error being delivered to each meridian

px would prefer posiion 4 as much less residual error, still varying but less than position 3

what is done after working out position 4 was better

rotate the trial cylinder towards negatuve markings of JCC by 10 degrees -0.75 × 160

now at position 5:

trial frame lenses; -3.00 / -0.75 × 160

JCC: +0.25 / -0.50 × 115

position 6:

-3.00 /-0.75 × 160

JCC: +0.25 / -0.50 × 25

positions 5 & 6

position 5 is preffered as less residual ametropia

both getting close together

what is done now that we know positoin 5 was preferred

rotate the trial cylinder towards the negative axis of JCC by 5 degrees

now at -0.75 × 155

position 7;

-3.00/-0.75 × 155

+0.25 /-0.50 × 20

position 8:

-3.00 /-0.75 × 155

+0.25 / -0.50 × 110

position 7 & 8

px will repsond saying they look the same

leave the axis in the same position

cylinder end point is -0.75 ×155

still have residual error but this is why we correct the cyl power

checking the cylinder power

axis markings of JCC ( principal meridians) are lined along the axis of trial frame cyclinder

red markings along axis= want more minus power

position 9 & 10

dont need to work out astigmatic decomp as the principal meridians are established from cylinder axis

px prefers postion 9 as the dots are positioned closer to 0 and fluctuates less

so would give more minus as 9 is where the red markings are lined up with the trial axis

position 11 and 12

given -1.00 as in position 9 and 10 the px preffered the minus

px says there is no preference nd it is the same

so leave it as a -1.00 × 155

JCC when no cylinder is in place after ret

JCC held at 90 and 180: px will prefer to 180

JCC then held at 45 and 135 and px prefers when jcc held at 135

so cyl axis is placed between 135 and 180 and start with a -0.25