Jackson cross cylinder practical 6 Flashcards
what is the JCC lens made of ?
- combination of two cylinders with their axis crossed at right angle to each other
how far are the axis in the JCC to the handle ?
- axis of the two crossed cylinders are 45deg to
handle - axis always at 45 deg to handle
what are the two types of JCC ?
astigmatic lens : +0.25/-0.50
comes in other powers
higher powers= bigger changes in astigmatism and as you alter axis - test if subject prefers steeper or shallower axis
what does axis depend on ?
axis depends on orientation - how you hold the handle
how do you hold JCC ?
- when checking power axis marking of cross CYL are along your cylinder axis
- when checking axis of correcting cylinder you hold with the handle along axis direction
what is BVS of JCC ?
0D
we are not changing sphere component
we are changing the magnitude and axis of astigmatism
why is JCC preferred method for refining cylinder power and axis ?
. can change correcting cylinder axis and power without changing lenses
. provides quicker and easier way to carry out testing
what do you have on the cross-cyl ?
. you have axis marking not directions which power acts
how does JCC work ?
. JCC work with standard cyl notation that the axis is the direction along which the power does not acts
. along positive axis we get negative cyl power and along negative axis we get positive cyl power
how to investigate cyl power ?
- assumes emmetropic or corrected eye
- put -1.00DC x 45 we create with this lens simple hypermetropic astigmatism and with axis at 45 both horizontals and verticals will be blurred
- accommodation should bring DLC on to the retina -
- we would expect a loss of 1-2 lines based on what we know about the effect of astigmatism on vision
when we use the cross-cyl
- hold 0.50 cross cyl , such that negative cyl axis is along the 45
- effect of cross cyl is to add 0.50 of power along 45 - remember negative cyl axis is where positive power is acting
- the negative power of cross cyl acts along 135 -
- 1.00 becomes -1.50
astigmatism is +0.50 - -1.50 = 2.00D
what happens when you twirl the cross cyl ?
-1.00DC x 45
.negative cyl axis along 135 and positive cyl axis along 45
- along 45 where negative cyl power is acting it goes from 0.00 to -0.50
- along 135 where we have to + 0.50 to -1.00 = -0.50
- difference is zero
what is the conclusion observed when changing cyl power ?
- cross cyl when held with its axis aligned with the axis of correcting cylinder has the ability to increase or decrease its power
. 0.50 cross cyl will increase or decrease a power by 1D
. 0.25 cross cyl will increase or decrease power by half D
how does cross cyl change the axis of correcting cyl ?
- assumes emmetropic eye
- prescription in trial frame generates 0.75 D of simple myopic astigmatism axis 180
- advantage of creating simple myopic astigmatism we can generate one clear direction
- must hold cross cyl with handle along the axis of correcting cyl
- in theory vertical direction of fan chart must appear clearest - line blur as you move away - if no then possible uncorrected astigmatism , misalignment of cyl axis
what happens when you hold cross cyl along the axis of cyl in trial frame ?
- the effect is to rotate the axis of the astigmatism
- flipping the cross-cyl alternately steepens and flattens the axis of the astigmatism
what does the magnitude of the axis shift depend on ?
- the magnitude of the axis shift depends on the magnitude of the cross-CYL and the magnitude of astigmatism
- higher power of cross CYL = greater axis shift
what happens if you have weaker correcting cyl in the trial frame ?
- bigger axis shift
what is is the effect of lower cyl powers compared to higher cyl powers ?
- lower CYL powers you make relatively large changes in axis but for higher CYL powers you make small
- visual deficit for incorrect axis when cylinder power goes up is much tighter - people respond much more quickly to visual deficit if there is an error in axis
what does obliquely crossed cylinder show ?
- SPH and CYL and axis are not completely independent
- when make a change for one of them you will get changes for all of them
how determine unknown astigmatic error with the help of cross CYL ?
- subject should’ve had astigmatic error
- must find BVS using ret or a letter chart
- we now have mixed astigmatism and DLC fairly close to the axis
- ask subject to look at target
- we try to determine astigmatic axis for correction - hold cross cyl with horizontal axis and ask subject to look at target and ask ( is it more clear in position1 , position 2 or same )
- if position 1 which is when negative axis of JCC is along 135 - this will tell us that negative correcting cyl for that astigmatic prescription is in the quadrant 90-180
- if position2 which is when negative axis of JCC is along 45 - this will tell us that negative correcting cyl for that astigmatic prescription is in the quadrant 0-90
- same - will tell us that axis is close to the boundary
which is 90 or 180 - in the next stage hold the JCC handle along the oblique axis 45 or 135
- ask px to look at target again ( position 1 or position 2 or same )
- if position 1 ( quadrant 90-180 ) vertically - negative correcting CYL in the middle of 90-135
- if position 2 ( quadrant 90-180 ) horizontally - negative correcting cyl axis is in the middle of 135 and 180
how to estimate power of correcting CYL ?
. DLC is on retina so can use our knowledge that astigmatism has half the effect of sphere
. 1D sphere loses you 3-4 lines , the 1D of astigmatism is half that 1-2 lines
. use negative cyl
what is refining axis based on ?
- the basic rule - if there is a preference from your subject in one of the positions of the cross cyl you rotate the axis negative correcting cyl towards that direction
- must hold cross cyl handle along the axis of correcting cyl even when it moves
- when you change direction you halve the step you are making in terms of axis
- refining axis is based on a staircase approach - at each reversal halve the change to home in on your target value
-change the axis by an amount appropriate to the x- CYL power and correcting cylinder power
what is summary of experiment ?
- magnitude of the cross-cyl affects the change in astigmatism when checking DC
- magnitude of cross-cyl and correcting cyl affect axis change when refining axis
higher correcting cyl = smaller axis change - axis change is based on obliquely crossed-cyls
- the JCC is normally used following ret although it is neat to see that we can start without a ret result by using our knowledge of visual optics
- in this experiment we haven’t changed SPH to keep DLC on retina because accommodation will do that for us
