Subjective Refraction- Refining Cylinder Flashcards
What must you ensure about the fellow eye when refining cylinder
The fogging of the fellow eye should remain in place and, for the purpose of the Refraction Certificate examination, if demonstrating subjective refraction of the cylinder only, adequate fogging must first be ensured.
What ophthalmic instrument is used to fine tune the cylindrical prescription?
the Jackson cross cylinder (JCC)
What is the JCC made up of?
a sphero-cylindrical (toric) lens in which the power of the cylinder is twice the power of the sphere and of the opposite sign. The JCC is equivalent to superimposing two cylindrical lenses of equal power but opposite sign with their axes perpendicular to each other.
Where is the handle of JCC located
The handle of the JCC is 45 degrees to the axes of the cyls. Since there are two perpendicular opposing cyls, an axis for the JCC is not denoted. The spherical equivalent (equal to the sphere plus half the cyl) of a JCC is therefore zero.
What is a 0.50 JCC consisting of?
If a 0.25 cyl is superimposed perpendicularly with a +0.25 cyl ,the net result is equivalent to -0.25/+0.50 (when transposed equivalent to +0.25/-0.50). This would be a 0.50 JCC, since the JCC is defined by the power of the cyl notation.
When to use which JCC
The 0.50 JCC is used if acuity is 6/12 or better, whereas the 1.00 JCC is used if acuity is worse than 6/12
How to identify a 0.50 JCC
0.50 JCC will have +0.25 written on the lens and, perpendicular to this, ~0.25 will be denoted.
How to identify a 1.00 JCC
1.00 JCC will have +0.50 written on the lens and, perpendicular to this, -0.50 will be denoted
How to check the cyl axis (as established by retinoscopy) with JCC
Hold the handle along the proposed plus axis.
Ask the patient to look at the letter O (or other types of circular targets such as two double rings).
Ask them:
Does the O look rounder and clearer with lens 1 [position 1- handle along axis] or lens 2 [position 2 - twist 180 degrees] or about the same?
If patient reports both sides are equally bad when checking cyl axis with JCC what does this mean?
this should be interpreted as meaning that position 1 is the same as position 2
For cyl axis refining, when working in plus cylinder if the patient prefers position 1 what do you do next?
rotate the cyl so the axis moves towards the plus cyl of the JCC when in position 1
For cyl axis refining, when working in plus cylinder if the patient prefers position 2 what do you do next?
rotate the cyl so the axis moves towards the plus cyl of the JCC when in position 2.
How much cyl rotation is needed when fine tuning the cyl axis?
depends upon the acuity and the strength of the cyl. If acuity is already good, only move the cyl by small amounts to avoid losing the good acuity. If the cyl is large, avoid large movements, since only a couple of degrees of movement of a large cyl can make quite a difference.
What is the bracketing technique with cyl axis refining?
you initially move the axis by 20 degrees, then re-check and move by 10 degrees, then 5 degrees, then 2 degrees to reach the desired end point. Never underestimate the importance of obtaining the correct axis for a high-powered cyl.
What does it mean if the patient reports position 1 is the same as position 2 in cyl axis refinement?
an end point has been reached and a satisfactory axis has been obtained. Now move onto refining the cyl power.
