OM t of c.iii - Vision Research Coordinating Center - Washington ...

2/22/96 Chapter 6 Vision Assessment page 6-6
Additional minus may result in no change or a degradation of the clarity of
these letters. When unfogging, monitor the patient's visual acuity with each
addition of -0.25 diopter until best visual acuity is achieved. This is
accomplished by having the patient read the smallest letters possible with
each addition of -0.25 diopter.
(5) Place the Jackson Crossed-Cylinder (JCC) in the phoropter for the eye being
tested. Isolate a line of letters 2 to 3 lines above threshold. Refine the cylinder
axis found in retinoscopy to an endpoint—when the patient reports equality
of visual acuity of the two JCC positions with the JCC axes 45 degrees on
either side of the endpoint axis.
(6) Following refinement of the cylinder axis, refine the cylinder power. The
endpoint for cylinder power refinement is the least minus cylinder for
equality of the two JCC positions or 0.25 diopter cylinder less than the
reversal power if equality is not obtained. Change the sphere component by
0.25 diopter of opposite sign for each 0.50 diopter cylinder change.
(7) Following cylinder testing, fog the eye by 0.75 diopter sphere and unfog in
0.25 diopter steps to the maximum plus sphere or least minus sphere which
allows the patient to read the most letters possible on the threshold visual
acuity line. Ask the patient to read the smallest line possible with each
addition of -0.25 diopter sphere until the threshold acuity is achieved. If a
+0.25 diopter sphere is added to the endpoint sphere, the patient will lose the
ability to read some or all of the letters.
(8) Occlude the tested eye, and repeat procedures three through seven on the
fellow eye.
(9) The refraction results are transferred to a trial frame for visual acuity testing.
For spherical equivalents exceeding ± 5.00 D, a vertex distance for each eye
behind the trial frame must be measured with a distometer and recorded.
If the sphere and/or cylinder value needed exceed that in the phoropter, the
manifest refraction must be accomplished using a trial frame.
6.3 Alternative Refraction Technique
If the patient’s manifest refraction visual acuity is compromised to an extent
where he or she does not respond to small changes in lens power, a determination of
what interval of lenses (the Just Noticeable Difference or “JND”) to use should be made.