lecture(3) phoropter.ppt

Purpose
 Allow the examiner to change lenses
efficiently and easily.

Groups of control
A. lens controls: one for spherical and one for
minus cylindrical lenses.
 spherical lens control is large wheel at each
side of the phorpter .
 minus –plano- cylider control is controlled
by two knobs one for the cylinder power and
the other for axis power.

B. Auxiliary lens knob/Aperture control:
determines what the patient looks through
when behind the phoropter.

C. Ancillary units: there are two or three
ancillary lenses can rotate in front of the
aperture .
 jackson cross cylinders unit
 Rotary prism unit
 Madox rod

D. Adjustment : the phoropter contains
controls for adjusting the phropter to fit the
patient .
 PD knob
 Leveling knob
 Vertex distance control
 Pantoscopic tilt control

 PRELIMINARY STEPS OF REFRACTION:
1. Place the patient in a comfortable sitting position behind the
phoropter.
2. Adjust the IPD to centre the patients eyes on the lenses
3. Level the instrument to perfectly horizontal position.
4. Adjust the back vertex distance to 12mm.
5. Starting point of refraction is determined by objective refraction in
any of these ways:
a. Retinoscopy
b. Auto refractometer readings
c. Glasses power

 STEPS OF SUBJECTIVE
REFRACTION PROPER
1. To test baseline visual acuity.
2. Establishing spherical power.
3. Cylindrical axis refinement.
4. Cylindrical power refinement.
5. Spherical power refinement.
6. Binocular balancing.

1. To test baseline visual acuity.
Test the visual
acuity of each eye separately
using Snellen‘s chart.

2. Establishing spherical power.
Start from one line above the visual acuity
line.
Occlude one eye.
“Fog” the eye with a plus lens.
Start decreasing the power by 0.25D each
time and ask the patient which is better.
Stop when the patient no longer feels any
improvement.

Use the least minus or the most plus power.
(If we do not give the patient the least minus
power, we leave the patient to accommodate
which can cause asthenopia. Hence it is said
that treat minus power like money. Give it to the
patient only in return of something, ie. Better
vision.)

FOGGING
Fogging means to make the eye artificially
myopic. Because in a state of hyperopia, the
patient tends to accommodate.
Hence we put enough plus lenses in front of
the eye such that accommodation is relaxed
and the refractive error is stabilized.

3. Cylindrical axis refinement.
Two methods:
1. Jackson cross cylinder (JCC)
2. Astigmatic fan test.

Jackson’s cross cylinder:
 Used to refine the axis obtained by
objective refraction (OR).
Place the JCC over the cylindrical
lens.
The axes should be at 45 and 135
degrees (the axis line on the JCC).
After setting the axis of cylinder as
per the axis obtained in OR, flip the
JCC and ask the patient which
position is better?

In minus cylinder, chase the red line.
In plus cylinder chase the white line.
Continue this for every 15 degrees in one
direction till direction changes.
And then fine tune it for 5 degrees in the other
direction.

Astigmatic clock dial:
 Fog the eye.
 Ask the patient to look at the
astigmatic dial and identify the
darkest and sharpest line.
 Add minus cylinders
perpendicular to this axis
progressively till all lines
appear equally blurred (since
eye is fogged).
 This helps to find the axis as
well as power.

4. Cylindrical power refinement.
 After finding the axis, place the JCC in the power
axes.
 On flipping the JCC ask the patient which is better?
 If white- add plus power.
 If red- add minus power.
 Continue till both appear equally blurred.
 For every 1D cylindrical power added or reduced
0.5D of spherical should be added or reduced
respectively.

5. Spherical power refinement:
 After the astigmatism axis and power is refined
the sphere power is refined.
 This is done once again by fogging method.
 This is then confirmed by:
 Duochrome test.
 Pinhole test.

DUOCHROME TEST::
 It is based on principle of chromatic aberration.
 Due to difference in wavelengths, in emmetropes
yellow light focuses on the retina while red behind
and green in front.
 Hence to an emmetrope both the sides appear
equally clear.
 If red is clearer- under corrected myopia or over
corrected hyperopia.

PINHOLE TESTING:
 If improvement occurs using
the pinhole, the prescription
is incorrect.

6. BINOCULAR BALANCING
 To equalize the accommodative effort in both
eyes.
 Fogging and alternate cover test
 Duochrome test with fogging
 Prism dissociation test.

Fogging and alternate cover test:
 With the best accepted lenses in the frame both eyes
are fogged with a 1D lens.
 A rapid alternate cover test is performed.
 Patient is asked to tell which image is clearer.
 If the eyes are in balance the patient will report equal
blur.

Prism dissociation test:
 After fogging, a 3 or 4 PD prism is kept base
down in front of right eye and base up in front
of left eye.
 A difference between the clarity of the upper
and lower lines is used to correct the error.

lecture(3) phoropter.ppt

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lecture(3) phoropter.ppt