refraction using a phoropter

REFRACTION USING A

PHOROPTER

A phoropter is a common name for an ophthalmic testing device, also called a refractor. It is commonly used by eye care professionals during an eye examination, and contains different lenses used for refraction, to measure an individual's refractive error and determine his or her eyeglass prescription.

PARTS OF A PHOROPTER

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

lenses3. 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. Retinoscopyb. Auto refractometer readingsc. 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 off 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 stabilised.

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 cylinderchase 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 focusses 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 BALANCINGTo equalize the accommodative

effort in both the 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.

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