CLINICAL REFRACTION

At the end of this lecture, students should be able to

✓ Define Clinical Refraction
✓ Describe the principle on which clinical refraction is based
✓ Outline the common techniques employed in clinical refraction
✓ Distinguish between Objective and Subjective refraction

Introduction
Clinical refraction is a process that is used to determine the optical power of a patient’s eye. The process allows
the examiner to arrive at one individualized prescription, from the universe of approximately 200,000 possible
prescriptions. The goal of refraction should be stated that it is both visual and functional: it is to identify the
lens that will allow the patient to achieve clear and comfortable vision, to which he/she will adapt rapidly, and
that will do no harm to the patient.

Principle
The principle used in clinical refraction is to render the retina conjugate with optical infinity through the
application of lenses in front of the eye.
Common technique

The determination of the refractive state of the eye involves both Objective and Subjective Refraction.
In objective refraction, the examiner determines the refractive state of the eye on the basis of the optical
principles of refraction without the need for subjective responses on the part of the patient.
In Subjective refraction, the examiner determines the refractive state entirely on the basis of the patient’s
subjective responses.

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The classical methods of objective refraction are keratometry and retinoscopy. In keratometry, the refracting
power of the cornea is determined in each of the two principal meridians. Keratometry therefore provides the
practitioner with information about the astigmatism of the eye but no information about spherical ametropia
(myopia or hyperopia). Retinoscopy provides the practitioner with information concerning both spherical
ametropia and astigmatism. There are various methods of retinoscopy depending on what is aimed to be
achieved.
Auto refractor has also been developed to objectively provide the optical power of the eye. The measurement
procedure basically involves sitting the patient comfortably at the machine, aligning the instrument with the
patient’s visual axis, using a monitor and pressing a button. The result can be printed out.
An objective measurement of refractive error is the only assessment available in patients who are unable to
cooperate in a subjective refraction, such as young patients. It is heavily relied upon when subjective responses
are limited (patients who do not speak the same language as the examiner) or unreliable (malingerers). In more
routine patients, it provides an objective first measure of refractive error that can be refined by subjective
refraction.

RETINOSCOPY

At the end of this lecture, students should be able to

✓ Define Retinoscopy
✓ List the types of Retinoscopy
✓ Describe the procedure for Static Retinoscopy

Introduction
Retinoscopy is an objective method of measuring the optical power of the eye. We use a retinoscope to
illuminate the inside of the eye and to observe the light that is reflected from the retina. These reflected rays
change as they pass out through the optical components of the eye, and by examining just how these emerging
rays change, we determine the refractive power of the eye. We describe retinoscopy as objective because we
evaluate the eye as an optical instrument, initially ignoring any information the eye transmits to the brain. Thus,
retinoscopy does not depend on the patient’s vision or judgment.

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The majority of retinoscopes in use today employ the streak projection system developed by Copeland. For
simplicity, we will confine this discussion to popular streak retinoscopes. Modern retinoscope have the
following the light source, condensing lens, mirror, focusing sleeve, current source (it could be dry or
rechargeable cell). Simply slide the sleeve of the instrument up or down to move the bulb. Moving the sleeve
up creates the plane mirror effect; while moving the sleeve down produces the concave mirror effect (this could
be reverse depending on the manufacturer).
Types of Retinoscopy
Below are some of the types of retinoscopy
1. Static retinoscopy
2. Dynamic retinoscopy
3. Cycloplegic retinoscopy
4. Mohindra retinoscopy

ASSIGNMENT: Read up the different types of retinoscopy

Points to note
• The retinoscope should rest firmly against your brow or spectacle frame, so you can keep the retinal reflex
in the peephole aligned with your pupil while you manipulate the scope.
• During retinoscopy, we observe the movements of the fundus reflex. In order to move the projected streak
across the fundus, you have to wiggle the scope. The streak is always moved perpendicular to its axis. For
example, when you place the streak axis vertical, you move it sideways. When you place the axis
horizontal, you move it up and down.
• Looking through the peephole in your retinoscope, you see these emerging rays as a red reflex in the
patient’s pupil. If you sweep the streak across the eye, the reflex you see will also move. If the emerging
rays have not converged to a point (the FP), the retinal reflex will move in the same direction as you move

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 the streak; this is called the with motion reflex (WITH). If the rays have come to the FP and diverged, the
reflex will move opposite to your movements; this is the against motion reflex (AGAINST)

Procedure (Static Retinoscopy)

1. Set the patient’s distance PD in the phoropter or trial frame. Position the phoropter or trial frame before
the patient so that the lenses will be in the patient’s spectacle place and make sure it is level.
2. Either:
a. Dial in the +1.50D retinoscope lens into the phoropter or place a working distance lenses in the
back cells of the trial frame (+2.00D for a 50cm working distance). This technique has the advantage
of relaxing accommodation for young patients or hyperopes.
b. Do not add a working distance lens. The working distance power (+1.50D or + 2.00D) usually
must later be subtracted from your final retinoscope result.
3. Switch on the Duochrome or a similar large target that is easy to see when blurred and does not provide a
stimulus to accommodation
4. Explain the test to the patient: I am going to shine light into your eye. Please do not look at the light.
Don’t worry if the chart is blurred.
5. Switch the room light off.
6. Hold the retinoscope with your right hand and observe with your right hand to examine the patient’s right
eye.
7. Sit at the same level to the side of the patient so that manipulation of the trial frame/phoropter is easy.
8. Position the streak so that it is vertical. Look through the aperture of the retinoscope and direct the light at
the patient’s pupil and you should see the red retinoscope reflex. Swipe the retinoscope streak across the
patient’s pupil and compare the movement of the reflex in the pupil with the movement of the retinoscope.
9. If you see WITH, add plus lenses. If you see AGAINST, add minus lenses.
10. Determine if the refractive error is spherical (the observed reflex has the same direction, speeds, brightness
and thickness in all meridians) or astigmatic (the reflex differs in different meridians).

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11. Determine the spherical component by ‘neutralizing’ (adding plus lenses to ‘with’ movement and minus
lenses to ‘against’ movement until the reflex fills the entire pupil and all perceived movement stops).
12. If there is astigmatism present. You will observe that neutralizing one meridian still leaves the other
meridian not neutralized.
a. You can continue to add a spherical lens until you get to the point of neutrality.
b. You can add a cylindrical lens to neutralize the second meridian. Make sure to put the axis of
the cylinder in same alignment with the streak.
13. Repeat the steps for the left eye
14. Measure the patient’s VA with the net retinoscopic results.

Recording
O.D -2.00/-1.00 x 90
O.S -2.25DS

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 KERATOMETRY

At the end of this lecture, students should be able to

✓ Define the process of the keratometry
✓ List the design of the Keratometer
✓ Outline the limitations o the keratometer
✓ Record Keratometric readings
✓ Calculate astigmatic power and axis for Keratometric readings

Introduction
Keratometry is an objective means of measuring the anterior cornea curvature. The goal or purpose of
keratometry is to assess the curvature, power and toricity of the cornea. Keratometry may also be used to assess
the integrity of the cornea/tear surface.

History of keratometer
The name of the instrument used is the Keratometer. The Keratometer is the most widely used instrument for
measuring the curvature of the anterior corneal surface.

Types of keratometer
There are basically two designs
▪ One position instrument – Bausch and Lomb (B&L) Keratometer
▪ Two position instrument – Javal Schiotz keratometer

B & L Keratometer Javal Schiotz keratometer

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Limitations of the Keratometer includes
• Assumption that cornea is symmetrically spherical or spherocylindrical
• Measures small area of the central cornea (3--‐4mm diameter)
• Less accurate measuring irregular surfaces: very flat or steep cornea, irregular astigmatism
• Does not quantify irregular astigmatism
• One--‐position instruments assume regular astigmatism
• Approximate focal point and refractive index used in calculation
• The use of paraxial optics to calculate surface power
• Limitations In detecting peripheral or Posterior keratoconus

Principle
The original keratometer was designed on the basis of the imagery of Ramsden and the doubling principles of
Helmholtz. It was used as a physiological optics instrument. The modifications by Javal and Schiotz in 1881
made it suited for clinical use.
The relationship which exists between the object size and image size of a convex mirror comes into play in
keratometry. Considering the following:
In keratometry, the cornea acts as the convex reflector and the object used is the Keratometer mire. When the
mire is illuminated the image can be seen on the cornea. Direct measurement of image size could have been
possible but for the smallness of the image and the micro eye movements taking place. Therefore, another
technique is used to determine the size of the image. This involves doubling the image and measuring it through
a telescope.

The mires which act as the object are places such that the image is received within the annular zone 3.0 mm in
diameter at the apex of the cornea. The size of the image is about 3.00mm and it is formed 4.00mm behind the
surface of the cornea. This image becomes a new object for the telescope system which magnifies it 1.304x.
The eye piece of the telescopes magnifies it further to 6.197x. Doubling is aided by the use of prisms and the

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separation of the doubled image depends on the distance of the doubling prisms from the objective of the
telescope.
The centers of the separated images are however fixed such that the gap between them is determined by the
size of magnification of the images. In a flatter cornea where the images are magnified they overlap, but in
steeper cornea where the images are minified, the images are separate. What is desired for measurement is a
point where the images just touch each other. Either the object size is varied to achieve this or the distance of
the doubling prism from the objective is varied and in each situation, the magnification of the image is altered.
In the first situation where object size is varied, the size of the object required to produce a constant image size
is measured. In the second situation where the distance of the doubling prisms is varied, the size of the image
required to achieve constant object size is measured.

Procedure
B & L Keratometer
1. Seat the patient comfortably in front of the Keratometer, and ask them to remove any spectacles. Sit
opposite the patient, across the instrument table and dim the room lighting.
2. Explain the procedure to the patient
3. Adjust the eyepiece of the instrument by directing the telescope to a distant object, turning the eye piece
anticlockwise as far as it will go and then turning the eyepiece clockwise until the black crosshair just
comes into sharp focus.
4. Adjust the height of the patient’s chair and the instrument to a comfortable position from both you and the
patient. Ask the patient to lean forward and place the chin in the chin rest and forehead against the head
rest. Occlude the eye not being tested by swinging the instrument’s occluder into place. Then adjust the
chin rest so that the outer canthus aligns with the head rest marker.
5. Ask the patient to look at the image of their own eye in the centre of the instrument and to open the eye
wide after a full blink. If a high refractive error prevents patients from seeing their own eye, then ask them
to look down the centre of the instrument.
6. Align the instrument so that the lower right mire image is centred on the cross hairs, and lock the instrument
into place.
7. Adjust the focusing of the instrument by turning the focusing knob until the mires are clear and the lower
right mire is no longer doubled
8. Measure the principal meridian that is closest to the horizontal first. Rotate the instrument so that the plus
signs are set ‘in step’.

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9. Measure the second principal meridian. Adjust the focusing knob to give the best focus for the minus signs
and then adjust the vertical alignment when until the minus signs are super imposed.

Javal Schiotz Keratometer

1. Set up the patient and the instrument as described above.
2. Move the telescope forward by adjusting the focusing knob appropriately. You may need to make minor
adjustment both horizontally and vertically to centre the mire images.
3. Ask the patient to blink and then keep the eyes as wide open as possible. Turn the knurled knob situated
below the arc until the staircase and block mires are just touching. You must simultaneously adjust the
instrument position to maintain focus of the mire images.
4. Adjust the position of the mires until they are just touching with no overlap.
5. Read off the angle of the arc from the degree scale of the instrument and the radius of curvature along this
meridian from the millimeter scale.
6. Turn the arc 900 and make necessary adjustment as described above.

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Recording
The results can be recorded with the radius of curvature of the horizontal meridian first followed by the
vertical as follows
O.D. 7.75 @ 175 / 7.60 @ 85
O.S. 7.70 @ 180 / 7.60 @ 90

Radius of curvature can be converted into corneal power using the formula
k = (n-1) / r
where, k = cornea power
n = refractive index of the cornea = 1.3375
r = radius of the anterior corneal surface (in metre, m)
so, k = (1.3375-1)/r = 0.3375/r
Alternatively, the results can be recorded in diopters,
O.D. 42.00 @ 175 / 43.75 @ 85
O.S. 43.50 @ 180 / 44.25 @ 90
The difference between the two powers equals approximate corneal astigmatism. Flatter meridian represent
the axis of minus cylinder power.
So for the recorded value above the corneal astigmatism is
OD 1.75 x 175
OS 0.75 x 180

KERATOMETRY
Lower Power @ High Power @ Corneal cyl
OD 42.00 @ 175 43.75 @ 85 1.75 x 175
OS 43.50 @ 180 44.25 @ 90 0.75 x 180
Base Curve HVID
OD 7.75 @ 175 / 7.60 @ 85
OS 7.70 @ 180 / 7.60 @ 90

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