measurement of lens power by lensometer

ARAVIND EYE CARE SYSTEMAravind Eye Hospital

& Postgraduate Institute of OphthalmologyMadurai, India

ARAVIND EYE CARE SYSTEMAravind Eye Hospital

& Postgraduate Institute of OphthalmologyMadurai, India

Measurement of lens power by Lensometer

FacultyAravind School of Optometry

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Corrective lenses

Lens type Lens form Corrective useSphere Convex ( + )

Concave (--)HypermetropiaMyopia

Cylinder & Sphero-cylinder

Convex ( + )Concave (--)

Hyperopic Astigmatism Myopic Astigmatism

Prism · Correcting squinting eyes· Relieving eye strain

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Measuring Lens Power

· Hand neutralisation by trial lens method.

· By Lensometer.

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Trial lens method

· View cross target at distance

· Hold lens on visual axis, close to eye.

· Align lens such that cross target is continuous.

· Move lens vertically along line of vertical limb of target

· Determine direction of horizontal limb movement

· Place trial lens flush with ‘unknown’ lens

· No movement = neutral

· Repeat the same for line of horizontal limb of target.

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Neutralization of lens power using trial lenses

Lens MovementsCross Target

“Against”“With”

Hand Neutralisation

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Hand Neutralisation of Toric Lenses

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Lensometer

· Measures the corrective lens power- Sphere- Cylinder and its axis- Prism.

· Optic center

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Eyepiece Spring ClipTable

Lens Stop

Axis Dial

Power Wheel

Optic Centre Marker

Components of a Focimeter

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Constructing a Focimeter

Zero Position Target at first principal focus

With Positive Power Lens Unknown lens at 2nd principal focus Distance between standard lens &

target is reduced

Telescope

Lens holder for unknown lens

Target

With Negative Power Lens Unknown lens at 2nd principal focus Distance between standard lens &

target is increasedStandard lens

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xx fofo f’of’o x’x’

Constructing a Focimeter

Telescope

Lens holder for unknown lens(holding negative lens in figure)

Standard lensLight Source& Moveable Target

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Calculation

· Movement of the target per dioptre

Fo = +25.00D

Target travel (mm) per dioptre:

x=1000 / Fo² = 1000 / 25² = 1.6mm per Dioptre

So, for a focimeter required to measure 20Dthe total required travel of target = 40 x 1.6 = 64mm

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Target system

· American optical system – crossed line target.

· European optical system – Ring dots system

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Focimeter Preparation

· Focus the eyepiece· Ensure that all the readings at Zero.· Calibration

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Line Target

1

3

5

Reticule scale

Prism dioptres delineation

Focimeter – crossed line target system

· Insert the spectacle.

· Determine the lens power.

· Mark the optical centre.

· Measure the power of the second lens.

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Reticule scale

Prism dioptre delineation

0180

90

Focimeter - Ring of Dots Targets

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Procedures· Focus the eyepiece by focusing the hair-line black reticule.

· Place the back vertex (the ocular side) of the unknown lens against the lens stop of the lensometer.

· Make sure both eye wires of frame, right & left, are touching the stage.

· Move the lens side to side to align with center the target that places the optical center of the lens at the stop.

· Notice that the target lines are a cross composed of two sets of lines oriented 90 apart.

· Try focusing the target lines.

· If all lines are in focus simultaneously, the lens is spherical.

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How to measure the cylindrical power and axis?

· If only one set is in focus (the other set 90 away is blurred), the lens is cylindrical.

· If this is the case, focus one set of lines with the power drum, while simultaneously

rotating the axis wheel so that the lines are not only clear, but they are also unbroken.

· Note the power on the drum.

· If the unknown lens is cylindrical, the set of lines 90 away will be out of focus.

· The power of the first set of clear lines is called the sphere reading.

· Now focus the other set of lines (90 away from the first set) so that they are clear and

unbroken.

· Note the new power on the drum & also note the axis on the outside wheel.

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How to measure the cylindrical power and axis?

· If the 2nd set of lines was focused at a lower + (more --) power, you have measured in

‘—’ cylinder form.

· Since our convention is ‘—’ cylinder form, get used to making the less ‘—’

measurement in 1st measurement. That would mean that the 2nd measurement would

be more +, thus allowing to record the reading in ‘—’ cylinder form.

· In this case (minus cylinder form), the first measurement is the sphere, and the

increase, or change in minus power you had to travel to the second, (higher minus

power) focus is the cylinder power.

· Record the values.

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- +2.00---

- +1.00--

- 0.00---

- -1.00--

+1.00 / -2.00 x 120Power wheelSphere setting

Power wheelCylinder setting

Axis

Example - Line Targets

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0180

90

0180

90

- +2.00---

- +1.00--

- 0.00---

- -1.00--

+1.00 / -2.00 x 120

Axis

Power wheelSphere setting

Power wheelCylinder setting

Example - Ring of Dots Targets

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Focus of cylindrical lens

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Measure the bifocal add

· Lift the front of the lens against the lens stop.

· Take a reading of one set of unbroken, focused lines through

the distance portion, then move the stage up until the bifocal

add is against the stop.

· Re-focus the same lines again, but in segment.

· The difference between the 1st & 2nd reading is the add power.

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2.0 Base Up

Vertical Prism

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1.5 Base In

Horizontal Prism

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3.0 up @ 150

Oblique Prism

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Focimeter Use - Sources of Error

· Failure to focus the eyepiece· Zero setting & axis alignment· Centration of the reticule & target

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Projection & Automatic Focimeters

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