Measurement of lens power by Lensometer
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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
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
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
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
Measuring Lens Power
· Hand neutralisation by trial lens method.
· By Lensometer.
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
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.
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
Neutralization of lens power using trial lenses
Lens MovementsCross Target
“Against”“With”
Hand Neutralisation
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
Hand Neutralisation of Toric Lenses
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
Lensometer
· Measures the corrective lens power- Sphere- Cylinder and its axis- Prism.
· Optic center
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
Eyepiece Spring ClipTable
Lens Stop
Axis Dial
Power Wheel
Optic Centre Marker
Components of a Focimeter
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
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
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
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
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
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
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
Target system
· American optical system – crossed line target.
· European optical system – Ring dots system
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
Focimeter Preparation
· Focus the eyepiece· Ensure that all the readings at Zero.· Calibration
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
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.
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
Reticule scale
Prism dioptre delineation
0180
90
Focimeter - Ring of Dots Targets
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
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.
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
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.
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
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.
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
- +2.00---
- +1.00--
- 0.00---
- -1.00--
+1.00 / -2.00 x 120Power wheelSphere setting
Power wheelCylinder setting
Axis
Example - Line Targets
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
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
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
Focus of cylindrical lens
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
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.
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
2.0 Base Up
Vertical Prism
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
1.5 Base In
Horizontal Prism
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
3.0 up @ 150
Oblique Prism
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
Focimeter Use - Sources of Error
· Failure to focus the eyepiece· Zero setting & axis alignment· Centration of the reticule & target
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I
Projection & Automatic Focimeters
A R A V I N D - M A D U R A I A R A V I N D - M A D U R A I