Lensometer

Optics of Lensometer
MD . Azizul Islam
Junior Optometrist
Oculoplasty Department
IIEI&H

Lensometer is also known as Focimeter or avertometer
(The main use of the lensometer is to measure the back
or front vertex power of a spectacle lens.
It is a device designed to measure the refractive power
prescription of a unknown lens. (spherical, cylindrical
with axis and also the optical center)
Types:-
 Manual
 Automated
LENSOMETER
Manual
Automated
N.B: The lensometer works on the Badal principle with the addition of
an astronomical telescope for precise detection of parallel rays at
neutralization.
The Badal principle is Knapp’s law applied to lensometers. IIEI&H

 The Badal principle (Ogle, 1968; Southall, 1964) is based on the observation that if the eye is placed at the
focal point of a positive lens, the virtual image of an object located between the lens and the anterior focal
point will always subtend the same visual angle.
 Badal optics are disclosed in which an aperture array at an optical system under test is interrogated for the
deflection of light between a detector array conjugate with the aperture array of the optical system under test
being examined. Excursion is measured in a plane normal to the axis of the Badal optics instead of observing
towards and away image focus along the axis of the Badal system.
 In the case of an objective refractor, the eye is illuminated at the retina with a test spot, preferably light in the
infrared. An image of a detector array is relayed by a relay lens through the Badal optics to the cornea of the
eye, the lens under test. Light emanating from the test spot on the retina passes through the eye lens at the at
the image of the detector array. This light undergoes excursion in accordance with the power of eye lens under
test at each aperture of the detector array.
 The extent of this excursion is determined by a moving boundary locus between the image of the detector
array and the detector array itself. This excursion is related directly to the eye prescription.
Badal principle
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General Description of the Lensometer
1. The eyepiece
2. Chrome Knurled Sleeve
3. Prism Compensating Device Knob
4. Lens Holder Handle
5. Marking Device Control
6. Gimbal
7. Ink Pad
8. Spectacle Table Lever
9. Spectacle Table
10. Power Drum
11. Locking Lever
12. Prism Axis Scale
13. Prism Compensating Device
14. Prism Diopter Power Scale
15. On-off Switch
16. Lens Stop
17. Cylinder Axis Wheel
18. Filter Lever
19. Lamp Access Cover IIEI&H

1. The Eyepiece : mounted in a screw-type focusing mechanism, the eyepiece
plays an important role in the accuracy of your readings. It is essential that the
eyepiece be focused to the individual eye of each user. It may be fitted with a
rubber guard to prevent scratching of the user’s own eyewear.
2. Chrome Knurled Sleeve: used to rotate the Reticle to orient prism base.
3. Prism Compensating Device Knob: Used to read prism amounts greater than
five prism diopters.
4. Lens Holder Handle :Used to hold a lens in place against the aperture.
5. Marking Device Control: Used to spot the lens at either the Optical Center or
Prism Reference Point (PRP).
6. Gimbal: A pivoting holder that holds the lens in place.
7. Ink Pad: Holds the spotting ink.
8. Spectacle Table Lever: Used to raise, or lower the level of the Spectacle Table.
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9. Spectacle Table: The resting place for the frame when neutralizing finished eyewear.
10. Power Drum :Hand wheel with numbered scale readings between +20 and -20
Diopters.
11. Locking Lever: Used to elevate or depress the position of the instrument for
individual’s height or posture.
12. Prism Axis Scale: Used for orientation of prism axis
13. Prism Compensating Device: Used to verify or layout large amounts of prism.
14. Prism Diopter Power Scale: Displays prism amount.
15. On-off Switch : Power switch.
16. Lens Stop : Aperture against which the lens rests.
17. Cylinder Axis Wheel: Used to orient or neutralize cylinder axis.
18. Filter Lever: Used to engage or remove green filter.
19. Lamp Access Cover: Provides access to change the lensmeter bulb.
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Optics of Lensometer (Manual)
Lensometer cross sectional view.
1 – Adjustable eyepiece
2 – Reticle
3 – Objective lens
4 – Keplerian telescope
5 – Lens holder
6 – Unknown lens
7 – Standard lens
8 – Illuminated target
9 – Light source
10 – Collimator
11 – Angle adjustment lever
12 – Power drum (+20 and -20 Diopters)
13 – Prism scale knob IIEI&H

Optics of Lensometer (Automated)
Lensometer cross sectional view.
1 – LCD Monitor
2 – Operated Switch
3 – Marking Level
4 – Lens Plate
5 – Lens Holder
5 – Nose Pad
6 – Memory add Switch
7 – UV –Measurement Switch
8 – Printer
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Optics of Lensometer (Automated)
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 Rotate the eyepiece until fully extended from the
instrument(usually by rotating the eyepiece as far as
possible in an anti clock wise direction).
 The graticule visible through the eyepiece will now
appear blurred. The eyepiece should then be rotated
in a clock wise direction until the target cross hairs
and the graticule just come into focus.
 Continued rotation of the eyepiece will force the
observer to accommodate in order to keep the
graticule in focus.
 With the power wheel at the zero position, the cross
hairs and the target should be in clear focus. Failure
to focus the eyepiece will result in incorrect
readings of lens power.
Procedure Focusing the Eyepiece
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 Focus the eyepiece of the lensometer for the examiner’s eye
 With the power wheel set on zero, turn the eyepiece as far
counter-clockwise as possible
 Then slowly turn it clockwise until the reticule first comes
into sharp focus
 Insert the spectacles
 If testing a pair of glasses, always check the right lens first
 Place the pair of glasses in the lensometer with the ocular
surface away from you
 The lens is held in place by the lens holder and is held level
on the lens table
 Center the lens by moving it so that the image of the
lensometer target is aligned in the center of the eyepiece
reticle
Lens Measurement Preparation
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If all the lines or dots of the target are in focus at a
given setting of the power wheel, the lens is spherical,
Marking the optical centre
1. Check that the centre of the lens coincides with the
centre of the target.
2. When this is so, the lens is correctly positioned and
the optical centre could be marked.
3. While there is no centre dot, the whole set of dots
can be ‘framed’ within the lines of the graticule to
locate the centre.
4. Repeat the same procedure for the other lens.
Determining the Lens Power
(Spherical Lenses)
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Step 1 (finding the sph: power) : Rotate the power wheel until one set of lines (stretched
dots) becomes clear. Start with the higher positive power (or lower negative power). The
axis drum will need to be rotated to ensure that the lines are unbroken. Note the power on
the power wheel.
Step 2 (finding the cyl: power): Rotate the power wheel until the second set of lines
(stretched dots)becomes clear. The second power reading minus the first reading will
give the power of the cyl (and its correct sign).
Step 3 (finding the axis) : Note the direction of the lines (stretched dots) at the second
reading.This is the axis. The rotatable line in the graticuleis used to line up with the
stretched dots to determine the axis.
Determining the Lens power
(Sphero-Cyllinder)
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Determining the Lens power
(Sphero-Cyllinder)
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Take Home Messages
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 Always focus the eyepiece before using the instrument
 Concentrate on the central circular orientation of dots
(especially in case of toric lenses)
 One meridian of the target should always be parallel to
the orientation of central small lines (in toric lens)
 Each dot at the center will not be round, instead they will
be small lines oriented in one particular meridian
 Except the protractor, everything will be blur when a lens
is placed
 The target moves with the movement of the lens
Instrument should not be used more on battery

References
Theory and Practice of Optics & Refraction, A. K. Khurana
Lensometry manual by Mr.Sunny Mannava (HCU).
Optical-Dispensing, Arvindo eye care (Gide)
Picture: Me,Book,Google .
IIEI&H

IIEI&H
For:Azizul islam
https://www.slideshare.net/AzizulIslam6

Lensometer

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