As Seen in clinical setup in nepal.

1. Lensometry
BY Anuja dhakal
Phase II,NEH

2. What is a lensometer?
• A lensometer or lensmeter is a standard
ophthalmic device sometimes referred to
as vertometer or focimeter.
• The techniques and procedures for operating
a lensometer are termed as lensometry or
focimetry. The most common use of
lensometer is to verify the accurate
configuration/prescription in a pair of eye
lenses by optometrists and opticians.

3. • “ A focimeter is used to measure the vertex
power of a lens.”

4. Who invented the lensometer?
• Antoine Claudet designed the
photographometer in 1848. This device was
used for measuring the intensity of
photogenic rays. In the following year, he
designed the focimeter that was used for
finding the correct focus in photographic
portraiture.
• In 1922 the first projection lensometer was
patened.

6. What is the lensometer used for?
• The lensometer helps orient appropriately and mark
uncut lenses, verifying the power of single vision,
bifocal and trifocal lenses, and checking the right
mounting of eye lenses in spectacle frames.
• The device is used by the ophthalmologists or
optometrists to evaluate certain parameters specified
on the patient’s prescription, such as cylinder, axis,
sphere, add, and in some cases, prism.
• This instrument is often used to verify the accuracy
of progressive lenses.

7. • It is also used to determine and mark the
center of a lens and several other physical
measurements that are essential for the
proper functioning of the lens.
• At times, the lensometer is used for a prior
examination of a patient’s spectacle power to
match with the previous prescription that the
patient was given.
• Measures ranges up to +20D to -20D.
• Contact lens power measurement.

8. Detection of lens type
• Possible to detect weather the lens is
spherical, Astigmatic,or a Prism.
• Ways to detect lens type:
central peripheral thickness
magnification / minification
movements

9. Hand neutralization
The power of an unknown lens can be determined by
neutralising it with another lens of known power.
Neutralisation is based on the fact that if you look at an
object through a convex or concave lens and move the
lens from side to side (right and left or up and down),
the image that you see through the lens will also move.
If the lens has no power - or has been neutralised by
placing a lens of equal power but opposite sign against
the unknown lens - then there is no image movement.

11. With movement

12. Against movement

13. 1)Spherical Lenses
• Spherical lenses causes no distortion of the
object seen, however when lens is moved
from ‘side to side’ and ‘up and down’ it
appear to move.
• Use lenses of opposite movements:
Against Movement: Use Concave Lenses.
With Movement: Use Convex Lenses.

14. 2)Astigmatic lenses.
• For a toric lens ,rotational motion is used to find
the axis
if observe “against motion” use plus cylinder axis.
If observe “with motion” use minus cylinder axis.

15. 3)prism
• A lens with a ground prism will displace the
cross image as follows:
Base up: displaces image downwards
Base down: displaces image upwards
Base in: displaces image outwards
Base out: displaces image inwards

16. Errors in Hand Neutralization
• Error can be induced in Hand held neutralizing
technique such as:
It is somewhat inaccurate for curved lenses of
more than about 2 diopter power.
Error upto 0.50D may be incurred with
powerful lenses.

17. Types of lensometer:
1) Manual lensometer
2)Automated lensometer

18. lensometer working principle
• A lens having a focal length (f) is used to image a target
(usually a crossed set of lines). The user then places the
spectacle lens (under test) at the lens’s rear focal point
(f).
• The light rays emerging from the spectacle lens then
pass into an eyepiece having an internal reticle. The
target is axially shifted simultaneously by the user until
it is in focus with the reticle. Then, a condition occurs
when the light rays emerging from the test spectacle
lens becomes collimated.

19. • The formula relates the target position d and
the power of the spectacle lens Φ:

20. DIAGRAMMATIC REPRESENTATION

21. 1)Manual lensometer
• Manual lensometer gives the accurate power
of a lens and were used in optical industries.
• A manual lensometer is portable and can be
carried anywhere , But a person needs to have
a better idea to measure the power of a lens.
(specially in case of a toric lens)

22. Working principle of manual lensometer.
• The working of a manual lensometer is
grounded on the Badal principle.
• According to the principle, when we place the
eye at the focal point of a positive lens, we can
detect an object’s virtual image, between the
anterior focal point and the lens subtending
the same visual angle.

23. Badal optics
The Badal principle is Knapp’s law
applied to lensometers.

24. General description of manual lensometer
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

25. Parts of a Manual Lensometer
1. Eyepiece
The eyepiece used for obtaining reading accuracy is mounted over a screw-
type focusing mechanism. A rubber guard is also placed n the eyepiece to
avoid the scratching of the user’s eyewear.
2. Prism Compensating Device Knob
The Prism Compensating Device Knob helps in reading prism amounts that
are greater than five prism diopters.
3.Chrome Knurled Sleeve (Lensometer Reticle Adjustment Knob)
The Chrome Knurled Sleeve helps in rotating the lensometer Reticle to
align the prism base.
4. Lens Holder handle
The lens holder handle is present to hold a lens in place alongside the
aperture.
5. Spectacle Table Lever
The spectacle table lever helps in raising or lowering the level of the
spectacle table according to the user.

26. 6. Spectacle Table
The spectacle table provides a resting place for the spectacle frame
when the lens power is being neutralized.
7. Marking device control (Lens Marker)
Lens markers are pins controlled by the handle and used to mark
the lens at the optical center or the prism reference point.
8. Power Drum
The power drum is a handwheel that has numbered scale readings
ranging from +20.00 to -20.00 Diopter. The scale of the reading
interval is in 0.12 diopters steps, and for higher powers, the interval
is of 0.25 diopters steps.
9. Prism Axis scale
The prism axis scale helps in the orientation of the prism axis.
10. Prism Compensating Device
Prism compensating device helps to verify a large amount of prism.
11.Prism Diopter Power Scale
The prism diopter power scale displays the prism amount.

27. 12. Locking Lever
The locking lever helps in elevating or depressing the
position of the instrument according to the user’s
height or posture.
13.Cylinder Axis Wheel
The cylinder axis wheel helps in orienting and
neutralizing the cylindrical axis.
14. Filter Lever
The filter lever can incorporate or remove the green
filter.
15. Lamp Access Cover
The lamp access cover is used for changing/altering the
manual lensometer bulb.

28. Methods to operate manual lensometer
• 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 .

29. • 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

30. Mires formed in manual lensometer
Target It is green in colour and appears when
the device Is switch on.
It shows the position of the optical center of
the lens. There is a ring of round dots at the
centre of the target.
This represents the power orientation of the
lens as it rotates with the lens rotation These
round dots become small lines oriented in
one direction in case of a toric lens

31. Determination of spherical power
• 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.

32. Determination of spherocylinder
• 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.

34. Determination of bifocal
• For measuring the distance correction of a
bifocal lens, we want to measure the lens at the
optical center.
• The optical center of a conventional bifocal lens is
just above the center of the horizontal line of the
bifocal lens and read the near correction from the
center of the lower segment.
• For determining add power, turn glasses around
to read from front vertex.

35. To determine the ADD power on a bifocal:
1) Flip the bifocal over so that the lens stop of the lensometer
touches the front surface of the lens.
NOTE: Remember the Base Curve and ADD segment are both
located on the FRONT surface of the lens.
2) Adjust the lens so it touches the distance portion of the lens.
usually start with the right lens.
3) Record the power reading using the lensometer as you would in
a single vision lens. The power reading is known as the Front Vertex
Power for the DISTANCE/carrier Rx.
4) Adjust the lens so that the lens stop is now touching the
ADD/NEAR SEGMENT portion of the bifocal lens.
5) Record the power of this segment using the same procedure as
before for the Distance Rx portion of the lens. This power is known
as the TOTAL NEAR Power.

36. 6) To determine the ADD power, use the following
equation:
TOTAL NEAR Power = Distance Rx + ADD power, thus
ADD power = Total Near Power - Distance Rx
NOTE: If the Distance Rx contains a cylinder, just take
the difference between the spherical portion of the Rx
from the Total Near Power.
7) Repeat for the other lens.
For example, if the Distance Rx was determined to be
+2.00-1.00 x 180, and the Total Near Power was +4.00
from lensometry, then the ADD power would be:
ADD power = Total Near Power - Distance Rx = +4.00 -
(+2.00) = +2.00 D.

37. Determination of trifocal
• As with the bifocal ,
read the distance
correction just above
the segment line, and
read the near
correction from the
center of the lower
segment.

38. Determination of progressive lens
• Locate the centre to make the reading accurate,
otherwise is similar to the simple lensometery. All
new pair contain sticker on it, which describe
geometry of lens.
• The add power is read through the lower/nasal
area of the lens Conventional progressive
addition is measured from front vertex, while free
form progressive from back vertex.
• However, automated is more suitable for this
purpose.

39. Determination of prism
• Prism moves (deflects) the lensometer target
away from the center of the reticle.
• The target is deflected in the direction of the
prism base.
• When viewing the mires, lines may be off-center.
• The rings inside the lensometer are measured at
1 prism diopter.
• The number of rings from the center of the
reticle that the lines are over is the amount of
prism in the glasses.

40. Important points to be remembered.
• 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

41. specifications of veatch Aries Lensometer
• Power range -20 D to +20 D
• Axis range 0° to 180°
• Prism range 0.25D to 7.00D directly,to 14.00D
with auxiliary prisms included
• Lens diameter 13 to 84 mm
• Light source High luminosity LED Dimensions
• Height 370 mm
• Length 500 mm
• Width 170 mm

42. Limitations of manual lensometer
• Although it is good for single vision lenses, but
may misguide in addition determination of
bifocal/trifocal lenses.
• Power varies with 0.25D difference don’t
calculate 0.12D difference.
• Difficult in case of progressive lenses.
• Can cause damage to the contact lenses.

43. 2)Automated lensometer

44. • Automated Lensometer offers a fast and easy
way to accurately measure all lenses including:
Single Vision, Bi-focal, Progressive, and Prism.
• Automatic measurements can be taken
quickly by simply positioning the lens on the
lens table.
• Progressive lenses are recognized and
measured automatically, with the rate of
addition and diopter change graphically
displayed.

45. Principle:
The degree to which a beam of light is
deflected as it passes through the lens depends
on the focal and prismatic power of the lens and
the distance from its optical centre
Working:
A square pattern of four parallel beam of
light is passed through the lens to be tested. It is
quick and easy, minimum expertise required to
handle .
Accuracy is variable with machine to machine. .

46. Features of automated lensometer
• UV transmittance measurement
• Pupillary distance measurement
• Measurement of soft and hard contact lenses
• Space saving vertical orientation
• Full color tilt-screen for easy viewing while standing or
seated
• Easily accessible menu to customize your settings
• Data output options including a built-in printer and
USB ports
• Can be configured to interface with EMR systems

47. Advantages over manual lensometer:
• It can used for accuracy in all conditions: Proper
centration/ decentration ,Axis Marking.
• Measuring power of near addition lens (bifocal,
trifocal, progressive) made easier.
• Some models also measure Abbe no.
• The lens also be measured with 0.01D or 0.12D
difference.
• IPD measurement of lens mounted spectacle.
• Less time consuming in clinical steup.

48. References
• CLINICAL OPTICS –American Academy of
Opthalmology-section 3.
• Theory and Practice of Optics & Refraction, A. K.
Khurana.
• Grosvenor PRIMARY CARE OPTOMETRY.
• Internet sources
https://lambdageeks.com/a-detailed-overview-on-
lensometer-working-uses-parts/
http://www.medilexicon.com/medicaldictionary.php?t=53289
http://www.allaboutvision.com/eyeglasses/eyeglass-
prescription.htm
https://en.wikipedia.org/wiki/Lensmeter

49. Thank you
thank you!