This document discusses methods for detecting the type and power of lenses. It covers hand neutralization techniques for spherical and astigmatic lenses using convex and concave lenses. It also describes using a Geneva lens measure and manual and automated focimeters. The manual focimeter involves aligning mires to determine spherical and cylinder power of single vision, bifocal, trifocal and progressive lenses. The automated focimeter uses a light beam to precisely measure lens parameters. Both tools have limitations, with the automated version providing more accuracy.

1. Detection of Lens Power
and Focimetery
Presented By:
Muhammad Amin
Tehseen Javaid
Sarfaraz Ahmed
Shayan Zia

2. Learning objectives
Detection of type of lens
Hand neutralization
Neutralize Convex lens
Neutralize Concave lens
NeutralizeToric lens
Geneva Lens measure
Manual Focimeter
Automated Focimeter

3. Detection of lens type
Possible to detect weather the lens is
oSpherical,
oAstigmatic, or a
oPrism
Ways to detect lens type:
Central/peripheral thickness
Magnification/ minification
Movements

4. Hand Neutralization

5. i_Spherical Lenses
Spherical causes no distortion of the cross, 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 Lens).
With Movement: Use Convex Lenses (against movement Lens).

6. Convex Lens Movements
Cross Moves downMoves up

7. Concave Lens Movements
Cross
Moves down Moves up

8. ii_Astigmatic Lens
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

9. 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.

10. Geneva Lens Measure

11. Lens Measure
It is defined as,
“A device for measuring the radii of the curvature of
a spectacle lens”.

12. Measurements of lenses:
It uses three(3) pins to measure the curvature of a spectacle lens.
The clock is calibrated at front and back side of the lens, in order to measure
the power of lens.
Refractive index calibrated is 1.523 of Crown glass.
Other material can also be calculated by applying the correction factor.

13. Draw backs of Geneva Lens Measure
oOnly Calibrated at Ref. index 1.523 for crown glass.
o0.25D errors may obtain.
oMay cause a scratch on plastic Materials.

14. Manual Lensometer (Focimeter)

15. Parts of Focimeter

16. Description of Focimeter
Definition:
“ A focimeter is used to measure the vertex power of
a lens.”
oGreen light is used to eliminate chromatic
aberrations.
oIt is portable.
oMeasures ranges up to +20D to -20D.

17. oSingleVision lenses
oBifocal lenses
oTrifocal lenses
oProgressive Addition lenses
oDetermine the Sphere power
oDetermine the Cylindrical lens power
oMark the optical centre
oContact lens power
oPower of prism
Uses of Focimeter

18. Method to operate:
The steps are:
• Adjust the eye piece.
• Align the Power Drum at zero, making sharply formed mires inside if not
make zero error correction
• Mount the glass with back
surface of the lens against the rest.
• Calculate the power by rotating the drum.

19. Mires Formed in Manual Focimeter

20. Measurement of spherical power
• More easy to determine the power if the power is spherical, keep rotating
the drum until clear dot circle/ mire is seen.

21. Cylinder lenses
• Rotate the drum to sharply focus lines on first meridian is the spherical
power.
• Repeat the process for second meridian is focus, difference of both is
cylinder and note the axis on graticule

22. Measurement 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.

23. Measurement ofTrifocal
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.

24. Measurement of Progressive lenses
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.

25. Measurement 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.

26. Limitations of Manual focimeter
oAlthough it is good for single vision lenses, but may misguide in addition
determination of bifocal/trifocal lenses.
oPower varies with 0.25D difference don’t calculate 0.12D difference.
oDifficult in case of progressive lenses.
oCan cause damage to the contact lenses.

27. Automated Focimeter
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.

28. 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. .

29. Important Uses:
It can used for accuracy in all conditions:
oProper centration/ decentration
oAxis Marking.
oMeasuring power of near addition lens (bifocal, trifocal, progressive)
oSome models also measureAbbe no.
oThe lens also be measured with 0.01D or 0.12D difference.
oIPD measurement of lens mounted spectacle.

31. References:
Books:
• CLINICAL OPTICS -ELKINGTON- Third Edition
• CLINICAL OPTICS –American Academy of
Opthalmology-section 3
• Internet:
• http://www.medilexicon.com/medicaldictionary.php?t=53289
• http://www.allaboutvision.com/eyeglasses/eyeglass-prescription.htm
• https://en.wikipedia.org/wiki/Lensmeter