Lensometry (Focimeter)

3rd Year Student Of Optometry at ISRA School Of Optometry
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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 Neutralize T oric lens Geneva Lens measure Manual Focimeter Automated Focimeter

3. 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

4. Hand Neutralization With Movement Against Movement

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 down Moves up

7. Concave Lens Movements Cross Moves down Moves up

8. i i_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 Against motion With motion

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 Only Calibrated at Ref. index 1.523 for crown glass. 0.25D errors may obtain. May 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.” Green light is used to eliminate chromatic aberrations. It is portable. Measures ranges up to +20D to -20D.

17. Single Vision lenses Bifocal lenses Trifocal lenses Progressive Addition lenses Determine the Sphere power Determine the Cylindrical lens power Mark the optical centre Contact lens power Power 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 Toric lenses Spherical Lenses Spherical Lenses Toric lenses

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 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.

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 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 .

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: Proper centration/ decentration Axis Marking. Measuring power of near addition lens (bifocal, trifocal, progressive) Some models also measure Abbe no . The lens also be measured with 0.01D or 0.12D difference. IPD measurement of lens mounted spectacle.

30. 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