geneva lens measure/ lens clock

details on how to use a geneva lens or lens clock for an ophthalmic lens power measurement

1. GENEVA LENS MEASURE Suraj Shil, B . Optom 1 st yr

2. PARTS N=1.523

3. BASIC INFORMATION Its an adaptation of the spherometer and an advancement over curve gauge Deflection of movable leg is measured on the dial directly as surface power Curvature of each surface is measured separately and algebraically summed up Correction factor for glass other than n=1.523 must be applied

4. SPHEROMETER Spherometer is an instrument used for precise measurement of the radius of curvature of a sphere or a curved surface

5. CURVE GAUGE Curve gauge is a brass template having curved edges with curvatures corresponding to diopteric values marked on the gauge Used in lens surfacing

6. WORKING PRINCIPLE of GENEVA LENS The device has two fixed pins and a movable pin in the centre When placed on a curved surface the central pin moves inward or protrudes outward A pointer activated by a system of gears indicates the position of the movable pin in relation to the fixed pins The central pin indicates the sag of the surface which provides direct reading of refractive power

7. WORKING PRINCIPLE

8. DIGITAL LENS CLOCK

9. USES SAGITTAL DEPTH RADIUS OF CURVATURE POWER OF LENS LENS THICKNESS

10. PICTORIAL

11. RADIUS From pythagorean theorem of right angles, y ² + (r-s)² = r² y² + s² = r² - r² + 2rs r = y²/2s + s²/2s² r = y²/2s + s/2

12. DIOPTRIC POWER Let the radius found be ‘r’, then F = n’ – n/r

13. THICKNESS Can be used to estimate thickness of a hard or gas permeable contact lens Lens placed concave side up on any hard surface Lens gauge placed on it so that outer prongs rest on table and centre prong touch lens If power and distance between the prongs are known, thickness of the lens is given by the sagitta

14. F =( n’- n)/r……..(1) r = y 2 /2s + s/2……..(2) From equation (1) & (2), F = (n’-n)/(y 2 /2s + s/2) THICKNESS

15. FOR CYLINDRICAL LENSES Hold the lens measure such that the center contact point of the lens clock is at optical center and the lens clock is perpendicular to the lens surface Lens clock is rotated at this point with all three contact points against the lens The indicator will show a changing value, because the surface has two curves The values of these curves are indicated when the lens clock shows its maximum and minimum values

16. FOR BIFOCALS For the main lens, none of the three contact points must touch or rest on the segment portion To measure the near portion accurately, all three contact points of the lens measure must fit within the width of the segment **Using the conventional lens measure, this is not always possible

17. SEG CLOCK Seg clock is designed as a conventional lens clock except that the three points of contact are closely spaced, allowing them to fit within the smaller area of the lens’ near segment

18. CONVERSION FACTOR Lens gauge calibrated for n=1.523, but can be used for materials of other R.I. Let, Geneva lens calculate, F c = n c -1/r , F c = surface power calculated n c = R.I. calibrated For true power, F t =n t -1/r , F t = true power n t = R.I. of the material

19. Since, r is same for both equations, n c -1/F c = n t -1/F t F t = F c (n t -1/n c -1) CONVERSION FACTOR

20. Let, lens gauge calibrated for n=1.523, but used on CR-39 plastic, of R.I.=1.498, then the conversion factor will be?? n t -1/n c -1 = 1.498-1/1.523-1 = 0.498/0.523 = 0.940 When, the surface power calculated will be multiplied to 0.940, it will give the true power of the lens CONVERSION FACTOR

21. CONCLUSIONS Important instrument in optics Use not confined only for calculation of surface power Can be used for lenses of different R.I. Can be used for spherical, sphaerocylinder and even bifocals

22. REFERENCES SYSTEMS FOR OPHTHALMIC DISPENSING (page: 406, 407, 408, 409) CLINICAL OPTICS (page: 27,28) OPTICS AND REFRACTION - A.K. Khurana REFRACTION AND LENS PRESCRIPTION – Monica Chaudhary (page: 35,36) https://en.m.wikipedia.org/wiki/Lens_clock YOUTUBE channel ‘GOODWIN COLLEGE ONLINE STUDIES’ – “Lens Clock Measurer”

23. THANK YOU