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geneva lens measure/ lens clock
- 1. GENEVA LENS MEASURE SurajShil, B. Optom 1st yr
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- 3. BASIC INFORMATION • Itsan 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 isan instrument used for precise measurement of the radius of curvature of a sphere or a curved surface
- 5. CURVE GAUGE •Curve gaugeis a brass template having curved edges with curvatures corresponding to diopteric values marked on the gauge •Used in lens surfacing
- 6. WORKING PRINCIPLE ofGENEVA 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
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- 12. RADIUS • From pythagoreantheorem of right angles, • y² + (r-s)² = r² • y² + s² = r² - r² + 2rs • r = y²/2s + s²/2s² • r = y²/2s + s/2
- 13. DIOPTRIC POWER • Letthe radius found be ‘r’, then • F = n’ – n/r
- 14. THICKNESS • Can beused 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
- 15. • F =(n’- n)/r……..(1) r = y2/2s + s/2……..(2) From equation (1) & (2), • F = (n’-n)/(y2/2s + s/2) THICKNESS
- 16. 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
- 17. FOR BIFOCALS • Forthe 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
- 18. SEG CLOCK • Segclock 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
- 19. CONVERSION FACTOR • Lensgauge calibrated for n=1.523, but can be used for materials of other R.I. • Let, Geneva lens calculate, • Fc= nc-1/r , Fc = surface power calculated nc = R.I. calibrated • For true power, • Ft=nt-1/r , Ft = true power nt= R.I. of the material
- 20. • Since, ris same for both equations, • nc-1/Fc = nt-1/Ft • Ft = Fc (nt-1/nc-1) CONVERSION FACTOR
- 21. • Let, lensgauge calibrated for n=1.523, but used on CR-39 plastic, of R.I.=1.498, then the conversion factor will be?? • nt-1/nc-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
- 22. CONCLUSIONS • Important instrumentin 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
- 23. REFERENCES • SYSTEMS FOROPHTHALMIC 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”
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