Lens surfacing.pptx

In this presentation we learn about - Lens surfacing

1. Prasad babar Second year b.optom Lens surfacing

2. It is a branch of Optometry which deals with the processing of an ophthalmic blank into an ophthalmic lens Lens surfacing

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4. Lens blanks come from the factory finished on the front but just raw plastic on the back Lens blanks

5. Instruments Spherical surfacing machine :- Cylindrical surfacing machine

6. Instruments Tool:-Convex Spherical (power 0.00 to 20.00) convex tool will generate minus power Lens Convex tool Tool:- Concave Spherical(power 0.00 to 20.00) Concave tool will generate plus power Lens convex surface Concave tool

7. Instruments Tool :- Convex Cylindrical (power 0.00 to 20.00) Concave Cylindrical (power 0.00 to 20.00)

8. Instruments Buttons :- This is the instrument which holds the lens throughout the surfacing. It is made up of casted Iron After some time tool loses its curvature We can give it curvature by doing filing Buttons are of two types :- Spherical Cylindrical Governor

9. Instruments Brass Templates These are used for measuring the curvature of tool as well as lens curvature while doing surfacing

10. Instruments Measurement of tool curvature with Brass Templates correct Incorrect 6.00 Brass template Convex tool 6.00 6.00 Brass template 6.00 Concave tool

11. Instruments Screw gauge :- It is use to measure the thickness and diameter of the lens and lens blank.

12. Instruments Geneva Lens Measure This is used to measuring the curvature of lens in diopter (calibrated for R.I. 1.523 i.e.. Crown Glass)

13. Raw Material Powders :- Emri -120 Emri -180 Aluminum Oxide Cerium Oxide Ferus Oxide Carborandum

14. Raw Material Lens Blanks :- 1. Unfinished single vision lenses blank 2. Semi-finished single vision lens blank 3. Semi- finished multifocal lens blank Polishing Pad

15. Blank Selection For Spherical lens we select base curve and accordingly we will generate other surface +4.00D -2.00D Lens Power= +2.00D

16. Blank selection Selecting correct Base Curve Manufacturers table Vogel’s Formula:- If spherical Equivalent is Positive B.C. = Sph . Equivalent +6.00D If spherical Equivalent is Negative B.C. = Sph . Equivalent/2 +6.00D

17. Blank selection Finding Minimum Blank Size :- When no decenteration required M.B.S. =E.D +2mm (For Edging) When decenteration is required M.B.S. =E.D +2(Decentration)+2mm

18. Tool Selection Mainly calibrated for crown glass (R.I - 1.523) Tool Curve = Back Vertex Power – Nominal BC For Toric lenses we use toric transposition Spherical Curvature Base curve / Cross Curve

19. Lens surfacing 1.Marking 2.Blocking 3.Roughing/grinding 4.Truing 5.Smoothing 6.Polishing 7.Deblocking

20. 1.marking To correctly locate the lens for different stages of surfacing ,certain markings are essential Optical center position,cylindrical axis,base apex direction in case of prism Done by marking with suitable ink or jugging at blocking stage

21. 2.Blocking Lens blank is attach to a button Bee vax or metal alloy Lens should cool before actual process starts otherwise waves/distortion will take place

22. 3.Roughing Surface of blank is brought to its approximate curvature by rotating blank against the tool of desired curvature Roughing abrasive – Carborandum Grain size of abrasive – 0.3mm Abrasive is mixed with water Machine holds lens in place using the block and pushes it against the tool which is covered with an abrasive pad

23. 4.Truing Surface is more closer to what we want More finer grain abrasive is used Aluminum oxide – grain size – 0.075mm Water is mixed in abrasive to make pest Surface is reduced more slowly

24. 5.Smoothing Surface is brought to about 0.1mm of its finished thickness The abrasive is used is more finer Grain size of abrasive - 0.01mm Water is mixed in abrasive to make pest Surface is more smooth

25. 6.Polishing Pest polishing pad on tool with help of adhesive Polish Iron oxide for manual machines Cerium oxide for semi or fully automated machines Grain size - 0.002mm

26. 7.De-blocking In this process lens is removed form button Hit button with wooden mallet Immerse lens in hot water bath Extra precaution should be taken while doing this step c Lens Wax Button Wooden mallet

27. Spherical lens All the procedure should be repeated for another surface with spherical tool

28. Sphero-cylindrical lens In case of Sphero-cylindrical lens Base curve should be grinded with spherical Tool Then use cylindrical tool to make toric surface Governor of cylindrical machine holds lens Lens moves side ways in Power meridian only

29. Advanced methods With help of glass molding method process time is reduced Material west is less Good quality production is achieved