Lens defects or lens fault ppt

lens fault ppt

1. Lens defects or lens fault PRESENTED BY- MAAZ UL HAQ C.L GUPTA EYE INSTITUTE

2. Introduction Ophthalmic lenses are dispensed daily and routinely to the wearers so it should be free from defects Defects can affect optical properties of lens and in turn affect image quality and visual comfort

3. Lens Defects or Faults Medium from which the spectacle lens are made , may suffer from defects, Usually they are not visible until the finished lens has been obtained and verified

4. Defects in ophthalmic lenses can be grouped in three major types: Defects of manufacturing process Surface defects Errors in power of the lens

5. Manufacturing Defects or faults

6. Defects occurring during manufacturing process: Now days due to technological advancement such manufacturing defects are rare. Presents within the surface These defects include the following: Colour Strain Bad metal Bubbles Feathers Veins Cloudiness

7. Colour Defect: Ophthalmic lens should be clear Generally arises if impure material is not removed during manufacturing process e.g. any stress of Iron oxide in Silica causes unwanted Green color Certain high index material ingredients will show slight yellowishness

8. Strain defect: It is usually caused if molten glass is allowed to cooled rapidly , the outer surface gets cooled much faster than inner portion, and this results in strain

9. Inclusion/Bad metal defect: These are stone and crystallites which occur in finished glass. In simple words these are foreign body or particle remaining in glass Crystallites are crystal bodies, which are formed when the cooling process is very slow.

10. Bubble defect: This is a defect which occurs during melting and fining process These are small droplets in the glass These are gaseous matter which does not escape from the material before glass cools Mainly Barium Crown and flint glass are very prone to this defect Also referred to as seeds, air bells or boils.

11. Feathers defect: These are the cluster/groups of bubbles. This is usually seen around bad material which gives an appearance of small feathers in glass

12. Veins/ striae (Defects) These are presented as streaks or uneven mixing of material causes lines in the glass.

13. Cloudiness (Defects) It is caused by precipitated material during cooling The precipitated material is usually sulfates and chlorates

14. Surface Faults or defects

15. Surface Faults These faults usually present on the surface of lens and are usually caused by inadequate polishing or improper handling of lens These defects are as follows: Pits or holes Grey Polishing burn Waves or rings Orange peel defects Generators marks

16. Pits or Hole (Defect ) It is usually caused frequently, when insufficient truing and smoothing process takes place on the grinding machine. These are remains of smoothing procedure on the surface of lens after polishing

17. Grey (defect) It is caused by incomplete polishing of lens blank After incomplete polishing is done surface appears to be grey in color

18. Polishing Burn During polishing if the lens surface is allowed to get too hot i.e. if it lacks lubrication between polishing pad and lens, the polishing burn can occurs.

19. Waves (Defect) It is a form of defect when unwanted changes in curvature occur It is an undeviation and ripple on lens surface

20. Orange Peel Defect As name suggest it is like peel of orange so defect is also called orange peel defect. Some say that this type of surface is a water polish The defect is a type of wave and sometimes looks like the peel of an orange.

21. Generator Mark The most common type of unwanted patterned surface marks are generator marks. Generator marks are parallel, curved marks across the surface of the lens, caused by generator wheel Light generator marks are normal after generating, but after fining and polishing they should not be visible

22. Errors in Power of lens

23. Errors in power of lens: These errors are produced while generating power of the lens. These include: Lab tool errors Excessive blocking pressure Inconsistent Base curve