lens defect-1.pptx..........................

Lens defect

1. Topic- Lens Defect By- Akanksha Deep Tutor, DOPT

2. Ophthalmic lenses are dispensed daily and r outinely to wearers so it should be free from defects Defects can affect optical properties of lens and in turn affect image quality and visual comfort Medium from which the spectacles lens are made, may suffer from defects Usually they are not visible until the finished lens has been obtained

3. It can be categories in Defects of manufacturing process Surface defects Errors in power of lens Lens formation https://youtu.be/xfhjWyAU9Cw?si=26g-w8ydfQh7Bvug

4. Materials used in lens

5. Defects of manufacturing process Rare due to advancement of technology Defects includes Colour Strain Bad metal Bubbles Feathers Veins Cloudiness

6. Defect in the material By transmission Bad material Bubble Feathers By shadowing Vein Colour Strain Cloudiness By reflection

7. Bad metal Also known as bad stock or stones These are crystallites which occur in finished glass. These are foreign body or particle remaining in glass Crystallited are crystal bodies, which are formed when the colling process is very slow. Material must be free from any inclusions or foreign particles that may appear in generally due to portion of the batch material remaining undissolved during the melt

8. Bubble defect Also known as seeds, air bells or boils. Occurs during melting and fining process formed by large volume of gas given off during melting period Seems as small droplets in the glass Crown & Flint glass are very prone to this defect

9. Feathers These are cluster/ group of bubbles Tis usually seen around bad material which gives an appearance of feathers in glass

10. Vein Fine thread or streak of glass Like a thin empty tube running through the glass These are caused by insufficient mixing of bad metal Cord – Heavy or pronounced vein Stria - A very fine vein

11. Colour Spectacle lens is made to have high degree of clearness and transparency It is defined as any unintended coloration in a medium which is supposed to be white generally arises from the use of impure materials in the composition of medium E.g. any stess of iron oxide in silical causes unwanted green color Best detected on a matt white paper

12. Strain If molten glasses allowed to cool too rapidly the outer surfaces solidify berfore the hotter inner portion. As the inner portion then cools, it tends to shrink setting up tension within the glass This stress is called strain May also arise if external forces are allowed to compressed the glass

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

14. Surface faults or defect These faults usually present on the surface of lens and are usually caused by inadequate polishing or improper handling of lens Pits or hole Grey Polishing burn Waves or ring Orange peel defect Generators marks

15. Pits or hole 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

16. Grey It is caused by incomplete polishing of lens blank Polishing process should be continued until all signs of greyness disappear

17. Polishing burn It is very common defect During polishing if the lens surface is allowed to get too hot, then by lack of lubrication between the polishing pad and lens, the polishing burn of glisters can occur

18. Wave It is a form of defect when unwanted changes in curvature occurs Can be attributed to faulty machine motion or by polishing on a badly smoothed surface or even over polishing

19. Orange peel defect Type of wave and sometimes looks like a peel of an orange

20. Generator mark The most common type of unwanted patterned surface marks are generator marks caused by generator wheel Light generator marks are normal after generating But after fining and polishing they should not be visible

21. Thank You