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Lens Defects and How to Avoid Them
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 we
arers so it should be free from defects
• Defects can affect optical properties of lens and in turn affect i
mage quality and visual comfort
3. Lens Defects or Faults
• Medium from which the spectacle lens are made , may suffer f
rom defects,
• Usually they are not visible until the finished lens has been ob
tained and verified
4. • Defects in ophthalmic lenses can be grouped in three major ty
pes:
1. Defects of manufacturing process
2. Surface defects
3. Errors in power of the lens
6. Defects occurring during manufacturing process:
• Now days due to technological advancement such manufacturi
ng defects are rare.
• Presents within the surface
• These defects include the following:
i. Colour
ii. Strain
iii. Bad metal
iv. Bubbles
v. Feathers
vi. Veins
vii. Cloudiness
7. Colour Defect:
• Ophthalmic lens should be clear
• Generally arises if impure material is not re
moved 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 rapi
dly , the outer surface gets co
oled much faster than inner p
ortion, and this results in strai
n
9. Inclusion/Bad metal defect:
• These are stone and crystallites whi
ch occur in finished glass.
• In simple words these are foreign b
ody or particle remaining in glass
• Crystallites are crystal bodies, which
are formed when the cooling proces
s 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 doe
s not escape from the material befor
e 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 bubb
les.
• This is usually seen around bad mate
rial which gives an appearance of sm
all feathers in glass
15. Surface Faults
• These faults usually present on the surface of lens and are usu
ally caused by inadequate polishing or improper handling of le
ns
• These defects are as follows:
i. Pits or holes
ii. Grey
iii. Polishing burn
iv. Waves or rings
v. Orange peel defects
vi. Generators marks
16. Pits or Hole (Defect)
• It is usually caused frequently,
when insufficient truing and s
moothing process takes place
on the grinding machine.
• These are remains of smoothin
g procedure on the surface of l
ens after polishing
17. Grey (defect)
• It is caused by incomplete polishi
ng of lens blank
• After incomplete polishing is don
e surface appears to be grey in c
olor
18. Polishing Burn
• During polishing if the lens sur
face is allowed to get too hot
i.e. if it lacks lubrication betwe
en polishing pad and lens, the
polishing burn can occurs.
19. Waves (Defect)
• It is a form of defect when unwa
nted changes in curvature occur
• It is an undeviation and ripple o
n lens surface
20. Orange Peel Defect
• As name suggest it is like peel
of orange so defect is also calle
d orange peel defect.
• Some say that this type of surf
ace is a water polish
• The defect is a type of wave an
d sometimes looks like the pee
l of an orange.
21. Generator Mark
• The most common type of unwante
d patterned surface marks are gene
rator marks.
• Generator marks are parallel, curve
d marks across the surface of the le
ns, caused by generator wheel
• Light generator marks are normal af
ter generating, but after fining and
polishing they should not be visible
23. Errors in power of lens:
• These errors are produced while g
enerating power of the lens.
• These include:
i. Lab tool errors
ii. Excessive blocking pressure
iii. Inconsistent Base curve
