Contact lens Verification- different techniques

1. RAJU KAITI
Optometrist, Dhulikhel Hospital, Kathmandu
University Hospital

2.  Checking quality and physical characteristics
for-
 Prescription use
 Contact lens fitting sets
 Research purposes
 Contact lens verification undergoes two stages,
laboratory and clinical.

3.  Laboratory
 During the final phase of manufacture, an overall parameter
check is performed to ensure the lenses do not differ significantly
from the parameters ordered by the practitioner.
 Clinics
 Verification of lenses upon receipt, rather than during the
dispensing visit, is advisable.
 Patients
 ‘on-eye’ fit is another indicator of whether a contact lens has been
manufactured to specifications.

4.  Ensure correct lens is dispensed
 Quality of manufacturing(as per quality standard)
 Assess changes in contact lens with wear
 To ensure that proper over-refraction and trial fitting
examination has been conducted, the accuracy of trial
sets used in the clinic should be determined.

5.  Rigid and soft lenses have similar parameters which
require verification by the practitioner.
 Radii of curvature
 Linear parameters
 Edge profile
 Power
 Lens quality
 Rigid and soft contact lenses should be hydrated in a
soaking solution for 12 - 24 hours before verification
procedures are conducted.

7.  Back optic zone radius
 Back central optic zone radius
 Back peripheral optic zone radius
 Front optic zone radius
 Front central optic zone radius
 Front peripheral optic zone radius

8.  Back optic zone diameter
 Back central optic zone diameter
 Back peripheral optic zone diameter
 Front optic zone diameter
 Front peripheral optic zone diameter
 Total diameter
 Bifocal segment size and position

9.  Central
 Edge
 Lenticular junction
 At any other specified point

10.  Axial and radial edge lift
 Edge shapes

11.  Back vertex power
 Front vertex power
 Near addition
 Prism and base direction
 Cylinder power
 Aberration

12.  Finish
 Polish
 Edge form
 Transitions
 Tint
 Material

14.  Radiuscope
 Keratometer (modified)
 Toposcope
 Moiré fringe deflectometer
 Radius checking device
 Topographical mapping system
 Electrical conductivity method
 Microspherometer

15. Drysdale’s principle

17.  Lens holder is filled with water
 Clean lens is placed centrally on holder, convex surface is in
complete contact with water
 Holder is placed on microscope stage and centered
 Microscope eyepiece is correctly adjusted
 By observing through microscope, target is imaged on surface
of lens

18.  Dial gauge reading is recorded
 Second focus at centre of curvature of surface is obtained
 Second reading is recorded
 Difference between two dial gauge reading gives radius of
curvature of surface
 Procedure is repeated twice and average of 3 reading is taken
 Radius is measured in different point of lens as it may vary

19. • Lens is measured in the dry state
• Front surface image eliminated with saline in lens mount
• Lens is centered concave side-up
• BOZR=distance between1st and 2nd focal plane

20.  same procedure for
determining the FOZR,except
that the lens is centered
convex side up on the
appropriate lens mount.
 The scale is reversed so that
the aerial image will be
focused before the real image.

22.  Air checking, which requires
the SCL to be dabbed with a lint-
free cloth and measured in a
semi-dehydrated state.
 The critical duration of such air-
checking is approximately one
minute depending on ambient
temperature and humidity.
 Requires a wet cell filled
with saline solution
 The readings are multiplied
by the refractive index of
saline to calculate the BOZR
IMMERSION

23.  The Keratometer which is used for measuring corneal
curvature can also be used to measure the BOZR of a
contact lens by using special attachments.

24.  Keratometer set-up is
modified with a lens holder
and prism or mirror
attachment
 Values derived are less than
the actual radii
 The same procedure for
measuring the cornea is used
for contact lenses

25.  Keratometer set-up is modified
with a wet cell and prism or
mirror attachment
 Values derived are less than
the actual radii
 Readings are multiplied by the
RI of saline to get the BOZR
 The same procedure for
measuring the cornea is used
for contact lenses

26. The principle of the thick lens
system to design the R-C Device
whose refractive index is the same
as the lens material.
The contact lens floats on a liquid
interface which has the same
refractive index as the lens
material.
The R-C device is used in
conjunction with the focimeter and
lens thickness gauge.

27. By combining the features of the photokeratoscope and a
built-in camera, polaroid images of the cornea are captured
and viewed with a computer monitor.

28.  Back vertex power (BVP)
 Front vertex power (FVP)
 Front and back surface radii of
curvature.
 Centre thickness.
 Refractive index.

29.  It measures BVP
 Projection focimeter-greater accuracy
 Nakijama-mounted lens in liquid cell and read power
of resultant contact lens-liquid lens on projection
focimeterBVP of soft contact lens can be measured in
air or liquid
 Clarity of focimeter image relates to optical quality of
lenses

30.  BVP in air is not equal to BVP in liquid
 If F1=BVP in air
 F2=BVP in liquid
 F1=kF2where k is compensation factor
 K=n2-n/n2-n1 where,
 n=refractive index of air
 ,n1=RI of saline
 .n2=refractive index of Hydrogel material

31.  Lens is cleaned and dried
 Lens must be centered concave side down on the
focimeter stop
 Reading is taken off the power drum/scale after
focusing the mires
 Back vertex focal length is measured from the plane
of the focimeter stop

32.  Power for each meridian is measured
 Cylinder value is derived from measuring the
difference in meridional powers

33.  The SCL is measured by air-checking it with the focimeter.
 The lens is dabbed with a lint-free cloth/tissue to remove
excess water.
 The SCL is centered on the focimeter support and the
procedure used to measure RGP lenses is followed.
 An SCL can also be measured by immersing the lens in
saline contained in a wet cell. Because the lens power is
measured in saline, the value has to be multiplied by four
(approximately) to calculate the true lens power in air.

34.  Procedure is same as for BVP but with lens convex
side down
 FVP measurements can be converted to BVP by using
a table with known center thickness and back optic
zone radius

35.  Lens diameters
 back optic zone diameter (BOZD)
 total diameter
 peripheral curve width (PCW)
 Lens thickness
 centre thickness (ct)
 edge thickness

36.  Diameters and linear
parameters
 Measuring magnifier
 V gauze
 Cast, dividers and
transparent rule
 Micrometer & spheres
Measuring magnifier
20 mm scale: used
for corneal lens

37. V gauze
Scale 6.00 – 12.50mm
Cast, dividers and
transparent rule
Micrometer
& spheres:
Measure
primary optic
diameter, sag
is determined

38.  Can be measured with all most all techniques
 Indirect method:

39.  Thickness verification
 Dial thickness gauze
 Contek edge thickness
gauze & computer
 Radial thickness:
perpendicular to front
surface of lens
 Axial thickness: parallel
to primary axis of lens
 Spectacle lens measure
 Radiuscope: no water
used
Thickness gauze Contek edge
thickness
gauze & computer

41.  Lens diameters
 total diameter
 FOZD
 Lens thickness
 centre thickness (ct)
 edge thickness

42.  Instruments:
 Projection magnifier
 Moiré fringe deflectometer
 10x loupe with graticule
 Electronic thickness gauge
 Pressure controlled gauge
 Electrical thickness gauge
 Radiuscope (modified)

43. Instruments/techniques:
• Edge molding
• Projection magnifier
• Ehrmann profilometer
• Palm test
• Radiuscope (modified)

45.  Surface defects
 Optical quality
 Lens impurities/deposits

46.  Scratches and lathe marks:
 Can cause:
 deposit build up
 poor wettability
 surface hydrophobicity
 Indicate over polishing during manufacture

47.  Instruments:
 Magnifying 10x loupe
 Projection magnifier
 Contact lens optical quality analyzer (CLOQA)
 Dark field microscope
 Moire fringe deflectometer

49.  White background test
 Variation in thickness

50.  Hydrogel contact lenses are flexible
 If exposed to atmosphere, they dehydrate and
alter their contour. Verification in air is
inaccurate due to-
 Shrinkage of Hydrogel on dehydration
 Accumulation of surface moisture
 So, artifact liquid cells are used to measure
parameters of soft lenses
 But RGP lenses can be measured in air

51.  Mandell 1974 recommend following procedure-
 Lens should be removed from its liquid using sterile spatula or soft
plastics protected forceps
 Lens is then placed on lint free tissue and tissue is folded over
uppermost convex of lens
 Both surface are blotted dry
 Lens is dried in air with forceps
 Lens surfaces are examined for smudges
 It is preferable to check lens within one minute

52.  %water content = mass of watermass of hydrated lens x 100
 Water content of Hydrogel contact lenses is measured by
sensitive microbalance
 Alternative methods-
 Refractive index
 Refractive indexes decreases as water content increases

53.  On-eye-examination of
dispensing lens
 VA assessment before and
after over-refraction.
 Always assess
 dynamic and static fit,
 surface wet ability & lens
quality,
 corneal integrity.

54.  At the end of the verification process, the real
indicator that an accurate and optimal fitting
has been achieved is evaluation of the lens in
situ.

55.  Ensure that contact lenses dispensed have the
correct parameters, are sterile and in good
condition.
 Ensure that optimum visual acuity is achieved by
the patient with the contact lenses.
 Ensure that the contact lenses fit satisfactorily.
 Provide instruction on care and maintenance.

56. THANK