This document discusses the verification process for contact lenses. It has two main stages - laboratory and clinical. In the laboratory, lenses are checked to ensure their parameters match what was ordered. Clinically, lenses should be verified upon receipt to ensure the correct lens was dispensed. Parameters like radius of curvature, diameters, thickness and power must be measured for both rigid and soft contact lenses using various techniques and instruments. On-eye verification is also important to assess fit and comfort. The goal of verification is to ensure patients receive high quality lenses that meet specifications and provide good vision.

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