RAJU KAITIOptometrist, Dhulikhel Hospital, Kathmandu University Hospital

Checking quality and physical characteristics for-

Prescription use Contact lens fitting sets Research purposes

Contact lens verification undergoes two stages, laboratory and clinical.

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.

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.

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.

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

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

Central Edge Lenticular junction At any other specified point

Axial and radial edge lift Edge shapes

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

Finish Polish Edge form Transitions Tint Material

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

Drysdale’s principle

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

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

• 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

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.

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

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.

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

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

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.

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.

Back vertex power (BVP) Front vertex power (FVP)

Front and back surface radii of curvature.

Centre thickness. Refractive index.

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

BVP in air is not equal to BVP in liquid If F1=BVP in air

F2=BVP in liquid

F1=kF2 where 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

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

Power for each meridian is measured

Cylinder value is derived from measuring the difference in meridional powers

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 multipl ied by four (approximately) to calculate the true lens power in air.

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

Lens diameters back optic zone diameter (BOZD) total diameter peripheral curve width (PCW)

Lens thickness centre thickness (ct) edge thickness

Diameters and linear parameters

Measuring magnifier V gauze Cast, dividers and

transparent rule Micrometer & spheres

Measuring magnifier

20 mm scale: used for corneal lens

V gauzeScale 6.00 – 12.50mm

Cast, dividers and transparent rule

Micrometer & spheres:Measure primary optic diameter, sag is determined

Can be measured with all most all techniques

Indirect method:

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

Lens diameters total diameter FOZD

Lens thickness centre thickness (ct) edge thickness

Instruments:

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

Instruments/techniques:

• Edge molding• Projection magnifier• Ehrmann profilometer• Palm test• Radiuscope (modified)

Surface defects Optical quality Lens impurities/deposits

Scratches and lathe marks: Can cause:

deposit build up poor wettability surface hydrophobicity

Indicate over polishing during manufacture

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

White background test

Variation in thickness

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

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

%water content = mass of water\mass 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

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.

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.

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.

THANK

YOU…