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Contact lens verification(raju)
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.
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Speaker Raju Kaiti, an optometrist at Dhulikhel Hospital, presents on the topic of contact lens verification.
Verification involves laboratory and clinical stages for lenses intended for prescriptions, fittings, or research.
Highlights the importance of verifying contact lenses during manufacturing and upon receipt for proper fit.
Focus on ensuring proper lens dispensing, assessing lens quality, and verifying trial fitting accuracy.
Verification parameters include radii of curvature, power, and edge profiles for both rigid and soft lenses.
Specifies various radii measurements for both back and front optic zones crucial for contact lens fitting.
Details the measurement of diameters such as back and front optic zone diameters and bifocal segment positions.
Defines where measurements should be taken on the lens, including central and edge points.
Measurement techniques for edge lift and shape characteristics of contact lenses.
Focus on back and front vertex powers, near addition, prism, and aberrations affecting lens performance.
Measures aspects such as finish, polish, and tint to ensure optimal lens surface quality.
Listing key instruments used for measuring contact lens parameters like radius and curvature.
Introduction to Drysdale's principle relevant to contact lens measurement.
Describes the method for measuring lens curvature using a microscope and lens holder.
Outlines a method for measuring the radius of curvature using dial gauge readings.
Details the procedure for measuring lens curvature when dry.
Explains a procedure for measuring the front optic zone radius with the lens positioned correctly.
Describes air checking method necessary for measuring soft contact lenses under specific conditions.
Explains how keratometers can measure corneal curvature and contact lens parameters with attachments.
Details on modifying keratometer setups for accurate contact lens measurements.
Employs wet cell measurements to achieve accurate back optic zone radius readings.
Discusses the refractive index principles applied through the R-C device for lens design.
Combines photokeratoscope functions with imaging technology for analyzing corneal data.
Focus on obtaining back vertex power and other critical measurements with precision.
Describes focimeter techniques for measuring back vertex power in differing mediums.
Explains the relationship between back vertex power in air vs. liquid through compensation factors.
Outlines steps for taking precise measurements of lens power using focimeters.
Describes how cylinder values are derived using differences in meridional power measurements.
Steps to air check silicone contact lenses for accurate power measures in saline conditions.
Methodology for converting front vertex power measurements into back vertex power.
Discusses the measurement of lens diameters and thicknesses essential for fittings.
Various instruments used for measuring lens diameters and linear parameters.
Further measurement techniques highlighting murky and specific radius checking methods.
Describes methods for verifying lens thickness using various gauging techniques.
Lists detailed thickness measurements specific to various lens types.
Highlights different instruments utilized for thickness evaluations of lenses.
Identifies concerns regarding surface defects that affect optical quality.
Explains how scratches can degrade lens performance and indicate manufacturing issues.
Describes tools for analyzing surface quality and optical performance of lenses.
Shows testing methods aimed at identifying variations in lens thickness.
Discusses the complexities of measuring hydrogel lenses due to their hydration properties.
Outlines the correct method for handling and verifying hydrogel contact lenses.
Procedures for calculating the water content percentage in hydrogel lenses efficiently.
Emphasizes the importance of on-eye examinations for verifying proper fit and comfort.
Summarizes the entire lens fitting verification to ensure optimal vision and fit.
Concludes the presentation, reinforcing the importance of proper lens dispensing and maintenance.
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Contact lens verification(raju)
- 1. RAJU KAITI Optometrist, DhulikhelHospital, Kathmandu University Hospital
- 2. Checking qualityand physical characteristics for- Prescription use Contact lens fitting sets Research purposes Contact lens verification undergoes two stages, laboratory and clinical.
- 3. Laboratory Duringthe 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 correctlens 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 andsoft 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 opticzone 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 opticzone 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 andradial edge lift Edge shapes
- 11. Back vertexpower 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.
- 17. Lens holderis 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 gaugereading 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 ismeasured 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 procedurefor 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 Keratometerwhich 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-upis 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-upis 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 ofthe 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 thefeatures of the photokeratoscope and a built-in camera, polaroid images of the cornea are captured and viewed with a computer monitor.
- 28. Back vertexpower (BVP) Front vertex power (FVP) Front and back surface radii of curvature. Centre thickness. Refractive index.
- 29. It measuresBVP 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 inair 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 iscleaned 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 foreach meridian is measured Cylinder value is derived from measuring the difference in meridional powers
- 33. The SCLis 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 issame 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 andlinear 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 bemeasured 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: Projectionmagnifier 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 andlathe marks: Can cause: deposit build up poor wettability surface hydrophobicity Indicate over polishing during manufacture
- 47. Instruments: Magnifying10x loupe Projection magnifier Contact lens optical quality analyzer (CLOQA) Dark field microscope Moire fringe deflectometer
- 49. White backgroundtest Variation in thickness
- 50. Hydrogel contactlenses 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 1974recommend 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 dispensinglens VA assessment before and after over-refraction. Always assess dynamic and static fit, surface wet ability & lens quality, corneal integrity.
- 54. At theend 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 thatcontact 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.