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Lens power measurement


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  • 1. Lens Power Measurement Walter Huang, OD Yuanpei UniversityDepartment of Optometry
  • 2. Methods of Lens Power MeasurementTrial lens hand neutralizationLensometer Much greater accuracy may be obtained with the lensometer than with trial lens hand neutralization
  • 3. Trial Lens Hand NeutralizationTwo lenses neutralize each other whenplaced in contact with each other so thatthe combined power of the two lenses isequal to zeroAn unknown lens is neutralized by aknown trial lens of equal power butopposite in sign
  • 4. Trial Lens Hand Neutralization
  • 5. Trial Lens Hand NeutralizationThis is performed in the absence of alensometerIt is used qualitatively as a means forestimation in many clinical and dispensingsituationsIt often involves simply identifying if it is aplus, a minus, or a toric lensIt more accurately estimates low powerplus and minus lenses than toric lenses
  • 6. Trial Lens Hand NeutralizationIt is used to measure the front vertexpower of the lens
  • 7. Trial Lens Hand NeutralizationView a large distant cross target throughthe lensHold lens on visual axis, at arm’s distanceAlign lens such that the cross target iscontinuousMove lens vertically, observe motion ofhorizontal lineMove lens horizontally, observe motion ofvertical line
  • 8. Trial Lens Hand NeutralizationFor a plus or a minus lens, linear motion isused to neutralize power If observe “against motion,” use plus lens If observe “with motion,” use minus lens
  • 9. Trial Lens Hand NeutralizationFor a plus or a minus lens, linear motion isused to neutralize power
  • 10. Trial Lens Hand NeutralizationFor a toric lens, rotational motion is usedto find the axis If observe “against motion,” use plus cylinder axis If observe “with motion,” use minus cylinder axis
  • 11. Trial Lens Hand NeutralizationFor a toric lens, rotational motion is used to findthe axis
  • 12. Trial Lens Hand NeutralizationPlace known trial lens against front surface ofunknown lensNo movement indicates neutralityA minus or plus lens (i.e., a spherical lens) hasthe same speed and direction of motion in boththe vertical and horizontal meridiansIn the case of a toric lens (i.e., spherocylindricallens), neutralize one limb of the cross target at atime
  • 13. Trial Lens Hand NeutralizationExample An unknown lens is neutralized in the horizontal meridian with +3.00D and the vertical meridian with +2.00D Prescription of unknown lens: +3.00 -1.00 x 180 Power cross:
  • 14. Trial Lens Hand NeutralizationLarge distant cross target
  • 15. Trial Lens Hand NeutralizationPlus lens
  • 16. Trial Lens Hand NeutralizationMinus lens
  • 17. Trial Lens Hand NeutralizationToric lens
  • 18. LensometryDefinition “Lenso” = lens “metry” = measurement of
  • 19. Lensometer
  • 20. Lensometer PurposeNeutralizing a pair of glasses To determine the prescriptionVerifying a pair of glasses To confirm the accuracy of fabricated glassesDuplicating a pair of glasses To determine the prescription, and the lab duplicates the exact prescription
  • 21. LensometerIt is used to measure the back vertexpower or front vertex power of the lens
  • 22. LensometerTo find the back vertex power, place theconcave side of lens against lens stop
  • 23. LensometerTo find the front vertex power, place theconvex side of lens against lens stop
  • 24. LensometerIn the case that the lens is a sphero-cylindrical prescription, the lensometer isused to determine the cylinder axisIt is used to locate the optical center of thelensThe lensometer is used to measure theamount of prism in the lens
  • 25. Lensometer SystemsObservation system (Keplerian telescopewith two plus lenses, an inverted targetimage) Objective lens Eyepiece (ocular lens) Reticle (concentric circles and cross hairs focused by eyepiece)
  • 26. Lensometer SystemsFocusing system (Badal lens system) Light source Target (cross hairs) Standard lens (+20.00D) Lens stop Power wheel
  • 27. Lensometer Schematic
  • 28. Observation SystemThe Keplerian telescope consists of anobjective lens, an eyepiece, and a reticleThe two plus lenses are positioned so thattheir two focal points coincide with eachotherThe unknown lens whose power is to bemeasured or neutralized is positioned atthe lens stop (the location of thesecondary focal plane of the standardlens)
  • 29. Lensometer OperationWith the instrument set at zero, anilluminated target (light source) ispositioned at the focal length of a plus lens(standard lens) usually a +20.00D lensDiverging rays of light from the illuminatedtarget are bent by the standard lens andparallel light emerges from focusingsystem into the observation system, whichis viewed through the telescope by theobserver
  • 30. Lensometer OperationWhen the lens of unknown power isintroduced, the image of the illuminatedtarget is thrown out of focus
  • 31. Lensometer OperationThe target is movableBy moving the target closer to or fartherfrom the standard lens, the refractivepower of the unknown lens can beneutralized Closer to standard lens for plus lens neutralization Farther from standard lens for minus lens neutralization
  • 32. Lensometer OperationThe physical distance forward orbackward that the target moves indicatesthe power of unknown lens for themeridian being measured
  • 33. Lensometer Anatomy Risley prism Lens holder Lens stop Axis wheeleyepiece Lens stage Power wheel
  • 34. Lensometer reticule target Sphere line3 2 1 Cylinder lines
  • 35. Lensometer
  • 36. Lensometer PreparationFocus the eyepiece of the lensometer forthe examiner’s eye With the power wheel set on zero, turn the eyepiece as far counter-clockwise as possible Then slowly turn it clockwise until the reticule first comes into sharp focus
  • 37. Lens Measurement PreparationInsert the spectacles  If testing a pair of glasses, always check the right lens first  Place the pair of glasses in the lensometer with the ocular surface away from you  The lens is held in place by the lens holder and is held level on the lens table  Center the lens by moving it so that the image of the lensometer target is aligned in the center of the eyepiece reticle
  • 38. Single Vision Lens MeasurementTo measure single vision lenses, eitherback vertex powers or front vertex powersmust be found
  • 39. Single Vision Lens MeasurementDetermine which part of the target is usedfor determining the spherical componentand which part of the target is used fordetermining the cylindrical componentRotate the power wheel until the lines (orthe spots) are in clear focusIf the power is spherical, all the lines (orspots) will be clearNote the power on the power wheel
  • 40. Types of Target
  • 41. Single Vision Lens MeasurementIf the spherical and cylindrical lines do notcome into focus at the same time, the lenshas a cylindrical componentRotate the power wheel until the sphericallines focus with the less minus (or moreplus) powerOrient the target rotation dial (axis wheel)so that the spherical lines are perfectlystraight
  • 42. Single Vision Lens MeasurementRead the power and record as thespherical component of the prescriptionFocus the cylindrical lines by rotating thepower wheel to more minus (or less plus)power (90 degrees away)The difference in power between the twoprincipal meridians is the amount ofminus cylinder power in the lensRead the axis of the cylinder from the axiswheel
  • 43. Single Vision Lens MeasurementExample +1.00 -2.00 x 120 Power wheel sphere setting
  • 44. Single Vision Lens MeasurementExample +1.00 -2.00 x 120 Power wheel cylinder setting
  • 45. Single Vision Lens MeasurementMark the optical center (OC) Make sure that the lens is centered and the spectacle is sitting on the lens table evenly Use the OC marker on the lensometer to spot the lens Three dots will be marked The center dot marks the OC The other two dots indicate the 0 to 180 horizontal linePosition and read the second lens
  • 46. Single Vision Lens MeasurementWhen both lenses have been measuredand marked, measure the distancebetween optical centers of the lenses(DBOC or geometric center distance)
  • 47. Multifocal Lens MeasurementTo measure bifocal and trifocal lenses,front vertex powers must be found This is especially true for lenses with high distance and near powers Again, front vertex power is measured by turning spectacles around with back surface of the lens toward the operator (i.e., the convex side of the lens against the lens stop)
  • 48. Multifocal Lens MeasurementMeasure the distance portion of multifocallenses, in the same way as with single visionlensesTurn the glasses around backward so that thetemples face the operatorFind the distance front vertex powerFind the near front vertex powerRecord the addition power (Add), which is thedifference between the distance and nearprescriptions
  • 49. PrismChanges direction of light going through alens
  • 50. Prism PurposeTo treat a binocular vision problemTo shift the visual fieldTo improve the lens appearance throughprism thinning
  • 51. PrismIt is often divided equally between the twolenses of spectacles for balance andaesthetic reasons
  • 52. Types of PrismHorizontal prism Base in (BI) Base out (BO)Vertical prism Base up (BU) Base down (BD)Oblique prism A combination of horizontal and vertical prisms
  • 53. Types of Prism and Lens
  • 54. Methods of Achieving Prism in LensGrinding Usually applied when a large amount of prism is required Lens is custom-made Optic center is often not on lensDecentration Usually applied when a small amount of prism is required With spherical prescription, it is easier to deal with decentering than with spherocylindrical prescription
  • 55. Prism MeasurementThe purpose is to verify if the prescriptioncontains the desired prismatic effectVerification of prism in prescription is donesimilarly to measurement of lens powerusing the lensometerThe only difference is in the means bywhich the target is positioned in thelensometer
  • 56. Prism Specification
  • 57. Prism VerificationTo verify the prescribed prism when theamount of prism is known The center of the illuminated target is positioned at the location on the circular mires corresponding to the prism required
  • 58. Prism VerificationExample 1: If the right lens calls for 2 prism diopter BU, then the illuminated target would be positioned at the “2” ring above the center of the mires
  • 59. Prism VerificationExample 1:
  • 60. Prism VerificationExample 2: If both lenses contain 1.5 prism diopter BO, then the target would be placed at the 1.5 ring to the left of the center of the mires for the OD lens and to the right of the center of the mires for the OS lens Mark the major reference point (MRP) with the OC marker and measure the distance between the MRP of the lenses (i.e, the patient’s distance PD)
  • 61. Prism VerificationExample 2:
  • 62. Prism MeasurementIn an unknown lens Patient may come in with a prescription that you are not sure if it contains prism in the lenses After power of the lenses are neutralized and the optic centers are marked, measure the distance between the optic centers (DBOC) If DBOC does not equal to the patient’s distance PD, then there is prism in the lens
  • 63. Prentice’s RuleP=d*F P = prism power (in prism diopters) d = decentration (in cm) F = refracting power of the lens (in diopters)
  • 64. Prentice’s RuleExample: Rx = -4.00DS OU Patient’s distance PD = 62mm DBOC = 72mm P=d*F = (72 – 62mm)/2 x 1cm/10mm * (- 4.00D) = 0.5cm * (-4.00D) = 2 prism diopters BI in each lens
  • 65. Prentice’s RuleExample:
  • 66. RecordingRecord the prescription for each lensseparatelyRecord the amount of induced prism ineach lens, if applicable
  • 67. RecordingExample 1 OD -3.00DS Add +2.00 OS -1.50 -0.75 x 180 Add +2.00
  • 68. RecordingExample 2 OD +2.50 -0.75 x 080 2 prism diopter BI OS +1.00 -0.25 x 110 2 prism diopter BI