Lens power measurement


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Lens power measurement

  1. 1. Lens Power Measurement Walter Huang, OD Yuanpei UniversityDepartment of Optometry
  2. 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. 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. 4. Trial Lens Hand Neutralization
  5. 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. 6. Trial Lens Hand NeutralizationIt is used to measure the front vertexpower of the lens
  7. 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. 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. 9. Trial Lens Hand NeutralizationFor a plus or a minus lens, linear motion isused to neutralize power
  10. 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. 11. Trial Lens Hand NeutralizationFor a toric lens, rotational motion is used to findthe axis
  12. 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. 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. 14. Trial Lens Hand NeutralizationLarge distant cross target
  15. 15. Trial Lens Hand NeutralizationPlus lens
  16. 16. Trial Lens Hand NeutralizationMinus lens
  17. 17. Trial Lens Hand NeutralizationToric lens
  18. 18. LensometryDefinition “Lenso” = lens “metry” = measurement of
  19. 19. Lensometer
  20. 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. 21. LensometerIt is used to measure the back vertexpower or front vertex power of the lens
  22. 22. LensometerTo find the back vertex power, place theconcave side of lens against lens stop
  23. 23. LensometerTo find the front vertex power, place theconvex side of lens against lens stop
  24. 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. 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. 26. Lensometer SystemsFocusing system (Badal lens system) Light source Target (cross hairs) Standard lens (+20.00D) Lens stop Power wheel
  27. 27. Lensometer Schematic
  28. 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. 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. 30. Lensometer OperationWhen the lens of unknown power isintroduced, the image of the illuminatedtarget is thrown out of focus
  31. 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. 32. Lensometer OperationThe physical distance forward orbackward that the target moves indicatesthe power of unknown lens for themeridian being measured
  33. 33. Lensometer Anatomy Risley prism Lens holder Lens stop Axis wheeleyepiece Lens stage Power wheel
  34. 34. Lensometer reticule target Sphere line3 2 1 Cylinder lines
  35. 35. Lensometer
  36. 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. 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. 38. Single Vision Lens MeasurementTo measure single vision lenses, eitherback vertex powers or front vertex powersmust be found
  39. 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. 40. Types of Target
  41. 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. 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. 43. Single Vision Lens MeasurementExample +1.00 -2.00 x 120 Power wheel sphere setting
  44. 44. Single Vision Lens MeasurementExample +1.00 -2.00 x 120 Power wheel cylinder setting
  45. 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. 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. 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. 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. 49. PrismChanges direction of light going through alens
  50. 50. Prism PurposeTo treat a binocular vision problemTo shift the visual fieldTo improve the lens appearance throughprism thinning
  51. 51. PrismIt is often divided equally between the twolenses of spectacles for balance andaesthetic reasons
  52. 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. 53. Types of Prism and Lens
  54. 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. 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. 56. Prism Specification
  57. 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. 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. 59. Prism VerificationExample 1:
  60. 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. 61. Prism VerificationExample 2:
  62. 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. 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. 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. 65. Prentice’s RuleExample:
  66. 66. RecordingRecord the prescription for each lensseparatelyRecord the amount of induced prism ineach lens, if applicable
  67. 67. RecordingExample 1 OD -3.00DS Add +2.00 OS -1.50 -0.75 x 180 Add +2.00
  68. 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