Presentation on theme: "ARAVIND EYE CARE SYSTEM Aravind Eye Hospital & Postgraduate Institute of Ophthalmology Madurai, India ARAVIND EYE CARE SYSTEM Aravind Eye Hospital & Postgraduate."— Presentation transcript:
ARAVIND EYE CARE SYSTEM Aravind Eye Hospital & Postgraduate Institute of Ophthalmology Madurai, India ARAVIND EYE CARE SYSTEM Aravind Eye Hospital & Postgraduate Institute of Ophthalmology Madurai, India Measurement of lens power by Lensometer Faculty Aravind School of Optometry
A R A V I N D - M A D U R A I Corrective lenses Lens typeLens formCorrective use SphereConvex ( + ) Concave (--) Hypermetropia Myopia Cylinder & Sphero-cylinder Convex ( + ) Concave (--) Hyperopic Astigmatism Myopic Astigmatism Prism Correcting squinting eyes Relieving eye strain
A R A V I N D - M A D U R A I Measuring Lens Power Hand neutralisation by trial lens method. By Lensometer.
A R A V I N D - M A D U R A I Trial lens method View cross target at distance Hold lens on visual axis, close to eye. Align lens such that cross target is continuous. Move lens vertically along line of vertical limb of target Determine direction of horizontal limb movement Place trial lens flush with ‘unknown’ lens No movement = neutral Repeat the same for line of horizontal limb of target.
A R A V I N D - M A D U R A I Neutralization of lens power using trial lenses Lens Movements Cross Target “Against” “With” Hand Neutralisation
A R A V I N D - M A D U R A I Hand Neutralisation of Toric Lenses
A R A V I N D - M A D U R A I Lensometer Measures the corrective lens power Sphere Cylinder and its axis Prism. Optic center
A R A V I N D - M A D U R A I Eyepiece Spring Clip Table Lens Stop Axis Dial Power Wheel Optic Centre Marker Components of a Focimeter
A R A V I N D - M A D U R A I Constructing a Focimeter Zero Position Target at first principal focus With Positive Power Lens Unknown lens at 2 nd principal focus Distance between standard lens & target is reduced Telescope Lens holder for unknown lens Target With Negative Power Lens Unknown lens at 2 nd principal focus Distance between standard lens & target is increased Standard lens
A R A V I N D - M A D U R A I x x fofo fofo f’of’o f’of’o x’x’ x’x’ Constructing a Focimeter Telescope Lens holder for unknown lens (holding negative lens in figure) Standard lensLight Source & Moveable Target
A R A V I N D - M A D U R A I Calculation Movement of the target per dioptre F o = D Target travel (mm) per dioptre: x =1000 / F o ² = 1000 / 25 ² = 1.6mm per Dioptre So, for a focimeter required to measure 20D the total required travel of target = 40 x 1.6 = 64mm
A R A V I N D - M A D U R A I Target system American optical system – crossed line target. European optical system – Ring dots system
A R A V I N D - M A D U R A I Focimeter Preparation Focus the eyepiece Ensure that all the readings at Zero. Calibration
A R A V I N D - M A D U R A I Line Target Reticule scale Prism dioptres delineation Focimeter – crossed line target system Insert the spectacle. Determine the lens power. Mark the optical centre. Measure the power of the second lens.
A R A V I N D - M A D U R A I Reticule scale Prism dioptre delineation Focimeter - Ring of Dots Targets
A R A V I N D - M A D U R A I Procedures Focus the eyepiece by focusing the hair-line black reticule. Place the back vertex (the ocular side) of the unknown lens against the lens stop of the lensometer. Make sure both eye wires of frame, right & left, are touching the stage. Move the lens side to side to align with center the target that places the optical center of the lens at the stop. Notice that the target lines are a cross composed of two sets of lines oriented 90 apart. Try focusing the target lines. If all lines are in focus simultaneously, the lens is spherical.
A R A V I N D - M A D U R A I How to measure the cylindrical power and axis? If only one set is in focus (the other set 90 away is blurred), the lens is cylindrical. If this is the case, focus one set of lines with the power drum, while simultaneously rotating the axis wheel so that the lines are not only clear, but they are also unbroken. Note the power on the drum. If the unknown lens is cylindrical, the set of lines 90 away will be out of focus. The power of the first set of clear lines is called the sphere reading. Now focus the other set of lines (90 away from the first set) so that they are clear and unbroken. Note the new power on the drum & also note the axis on the outside wheel.
A R A V I N D - M A D U R A I How to measure the cylindrical power and axis? If the 2 nd set of lines was focused at a lower + (more --) power, you have measured in ‘—’ cylinder form. Since our convention is ‘—’ cylinder form, get used to making the less ‘—’ measurement in 1 st measurement. That would mean that the 2 nd measurement would be more +, thus allowing to record the reading in ‘—’ cylinder form. In this case (minus cylinder form), the first measurement is the sphere, and the increase, or change in minus power you had to travel to the second, (higher minus power) focus is the cylinder power. Record the values.
A R A V I N D - M A D U R A I / x 120 Power wheel Sphere setting Power wheel Cylinder setting Axis Example - Line Targets
A R A V I N D - M A D U R A I / x 120 Axis Power wheel Sphere setting Power wheel Cylinder setting Example - Ring of Dots Targets
A R A V I N D - M A D U R A I Focus of cylindrical lens
A R A V I N D - M A D U R A I Measure the bifocal add Lift the front of the lens against the lens stop. Take a reading of one set of unbroken, focused lines through the distance portion, then move the stage up until the bifocal add is against the stop. Re-focus the same lines again, but in segment. The difference between the 1 st & 2 nd reading is the add power.
A R A V I N D - M A D U R A I 2.0 Base Up Vertical Prism
A R A V I N D - M A D U R A I 1.5 Base In Horizontal Prism
A R A V I N D - M A D U R A I 3.0 150 Oblique Prism
A R A V I N D - M A D U R A I Focimeter Use - Sources of Error Failure to focus the eyepiece Zero setting & axis alignment Centration of the reticule & target
A R A V I N D - M A D U R A I Projection & Automatic Focimeters