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Measurement of lens power by Lensometer. Faculty Aravind School of Optometry. Corrective lenses. Measuring Lens Power. Hand neutralisation by trial lens method. By Lensometer. Trial lens method. View cross target at distance Hold lens on visual axis, close to eye.
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Measurement of lens power by Lensometer Faculty Aravind School of Optometry
Measuring Lens Power • Hand neutralisation by trial lens method. • By Lensometer.
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.
Hand Neutralisation Neutralization of lens power using trial lenses Lens Movements Cross Target “Against” “With”
Lensometer • Measures the corrective lens power • Sphere • Cylinder and its axis • Prism. • Optic center
Components of a Focimeter Eyepiece Spring Clip Table Lens Stop Axis Dial Optic Centre Marker Power Wheel
Constructing a Focimeter • Zero Position • Target at first principal focus Telescope Lens holder for unknown lens • With Negative Power Lens • Unknown lens at 2nd principal focus • Distance between standard lens & target is increased Standard lens • With Positive Power Lens • Unknown lens at 2nd principal focus • Distance between standard lens & target is reduced Target
x fo f’o x’ Constructing a Focimeter Lens holder for unknown lens (holding negative lens in figure) Light Source & Moveable Target Standard lens Telescope
Calculation • Movement of the target per dioptre Fo = +25.00D Target travel (mm) per dioptre: x=1000 / Fo² = 1000 / 25² = 1.6mm per Dioptre So, for a focimeter required to measure 20D the total required travel of target = 40 x 1.6 = 64mm
Target system • American optical system – crossed line target. • European optical system – Ring dots system
Focimeter Preparation • Focus the eyepiece • Ensure that all the readings at Zero. • Calibration
Reticule scale 1 3 5 Prism dioptres delineation Line Target Focimeter – crossed line target system • Insert the spectacle. • Determine the lens power. • Mark the optical centre. • Measure the power of the second lens.
Reticule scale 90 180 0 Prism dioptre delineation Focimeter - Ring of Dots Targets
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.
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.
How to measure the cylindrical power and axis? • If the 2nd 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 1st measurement. That would mean that the 2nd 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.
Example - Line Targets +1.00 / -2.00 x 120 Power wheel Sphere setting Power wheel Cylinder setting Axis - +2.00 - - - - +1.00 - - - 0.00 - - - - -1.00 - -
Example - Ring of Dots Targets +1.00 / -2.00 x 120 Power wheel Sphere setting Power wheel Cylinder setting Axis 90 90 180 0 180 0 - 0.00 - - - - -1.00 - - - +2.00 - - - - +1.00 - -
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 1st & 2nd reading is the add power.
Vertical Prism 2.0 Base Up
Horizontal Prism 1.5 Base In
Oblique Prism 3.0 up @ 150
Focimeter Use - Sources of Error • Failure to focus the eyepiece • Zero setting & axis alignment • Centration of the reticule & target
