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DispensingBSc (6).pptx

  1. Dispensing (6) Dr. Atif Babiker Mohamed Ali Faculty of optometry 2023
  2. The Lensmeter • A lensmeter or lensometer, also known as a focimeter is an ophthalmic instrument used for measuring lenes power. • It is mainly used by optometrists and opticians to verify the power of spectacle lenses, to properly orient and mark uncut lenses, and to confirm the correct mounting of lenses in spectacle frames. • Lensmeters can also verify the power of hard contact lenses, if a special lens holder is used.
  3. Uncut lens & spectacle
  4. Types of Lensmeters • There are Auto-lensometers , electronic fully objective that work very well and are very convenient. • A manual lensometer must always be kept on hand in case the auto-lensometer is not working. • Every optometrist and technician must know how to use a manual lensometer to read a pair of glasses.
  5. Types of Lensometers
  6. Optics of manual lensometers • The lensometer measures the vertex power of the lens through the back focal length. • A lensometer like an optical bench consisting of • A. An illuminated moveable target • B. A powerful fixed lens • C. A telescopic eyepiece focused at infinity.
  7. A simple lensmeter cross sectional view • 1 – Adjustable eyepiece • 2 – Reticle • 3 – Objective lens • 4 – Telescope • 5 – Lens holder • 6 – Unknown lens • 7 – Standard lens • 8 – Illuminated target • 9 – Light source • 10 – Collimator • 11 – Angle adjustment lever • 12 – Power drum (+20 and -20 Diopters) • 13 – Prism scale knob
  8. Parameters of a lensmeter • The parameters of a lensmeter are the values specified on the patient's prescription: sphere, cylinder, axis, and in some cases, prism. • There are some variations in the design and operation of lensmeters. • some have the diopter scale on the diopter wheel, and others have a scale inside the eyepiece head.
  9. Target of Lensometer
  10. How to read glasses • To read glasses on the manual lensmeter you must understand the different scales and parameters. • The Diopteric number line used starts at zero and extends in the plus direction for about 20 diopters, and in the minus direction for about 20 diopters. • Each diopter is divided into half, quarter, and eighth diopters. • As in the case with many ophthalmic instruments, positive numbers are in black, and negative numbers are in red.
  11. Adjust the eye piece • The first step in lensometry is to adjust the eyepiece. • This step is necessary to insure the accuracy of measurement. The eyepiece is adjusted for small refractive error and to account for any accommodation. • Turn the eyepiece toward plus (counterclockwise) until the reticule (cross lines) becomes blurr. • Turn the eyepiece back in the minus (clockwise) direction, slowly, until the reticule just comes into focus, then stop. This is the correct position for to start measurement.
  12. Measuring a spherical, single vision lens • A single vision lens should be inserted and ensure the optical center is adjusted so that the mires at center in the reticule. • The mires are brought into sharp clear focus by adjusting the power wheel. You would then read the lens power from the power drum. • In spherical lenses there is no change in shap and size of mires (target). • Usually the single line and the triple line both will come into focus at the same time( often 180 &90 degrees).
  13. Measuring cylindrical lens • Using the power wheel and the axis wheel to put the mires into focus. • By rotating the axis wheel put the single line mire in focus, in this state the diopter power on the power wheel is at zero. • Now rotate the power wheel, the triple line becomes in focus and note the power reading, this is cylinder power and direction of triple lines is the axis of cylinder. • The plus direction reads in plus cylinder notation. The minus direction reads in minus cylinder notation.
  14. Vertical and Horizontal Axes
  15. Measuring sphero-cylindrical lens • When the power wheel rotated one of mired appeared to be clear. So the axis wheel should be rotated until the single mire becomes clear and adjust the power wheel to the sharp focus( e.g. +2.0D this would be the sphere power) suppose the single line at 90. • However, the triple line were in 180 and rotate the power wheel again until the lines in sharp focus let e.g. power is +2.5D. • Then (+2.5 – 2.0) = 0.5 axis 180 • The final power is +2.0/+0.5CX180