Lensometer: Measuring Spectacle Lenses

By /

OPTOMETRY · SEMESTER 1

Lensometer: Measuring Spectacle Lenses

Ophthalmic Dispensing Theory

START READING NOTES

FIACLE- AUST

  • KCMC- School of optometry

LENSOMETRY

Lensometry is lensometer.

  • Use to check
  • Spherical lens power
  • Cylindrical lens power
  • Axis
  • Reading add
  • Prism power and direction
  • Optical center of lens
  • Contact lens power
  • Use to mark
  • Actual optical center
  • Required optical center when prism is present

Axis direction

PARTS OF LENSOMETER

  • Eyepiece
  • Chrome knurled sleeve
  • Prism compensating device knob
  • Lens holder handle
  • Marking device control
  • Gimbal
  • Ink pad
  • Spectacle table lever

Spectacle table

10. Power drum

  • 11. Locking lever
  • 12.Prism axis scale
  • 13.Prism compensating device
  • 14.Prism diopter power scale
  • 15. On – off switch
  • 16. Lens stop
  • 17. Cylindrical axis wheel
  • 18. Filter lever

19. Lamp access cover

Focus the eyepiece [accommodation and refractive error]

  • Focus the mires
  • Power drum sets on zero
  • If testing a pair of glasses,always check the right lens first

SET UP

In Focus

  • Out of Focus

FOCUSING THE EYEPIECE

User’s eye becomes part of optical system

  • Any uncorrected refractive error in eye will affect actual measurement of lens power
  • Also, user can accommodate to bring target into focus
  • Operator uses Lensometer for long periods – fatigue may affect ocular condition

Importance of focusing eyepiece -_x000b_

SPHERICAL LENS

CYLINDRICAL LENS

VIEW OF TARGET

Focus the eyepiece

  • Power drum sets on zero
  • Place lens on lens stop with back(concave) surface against lens stop
  • Centre the lens in the lens stop

Procedure for spherical lens

Rotate the power wheel until target comes into sharp focus

  • Rotate power drum towards + or – till mires are focused

Read power of lens from power wheel & record

Focus the eyepiece

  • Power drum sets on zero
  • Center the lens
  • First clear the mires in one meridian & record the reading in the same axis
  • Eg:- -2.00×10º
  • Then, clear the mires for other meridian
  • Eg:- -4.25x 100º
  • Then transpose in sphero-cyl form
  • -2.00\-2.25 x 100º

PROCEDURE FOR CYLINDERICAL LENS

Segment side should be placed against lens stop

  • For back surface segment (E.g. Ultex) –
  • Add = BVP of near portion – BVP of distance portion
  • For front surface segment (E.g. Fused & Executive) –
  • Add = FVP of near portion – FVP of distance portion

Procedure for add power

By centering the target in the crosshair or center circle of the reticule of the lensometer.

MARKING OPTICAL CENTER

For Rigid:-

  • Change the lens stop
  • Focus the eyepiece
  • Clean,rinse & dry the lens
  • Center the lens
  • Back(concave)surface against lens stop
  • Obtain clear mire image
  • Record BVP

PROCEDURE FOR CONTACT LENS

For Soft:-

  • Direct measurement

Wet cell method

Focus the eyepiece

  • Blot dry with lint free tissue
  • Air dry for 10 sec
  • Place lens with convex side against the lens stop
  • Center the lens
  • FVP is measured
  • Within four mins of removing from bottle – power tends to increase.

Direct measurement

Lens is immersed in a small chamber filled with saline

  • Entire system is placed on the lens stop of the lensometer
  • Measures BVP or FVP.
  • Multiply by conversion factor – compensation for difference in refractive indices.
  • Not very accurate – varies with refractive index of lens.

WET CELL METHOD

Steepness of contact lens in relation to lens stop

  • Improper placement of lens on stop.
  • Spherical aberration in contact lens.

Sources of error

Target displaced from optical center of instrument (though in sharp focus)

  • Amount of displacement = power of prism
  • Measured by graticule (1 unit = 1 Prism D)
  • Direction of displacement = direction of base of prism (telescopic system inverts image)

Procedure for prism power & direction

E.g. Mires displaced temporally on a scale of 2

  • Reading is 2 Prism base out

(1 unit = 1 Prism D )

Eyepiece not focused

  • Power drum not at zero

Improper centration of reticule or target Inspite of no zero error high powers give too plus or too minus readings – lens stop is in incorrect position.

Eg:- +20.00+21.00, -20.00  -18.00

Inspite of no zero error high powers read too high or too low – standard lens likely to be incorrect

Eg:- +20.00+21.00 , -20.00 -21.00

Sources of error in lensometry

Position of lens vertex.

  • Misalignment of marking system .
  • Mental arithmetic errors by the operator.
  • Filter

Drum backlash.

Clinical Optics

  • Bennett’s- ophthalmic prescription work
  • CLAO Volume-I
  • Optometric instrumentation

References….

Treatment

Preserving or restoring vision and improving appearance may involve one or more of the following:

  • glasses to aid in focusing and straighten the eye(s)

patching to force infants and young children to use and straighten the weaker eye eye drops or ointments as a substitute for patching or glasses or to make glasses more effective surgery to tighten, relax, or reposition eye muscles medication injected into an overactive eye muscle to allow the opposite muscle to straighten the eye vision training (also called eye exercises)

Scroll to Top