Subject 7: The lens-meter: Measuring Posterior Vertex

Power .

• Description of the lens-meter• Calibrating the instrument• Measuring Posterior Vertex Power

– Measuring a spherical lens– Measuring an astigmatic lens

• Marking the lens with a lens-meter

Description of the lens-meter

•Measures the front power or posterior vertex of a lens

•(1) Eyepiece

•(2) Reticle dial

•(3) Marking Device Control

•(4) Lens Holder Handle

•(5) Spectacle Table Lever

•(6) Spectacle Table

•(7) Power light

•(8) Power switch

•(9) Locking Lever

Description of the lens-meter

•(10) Clamps

•(11) Marking device

•(12) Lens Stop

•(13) Power Drum

•(14) Cylinder Axis Wheel

•(21) Protractor Scale

•(22) Reticle

•(23) Target

•(24) Power scale

Calibrating the Instrument

• Adjusting the eyepiece: With the lens-meter off and with no lens on the lens stop: turn (1) the focus all the way counterclockwise. Then turn it clockwise until the image in the reticle is in focus.

• Calibration test: With the instrument on, turn the power drum (13) until the illuminated target is sharp. The power scale will read 0,0 D.

Measuring the back vertex power of a spherical lens

• To determine the bvp of a spherical lens, only one focusing is required.

• After placing the lens in question, with the back vertex against the Lens Stop, the target will appear unfocused.

Turning the power drum (13) bring into focus the two meridians of the target, and obtain the corresponding reading for the surface power.

Measuring posterior vertex power in an astigmatic lens

• Astigmatic lenses present two powers in two principal meridians, the lens-meter will provide two readings corresponding to these powers.

• The target is oriented with the cylinder axis wheel (14), at the same time that it is focused (13).

• Once part of the target is focused, rotating (13) will provide the perpendicular focus.

Marking a lens with a lens-meter

• The central point coincides with the OPTICAL CENTER of the lens.

• The straight line which contains the three points is the REFERENCE

• Once the target is clean and centered over the reticle, proceed with marking with the three markers.