How to use a Lens clock to determine the base and the power!

A lens clock is a mechanical dial indicator that is used to measure dioptric power of a lens. A lens clock measures the curvature of a surface, but gives the result as an optical power in diopters, assuming the lens is made of a material with a particular refractive index.

It is called as Geneva Lens Gauge, Lens measure or Lens Clock

Appearance

The lens clock has 3 legs

• ·         The 2 outside legs do not move
• ·         The center one moves in and out
• ·         The difference in height position of the center leg and the 2 outside leg is sag for the arc of a circle

**Doesn’t show the actual sag measurement but shows the dioptric value for the surface power

Using The Lens Clock

• ·         Place the clock on a flat surface, so that all 3 pins are equal, your clock should measure zero– If not, your lens clock is defective
• ·         The lens clock must be held perpendicular to the surface of the lens
• ·         Tilting the clock by 10° from the perpendicular, can create as much as 2 diopters of error in your reading.

Base Curve Determination

• ·         Defined as the beginning curve upon which the net power is based
• ·         The lens clock can be used to measure this
• ·         Modern lenses have spherical front surfaces (F1)– The base curve will be the lens clock reading  of the front surface of the lens
• ·         Back surface is called (F2)
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• ·         When measuring the F1 of the lens, you will need to read the black scale
• ·         When measuring the F2 of the lens, you will need to read the read scale
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• ·         If there are more than 1 curve on the front surface, the lens is either warped or is a plus-cylinder lens form
• ·         The base curve is the least curved of the 2 readings

Index

• ·         The lens clock is designed for materials where n = 1.53 (crown glass)
• ·         Measuring a lens where n > 1.53 – The lens clock will read too LOW
• ·         Measuring a lens where n < 1.53 – The lens clock will read too HIGH

Nominal Power of a Lens

Since the lens clock directly measures the surface values of a lens, we can use it to approximate the power of lenses

– Only works for materials with index of refractions close to 1.53

• ·         F1 measures +6.00D
• ·         F2 measures -4.00D
• ·         Ft = +2.00D (Power)

Power Determination

The lens clock can be used to measure sphere and cylinder power

1. Hold the lens clock so that the center leg is at the center of the lens and perpendicular to the lens surface

2. Rotate the lens clock around the center leg

3. If the needle on the lens clock remains unchanged, the surface is spherical

4. If the needle shows a change in value, the surface is toric with 2 separate curves

5. Read the maximum and minimum values

(The orientation of the three legs where the maximum and minimum readings are will correspond to the major meridians of lens power)

·         Modern lenses are of Minus cylinder form. So while measuring the power of the lens you might find cylinder in F2 and the F1 will always be SPHERICAL

1.      When rotating the lens clock on the front surface of a lens, all meridians read +4.00D. On the back surface, the clock reads -6.25D Then the power will be

(+4.00) + (-6.25) = -2.25D

2.      When rotating the lens clock on the front surface of a lens, all meridians read +6.50D. On the back surface, the clock reads -7.50D when the 3 legs are along the 180° meridian, and -6.00D when the 3 legs are along the 90°meridian.  Here you can calculate the nominal power of the lens if the lens made in minus cylinder form as given below.

Ref: Dr. Long D. Tran, Wikipedia