determining rx with a lens clock

[ammoe2]

:D
Can anyone tell me how to determine a prescription using a lens clock? One of the residents here needs to know for an exam. I though he was joking as I always rely on the lensometer

Thanks in advance

[Robert Wagner]

Good day,

In using a lens clock to determine a power of a lens is very easy.

1. make sure your lens clock is "0'd" meaning on a plain base surface it reads 0.

2. Take a reading on the front surface of the lens.

3. take a reading on the back surface of the lens.

4. Add the two together and you will have the answer.

If you need to find out cylinder with a lens clock you can find the lowest number on the back surface and rotate the lens clock and the the difference is the cylinder power

I hope this helps

Robert
;)

[chip anderson]

Robert:

I'm afraid you can't do it very accurately anymore as most lens clocks are set for spherical crown glass index lenses. We don't use this much anymore as the indices vary so much and aspheric surfaces don't clock well.

Chip

[Oha]

There is a formula to compensate for the correct index of refraction- Im sure Darryl would know it.

And dont forget that thickness too, especially in higher plus prescriptions, can effect the results.

[Darryl Meister]

Most lens clocks are calibrated for a 1.530 reference (or "tooling") index. To convert the surface power measured with a lens clock to the correct, or actual, refractive surface power use:

Actual Power = (Index - 1) / 0.530 * Measured Power

So, if you measured 6.00 D on a polycarbonate (index = 1.586) lens, your actual surface power would be:

Actual Power = (1.586 - 1) / 0.530 * 6.00
Actual Power = 6.63 D

Remember for concave surfaces to use the red or negative numbers on your lens clock. Also remember to hold your lens clock perpendicular to the surface to get an accurate reading. Lens clocks will only give you correct readings on spherical and toric (with cylinder) surfaces. Aspheric and progressive lenses cannot be measured accurately with lens clocks.

Best regards,
Darryl

[chip anderson]

One more small pearl on Geneva lens clocks. Each mark on the dial is .01 mm depression of the center pin relative to the two other pins. It can therefore be used as a thickness gauge on contact lenses resting convex down on a flat surface.

Chip

[Shutterbug]

And just a additional note - this procedure is not going to give you an accurate rx, just an estimate. You're still going to need a lensometer if you want accuracy. I'm not sure why they would even include this procedure on a test. It would be more practical to discuss measuring slab-off prism with a lens clock.

:D

shutterbug

[DC Optix]

Raising this one from the dead...


Need some help. When using the lens clock in this way, on a toric back surface, how do you determine which curve from the back to use together with the front curve to determine the spherical power?

Thanks in advance!

[rbaker]

Neutralize the unknown lens with trial lenses. Its very accurate and index of refraction is not an issue.

[chip anderson]

First determine which axis you want to be the sphere power, + or - cyl. Then just subtract the back curve on that axis from the front curve. This will leave some error if one doesn't know the index. Or as suggested you can neutralze. If plus you can determine the focal length in each axis then you will know. If minus you have to neutralize the focal length with a plus lens of at least equal or greater power..

Chip

Guess the old lens bench concept just isn't taught anymore.

[DC Optix]

Please forgive the ingnorance :(

How do I determine the axis using ONLY a lens clock?

The lens clock will be the only tool available and I will have to estimate the Rx using only the surface curvature readings. Lets say I clock the front and it's +6.25 and find the back to be -6.75 and -7.25. I'm unsure as to which backside reading I use in combination with the front reading to calculate the spherical power.

Or am I completely wrong on how to calculate it?

Thanks for everyone's help...this just isn't sticking for some reason :hammer:

[Bob MacNeil]

Yes, I agree, trial lens each meridian, and they can learn the added benefit of finding prescribed prism.

Bob

[DC Optix]

Now THAT is a resurrection from the dead! LOL

[loose-screw]

I am studying for my ABO exam in May and that is where the optical cross comes into effect
one reading at 180 meridian and one at the 90 meridian on the back side and using the front curve to subtract the difference