lens clocks / plastic + glass BC

[harry888]

Is my memory faulty?

I seem to remember that there were two different types of lens clocks about 20 years ago, one calibrated for plastic and one for glass. So a 6 base was a different curve for the different materials because of the difference in index of refraction.

Now it seems all lens clocks are the same, so wouldn't the power assumptions be erroneous?

Harry

[LENNY]

What is the lens Clock?

Does it have something to do with a Coocoo Clock?:D

[Clive Noble]

The 'lens clock' Ahh, yes, it was an amzing invention about 20 years ago from Casio and another Far Eastern lens manufacturer where they managed to laminate the face of a clock within the structure of an ophthalmic lens, I suppose a bit like polarized lenses.

When the lens was viewed at a certain angle the reflected light would show the face of a clock and it could be set to show the time for either the patient or someone looking at him.

I'm attaching an image of a pair I made then. It still keeps good time I still have a pair of these lens blanks left, I do believe the last ones in the world, and plan to make them into my new Rx in one week's time.

[rbaker]

A given lens clock is calibrated for a specific index of refraction. The radius of curvature of a lens has nothing to do with it’s index of refraction.

[Chris Ryser]

Now it seems all lens clocks are the same, so wouldn't the power assumptions be erroneous?
Harry

All about lens clocks on Visioncare Products at:

http://www.visioncareproducts.com/05...ent_clock.html

[chip anderson]

Don't know about the other brands but I do remember that a Geneva Lens clock would show each small mark as 1mm deviation on it's face. It could actually lbe used as a contact lens thickness guage. I suspect that all of these are just a mm guage with a differen face on them. Of coures if the non-movable points were spaced differently, the readings for curvature would be different.

Chip

I have similar old guage that measures contact lens radius manually.

Chip

[SpecialT]

Now it seems all lens clocks are the same, so wouldn't the power assumptions be erroneous?

Harry

You are correct in what you are saying but it doesn't matter much because these days you aren't calculating the power of the lens. Lensometers are much better for this, of course. Because they are used pretty much only to check for consistency between two pairs of specs or between the right and left lens, it really isn't an issue. But if you are going to read the difference between the front and back surface and calculate the power of the lens then you must consult the equations that adjust for index.

[chip anderson]

Of course it wouldn't cost but a few cents more to have multiple scales printed on the same clock~face for many indices. Then if the index of the material is known, the same clock could be used on all materials.

Chip

[SpecialT]

Excellent suggestion Chip!

[Darryl Meister]

Virtually all lens clocks were calibrated for an index of 1.530 (i.e., the standard "tooling" index).

[Darryl Meister]

By the way, you can convert this into a different index (n) by multiplying the lens clock reading by (n - 1) / 0.53.