Drum Values?

[HarryChiling]

When ordering GP lenses the parameters are sometimes refered to as drum values. Does anyone know why?

[Diane]

Harry,

Oftentimes practitioners order bitoric GP lenses based on drum readings rather than giving an Rx to the lab and just the K's, and letting them design the lenses. Bitorics are fairly easy to design and fit.

For example, say you have an Rx of -2.00 - 3.50 X 180. K's 43.00@180 and 46.50@090. That's with the rule astigmatism and also corneal astigmatism and has too much corneal toricity for a spherical lens to fit well on the cornea. So you would order a bitoric lens. Going 0.50 flatter than K, and after vertex compensation, you would order the lenses as
42.50 – 1.50
46.00 – 4.62

Placed on an optical cross they would have corneal power 42.50 and lens power at 180 degrees and corneal power 46.00 and lens power -4.62 at 090 degrees.


Hope this helps.

Diane

[Dave Nelson]

I have never heard them referred to as drum values, but I have heard the control knobs on keratometers reffered to as drums. Must be some connection there.

[HarryChiling]

Thanks Diane and Dave,

The reason I asked is that my book refers to them as drum values and I made a program for an optometry school and refered to the button to calculate values as "calculate drum values", the proffesor asked and I don't have a clue why, other than what Dave has suggested by being a reference to the keratometer.

[Darryl Meister]

Drum power does indeed refer to either the knobs on a keratometer or the power knobs on a manual focimeter, depending on the context.

[Diane]

Drum power does indeed refer to either the knobs on a keratometer or the power knobs on a manual focimeter, depending on the context.

You win the prize. Both instruments have power drums and refer to the value of each.

Diane