A device used for checking eyeglass lenses to make sure they are the right prescription. The lensometer can check the sphere, cylinder, or axis of a lens. Its parts are:

    The Eyepiece; which is just like the eyepiece on a microscope.

The Marking Device, which is used to mark a lens with a water soluble marker.

The Lens Stop; for holding the lens in place.

The Aperture; this lets light through to the lens.

The Spectacle Table; eyeglass frames rest here.

The Power Scale; which is a dial on the side that lets you check to see what power the lens is.

The Axis Drum; which you use to check to see what the axis of the lens is.

The Power Switch; for turning the lensometer on and off.

If you look into the lensometer you will see three lines bisected by two thinner lines. When a lens is placed in front of the aperture it will blur these lines. When the lensometer is adjusted to the correct RX the lines will no longer blur.

Ideally when you check the lens against its prescription it should match up to the readings of the lensometer. If that is not the case then the lens are defective and need to be remade. At any optical retail store a lensometer is used to check glasses before they are issued to the customer. This is done whether or not the store has an on-site lab. According to the U.S. Vision Technical Guide the procedure for checking a lens is thus:

    "Always adjust the eyepiece:

1. Set the axis drum to the indicated axis in the RX.

2. Set the power scale to the sphere power of the RX.

3. Set the glasses on the spectacle table, right eye first, and attempt to center the glasses vertically. You may have to turn the power scale a little in either + or - direction, but as this point (sic), the sphere lines should come into sharp focus. Be careful not to slide the glasses with the lens stop against the lens.

4. Next move the power scale the amount indicated in the cylinder of the RX. The cylinder lines should come into sharp focus."

And then you repeat with the left eye.

To measure bifocals you will need to check the Add Power of the RX. There are two types of bifocals available at U.S. Vision Stores: the no-lined or progressive bifocal and the lined bifocal. The old executive bifocal has been discontinued (this was the one invented by Mr. Ben Franklin). Now, the reading area is the only part of the lens that needs to be checked with the Add Power. The distance area is measured as described above. With lined bifocals all you need to do is measure below the line on the lens. On progressives it's a bit more difficult to find the reading area and you may have to use the laser etchings present in all progressive lens. The etchings are hard to find, being nearly invisible and very small, often they will need to be marked with a water soluble marker. Add the Add Power to the Sphere and Cylinder Power and you have found the correct RX for the reading area.

That's the basics for the lensometer. If you want one of these things become friends with an Optometrist and begin a life of joy with your new, uh, toy.

Sources: U.S. Vision Technical Guide. Lensometer Usage Guide. Pg. 33 to 37. I suspect that this resource is individually sent to U.S.V. stores, it does not have a copyright notice anywhere on it.

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