Fig. 1: Verifying the Lens Clock’s Accuracy
The first thing to do before using a lens clock is to verify it reads zero when pressed against a flat surface.

ECPs rely on invaluable tools like a lens clock, salt pan, hand stone, and distometer on a daily basis.

With all the hoopla about the electronic and computerized optical and ophthalmic gizmos that have come to the market in the last few years, it’s easy to forget some of the low-tech devices eyecare professionals (ECPs) rely on every day. These little wonders provide important information that help ECPs provide quality eyewear to patients. Here are a few of them, along with tips on how to properly use them.

The lens clock is a useful low-tech instrument for measuring lens curvature and is most often used these days for determining the base curve of a lens (see “Tech Tips” for more on measuring a lens’ base curve). While the lens clock is not a highly accurate device (a spherometer is much more so), it can give you reasonably accurate results if you use it properly.

Fig. 2: Holding the Lens Clock
To begin the reading process, grip the lens clock securely with your pointer finger and thumb.

The first thing to do before using a lens clock is to make sure it reads 0.00D when pressed against a flat surface. You can do this by holding the clock’s three pins perpendicular to a surface you are certain is completely flat, like a tabletop or window pane, and verify that the clock’s reading is zero (Fig. 1). When you’re sure the clock is accurate, it’s time to read a lens.

To begin the reading process, grip the lens clock securely with your pointer finger and thumb (Fig. 2). Fold your arms and press them against the sides of your body while you place the lens and the clock in front of you. This helps stabilize your hands. With the lens clock facing up (so you can read it), hold the lens as perfectly perpendicular to the lens clock as you can, then press the lens against the clock to take the reading. Turn the lens 90˚ to determine if the surface has cylinder or progressive optics. If it does, the lens clock’s needle will waver as you turn the lens.

Fig. 3: Creating a Ridge in the Salt Pan
To create a ridge in the salt pan, use a large serving spoon or similar tool to push the glass beads toward the center of the salt pan.

Arguably as low-tech a gizmo as you can get is the salt pan. It’s essentially nothing more than a modified toaster but with the intention of warming instead of scorching what goes into it. Salt pans get their name from the substance the device uses to warm a frame—salt. Traditionally, opticians used larger-sized kosher salt for this purpose. Today, most offices and labs use glass beads as the warming medium because salt is corrosive and can leave a pasty residue on a plastic frame. Since every lab person has his own favored temperature for warming frames, having an adjustable temperature control is a good option on a frame warmer.

Using glass beads makes heating specific areas of the frame easy if you mound them into a peak (Fig. 3). By doing this, you create a ridge into which you can place a specific portion of the frame. Think of this as spot-heating the frame. This technique is ideal for heating the bridge of a plastic frame or the endpiece. To create the ridge, use a tablespoon, large serving spoon, or similar tool to push the glass beads toward the center of the salt pan. Once you have them in this configuration, plunge the frame part you wish to warm into the peak of the ridge.

Fig. 4: Lens Position on the Hand Stone
This lens is properly positioned to have a safety bevel applied on the hand stone.

Another low-tech device that comes in handy in the lab is the hand stone. Sure, modern edger systems have an automatic safety beveling feature and a host of cool bevel settings, but you’re not going to risk losing a lens’ centration to remove a minor chip or reduce the lens’ diameter by 0.25mm or 0.50mm by re-blocking and re-edging it. For those jobs (and others), the trusty hand stone is the ticket.

Hand stones come in two varieties: with a ceramic wheel or an industrial diamond wheel. The ceramic models tend to grind a lens softer and more slowly than diamond wheels, although once you get used to either version, you’ll find them acceptable.

There are a few tricks to good hand beveling. The first is to ensure that the stone stays constantly wet during the grinding process. A wheel that dries out can cause the lens material to melt and deform. The wheel also builds up debris quickly this way. Another important factor is holding the lens at the proper angle to the grind wheel (Fig. 4). Holding the lens at an improper angle to the wheel for the operation you’re attempting to perform (like making a lens a little smaller in diameter using a standard bevel) will result in a ruined lens. Once the lens is held in the proper position, twirling the lens with a continuously slow rotating motion is the key. What you’re trying to do is imitate the action of an automatic bevel edger. Taking a long grind swipe at the lens, raising the lens from the wheel, and repeating this scenario can easily result in a deformed bevel or edge grinding. Try to keep the lens in constant motion as you hand edge it.

Fig. 5: Measuring with the Distometer
With the non-movable foot resting on the back surface of the lens, measure vertex distance by pressing the plunger to move the instrument’s movable foot.

With so many free-form progressive lenses asking for vertex distance measurements these days, the distometer is making a comeback. Used decades ago for measuring the vertex distance of lenses for aphakic spectacle lenses, the distometer fell out of use when intraocular lens implants replaced spectacle lens correction of aphakia.

This undemanding gadget measures the distance from the back surface of a lens to the center of the cornea. To use it properly, the eyewear being measured must be properly adjusted for the wearer. The distometer is positioned between the back of the lens and the person’s eye (Fig. 5). In this position, the device’s non-movable tip is placed against the back of the lens while the patient is asked to close their eye. The optician then presses the plunger, which activates the movable foot. Once the movable foot touches the person’s closed eyelid, the vertex distance measurement is read on the instrument’s scale. One millimeter is added to the reading to compensate for the person’s eyelid thickness.

Ed De Gennaro is Director, Professional Content of First Vision Media Group.


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