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Progress Detected
This OLA-sponsored article investigates the ways to successfully order and dispense progressive addition lenses
By Alex Yoho, ABOM

Above: Examples of typical cutout charts. Most PAL cutout charts can be used to quickly check the PD and fitting height of the final product. They can also be used to check the current PD and fitting height once the lens reference points (pupils) have been dotted

Has your office been plagued by ordering progressive addition lenses (PAL) only to have the lab call and tell you that they just won't work?

Well, you're not alone.

One of the most common callbacks for labs involves progressives. This leads to frustrated patients who must return to your office and select a new frame or be disappointed with the second-best lens style.

Here's a look at some basics dispensers should know before ordering progressive addition lenses.

UNCOVERING LENS BLANKS

• Blank basics. When the lab begins with a progressive lens, it is in blank form. The lens blank consists of a round disk of lens material anywhere from an inch thick for blanks with flat front curves used for high-minus lenses to about a quarter of an inch for blanks with steeper curves used for high-plus lenses.

Semi-finished blanks have the front surface already finished with a high polish and complex curves to achieve the progressive addition. Some of the newest, most advanced lenses begin as rough blanks on both sides which are cut and polished with freeform lens processing technology.

• Base curves. Some ECPs think that keeping the base curve of a new lens the same as the patient's old glasses is key for comfort, regardless of prescription changes. This may not be possible. But, it's no problem because PALs are designed to the optimum base curves for the prescription and should not be changed.

Top to bottom: Checking the cutout potential and usable near area of a short corridor lens; showing the proper places to check progressive lenses; typical markings show how the progression is decentered within the blank; a cutout chart for one of the new wavefront designed lenses; this lens with a dot at the patent's pupil center will not process without cutout problems since the frame edge falls outside the largest concentric circle representing the largest possible blank

• Blank size. It's important to know that semi-finished blanks are supplied to the lab as is. The progressive addition is fixed in a specific position on the lens blank and cannot be changed.

The dispenser must be sure the blank is large enough in diameter so that the lab can cut the lens shape out of the blank without compromising the "seg" height or the PD.

INVESTIGATING CUTOUTS

Checking to see if a particular progressive lens will cut to the shape of the frame with the correct fitting height and PD can be accomplished quite easily.

PALs have cutout charts supplied by your lab or the manufacturer. Be sure to keep a chart for every type of progressive you prescribe.

• Adjust the frame. To check the cutout potential for a given frame, place the eyewear on the patient and take the time to adjust it the way it actually will be worn. Using a marker, place a dot directly in front of the patient's pupil.

• Eye to eye. To do this, look eye to eye with the patient. For example, close your right eye and look with your left eye at the patient's right eye and vice versa. Be sure you are at eye level with the patient in a normal straight ahead gaze. This method will give you an accurate fitting point, which you will use for the fitting height.

• Dot on fitting cross. Next, place the frame with dotted lenses face down on the cutout chart with the dot exactly on the fitting cross of the chart being sure the frame is situated straight. Each circle represents a specific product or base curve for a particular power range, so you should be sure that the one you need will properly cut out.

To do this, look at the circles around the layout diagram and check if it is completely outside the frame aperture. If the circle falls inside of the aperture of the frame, it will not cut out and a different frame must be selected.

SOLVING POWER MYSTERIES

When inspecting a lens, you may not get the full add power if you have cut the lens right at the center of the near reference point. It will not read the full add if you can't get the center of the near reference point in the center of the lensometer.

Many PALs have been dispensed this way with fine success. If the best reading you can get on a PAL is .25 weak, rest assured that the add is what it should be. It is just cut off too close to the edge to get a good reading.

Even those lens manufacturers that previously recommended ordering the add power .25 stronger to compensate for the weaker reading area are now adamant it should not be done.

The reverse of the above can also occur. In a narrow frame, the minimum fitting height fills the majority of the lens with the progression, leaving little room for any usable distance vision. That's where short corridor PALs come into play.

• Above, not at, fitting cross. When checking in a pair of progressive addition lenses, it is critical that you check them at the points on the lens that the manufacturer of the lens indicates on the cutout diagram. Generally, you should check the distance power about 10 millimeters above the fitting cross, not at the fitting cross itself.

• Prism pointers. On higher powers, you may need to introduce a compensating prism to the lensometer to bring the target closer to the center. When checking for prism, use the prism reference usually located midway between the two outer reference marks, usually 2mm to 4mm below the fitting cross and directly below it.

If it is a plus lens, you may find equal base down prism in each lens. This is due to prism thinning, which makes the lens thinner. Prism thinning should not be done on minus lenses.

The near point is the only place you will find the full add power. All of these checks are best done on a manual lensometer. As you inspect more and more lenses, you will gain knowledge of how the PAL should appear in the lensometer. It will pick up slight blurs which are normal due to the width of a standard lensometer aperture. It is also the reason you may get strange reading on an auto lensometer.

Now you can fit progressive addition lenses with the best of them, and say good bye to frustrated patients and callbacks from the lab.

If you have further questions, be sure to pick up the phone and call your partners at your OLA lab. They will be happy to walk you through any situation that you might encounter.