Saturday, 18 January 2014

Measurement and Dispensing of Progressive Lenses


Progressive addition lenses are made with the help of specially designed front surface curves. These changing surface curves cause the lens to gradually increase in plus power, beginning in the distance portion and ending in the near portion. These variable-powered progressive addition lenses should, according to design, permit clear vision at any given viewing distance merely by positioning the head and eyes.




PROGRESSIVE LENS CONSTRUCTION

Like a segmented multifocal, a progressive addition lens, or PAL, has certain distinct areas to the lens. But those areas in a progressive lens are not visible. The upper portion of the lens is basically the distance portion. The near portion of the lens, where the full near addition power is found, is down and inward. In between the distance and near portions is a progressive corridor where the power of the lens is gradually changing.

FRAME SELECTION

When choosing a frame for someone wearing a progressive addition lens, there must be enough room for the progressive zone and near portion. Because these areas are not visible like a bifocal segment is, they may be unintentionally cut off. This was a problem when progressive lenses were first introduced in the United States. At that particular time, many frames had narrow vertical dimensions. When progressive lenses were dispensed in these frames, much of the near portion was cut off. Since people could not see very well up close, dispensers falsely concluded that the lenses were no good. So frame selection is an important part of fitting progressives.

Here are some important points to keep in mind:

 1. The frame must have sufficient vertical depth. Each lens type has a manufacturer-recommended minimum fitting height. The recommendations of the lens manufacturer should be followed.  Standard minimum progressive addition lens fitting heights will vary, going down to a low of about 18 mm. If there is not enough vertical depth to allow the minimum fitting height, then either a different frame must be chosen, or a special short corridor lens that is designed for frames with a narrow vertical dimension should be used. Otherwise there will not be enough reading area left.

2. The frame must have sufficient lens area in the lower nasal portion where the near progressive optics are found. Sometime the frame has a large enough “B” dimension, but the shape is cut away nasally. Aviator shapes are an example of this type of frame.

3. The frame should have a short vertex distance. The closer the frame is to the eyes, the wider the field of view will be for both reading and distance vision.

4. The frame must be able to be adjusted for pantoscopic angle when facial structure will allow. A 10- to 12-degree angle is recommended. The intermediate and near fields of view are effectively wider when the progressive and near zones are closer to the eyes.

5. The frame must have sufficient face form. This also allows a wider viewing area through the progressive corridor.

CHOOSING THE RIGHT TYPE OF PROGRESSIVE

Most progressive lenses are made for general purpose wear since the majority of wearers only have one pair of glasses. Although general purpose progressives work for most people, here are some additional considerations:

1. What type of general purpose progressive is appropriate? It is possible to choose a certain type of general purpose progressive to fi t the needs of the wearer.

2. Does the wearer have a significant amount of cylinder power in the prescription? If so consider using a lens design that is atoric. Using such a design will reduce the amount of unwanted distortion that will otherwise be present in the periphery of the lens.

3. If the vertical “B” dimension of the frame is small, choose a short corridor progressive lens. A short corridor lens is still used for general purposes, but is meant for this type of frame.

4. Does this person use a computer a lot? Do they work in a small office environment where intermediate vision is important? If so they may need a near variable focus occupational progressive lens. This type of lens is made for closer viewing distances through the top of the lens and has both a wider intermediate progressive corridor and a wider near-viewing area. An occupational progressive lens should not be used as a person’s only pair of glasses, unless this person does not need a distance prescription and would otherwise only be wearing reading glasses. These lenses should be considered for a second pair of glasses.





MEASUREMENT AND ORDERING

A progressive addition lens has a rather narrow progressive corridor linking the distance and near portions of the lens. It is through this corridor that intermediate vision takes place. Unless the eye tracks down the exact center of this corridor, the lenses do not work very well. Therefore PD measurements must be taken for each eye individually and an exact vertical height specified for each eye.

To help make sure the progressive corridor is where it should be, the manufacture uses a fitting cross. The fitting cross is usually 4 mm above the start of the progressive corridor and is intended to be placed exactly in front of the wearer’s pupil center.

Standard Method for Taking Progressive Lens Fitting Measurements

1. Measure monocular distance PDs. The recommended method is to use a pupillometer.

2. Fit and fully adjust the actual frame to be worn. This includes pantoscopic tilt, frame height, vertex distance, face form, and nosepad alignment. Make certain the frame is straight on the face. If the temples are not adjusted, hold the frame in place while measuring so that it will not slip down the nose.

3. If the frame does not contain clear plastic lenses or the wearer’s old lenses, place clear (nonfrosted), transparent tape across the eyewire of the empty frame.

4. The dispenser is positioned with his or her eyes at the wearer’s eye level. With the wearer looking at the bridge of the fitter’s nose, the dispenser draws a horizontal line on the lens or tape. The line should go through the center of the pupil. This is done for both right and left eyes.

5. Place the frame on the manufacturer’s centration chart and move it left or right until the bridge is centered on the diagonally converging central alignment pattern. Then move the frame up or down until the marked horizontal pupil center lines are on the chart’s horizontal axis. Mark the previously measured PD for each eye as a vertical line that crosses the horizontal one.







6. For first one lens, then the other, read the fitting cross heights from the chart. Record these fitting cross heights and the monocular PDs on the order form and in the wearer’s record.






7. Check the size and shape of the frame on the lens picture portion of the centration chart. Do this by placing the frame on the lens blank circles of the centration chart so that the cross on the glazed lens overlaps the fitting cross on the picture.





8. Send the frame to the laboratory with the marks still on the lenses or tape.


DISPENSING PROGRESSIVES

Validation on the Patient

Once the prescription has proved to be correct, it is adjusted to fit the wearer. Normal frame fitting rules apply. In addition, to provide the maximum possible field of view, adjust the frame for:

1. A small vertex distance

2. Adequate face form

3. A maximum pantoscopic tilt that still looks appropriate for the wearer.

 With the visible markings still on the lenses, also check the following:

1. The fitting crosses should be in front of each pupil center. (Ensurance of the placement of the fitting crosses is especially important when the two eyes are not at an equal vertical height.)

2. The horizontal dashes on the lenses should be exactly horizontal and not tilted.

Removing the Visible Markings

The visible marks that are on a progressive addition lens when it comes back from the laboratory are non-water soluble. To remove them, use alcohol or an alcohol swab. Sometimes these marks can be stubborn. Some say that stubborn markings will come off easier if the lens is first heated in the hot air frame warmer. The alcohol may work better on the heated mark.

Instructing the Wearer at Dispensing

Adapting to progressive lenses can be made easier for a new wearer if the characteristics of the lenses are demonstrated at the time they are dispensed.

To demonstrate the full range of progressive lens versatility, hold a near-point chart at eye level at an intermediate distance. Instruct the wearer to look directly at the near-point card through the distance portion. Next ask the patient to tilt his or her head back until the letters on the card are clear. Gradually, bring the card closer to the eyes as the head is tilted still farther back, demonstrating the full range of viewing available.

TROUBLESHOOTING PROGRESSIVE PROBLEMS

Most problems encountered by progressive lens wearers are a direct result of basic fitting principles being ignored. Here are a few typical errors that should never occur, but do.

One monocular PD is correct; the other is wrong. This happens when the monocular PDs are done with a ruler or by marking the PD measurements on the lenses, and the fitter uses only one eye to measure both lenses.

The PD is given as a binocular PD, rather than as two monocular PDs.

Fitting cross height is measured for one eye, and the same measurement is written down for both eyes. Fitting cross heights must be individually measured for both eyes.


When a wearer does come back with a complaint, the most straightforward way to check for possible problems is to first put the progressive markings back on the lenses and see if they are correct in relation to where they should be when the prescription is worn. Often the problem will be obvious.




To verify distance power on a progressive addition lens, the lens must be positioned
with the arc around the lensmeter aperture as shown. This ensures that the power
reading will not be affected by the changing power in the progressive zone.







To verify prismatic effect, the lens is verified at the prism reference point (PRP)
located by the central dot directly below the fitting cross.







When both distance and near powers are low, the near power may be verified using the back vertex power as shown in the figure. In any case the near power must be read through the
near circle. The correct method, however, is to find the near add using front vertex powers.