Tuesday, February 18, 2014

[Optometry Student Share] Specialty Progressives - PART ONE

For years bifocal and trifocal lenses were worn by the majority of presbyopic spectacle lens wearers. Yet they were not able to satisfy all the visual needs for every wearing situation. As a result, a number of segmented specialty lenses developed.

Even though progressive lenses are clearly overtaking segmented multifocal, it is also unrealistic to think that general purpose progressives are able to fulfill everyone’s specialized needs any more than segmented lenses could. If a progressive lens is truly for specialized tasks and will not be used for full-time wear, the lens may be called an occupational progressive lens and may be abbreviated OPL. 

Progressive addition lenses as a general category are often abbreviated as PALs.


The short corridor category of specialty progressives is really a subcategory of general purpose progressives. The thing that makes this lens unique is that it is designed to allow a progressive addition lens to be worn in a frame with a small vertical dimension. Regular progressive lens corridors are too long. Too much of the near portion of a regular progressive lens is cut off when the lens is edged for frames with narrow B dimensions.

The short corridor progressive has a faster transition between the distance and near portions of the lens. This means that the wearer is quickly into the near portion when looking downward. Because the transition is short, near vision is suitable. Yet it is only logical that there will be some sacrifice of the otherwise larger intermediate portion. 

When choosing a short corridor progressive, be certain that the minimum fitting height is suitable for the frame. Even short corridor progressives can come up short on near viewing if the frame is exceedingly narrow. Short corridor progressives are fitted in the same manner as regular progressive lenses. Monocular PDs are needed, and the fitting cross is placed in the center of the pupil. Some examples of short corridor progressives are shown in the table below:

These are only a small number of short corridor lenses available. It is not meant to be an inclusive list, nor it will be a current list.


Near variable focus lenses started out as a replacement for single vision reading glasses. This lens also goes by other names, including,small room environment progressives, reader replacements, or simply OPLs. Over time the lens has become the lens of choice for someone working in a small office where intermediate and near vision are the primary viewing needs. To get an idea of how the lenses are constructed, take the example of a prescription that has no power in the distance and a +2.00 D add. The normal progressive addition lens would have powers as shown in figure below (with no power in the upper [distance] portion). Power gradually increases until it reaches the prescribed +2.00 D add power in the lower near portion.
Drawing of a simplified progressive lens with Plano distance and +2.00 add. "Power range" of this lens is full two diopters.

In occupational progressive lens (OPL), when a prescription with Plano distance power and a +2.00 D add is placed in near portion, a variable focus lens having a 1.00 D power range, the power difference between upper and lower portions is less. The progressive zone is also lengthened. This makes the progressive zone wider and reduces the intensity of peripheral distortion.

This simplified drawing of the lens structure, based on the same prescription, can be compared with the standard progressive in figure below:

This is usually not the case with most near variable focus lenses. The farthest distance that people (who work in small office environments) need to see clearly might be the distance of someone sitting across the desk from them. They also need a clear view of a computer monitor placed at an intermediate viewing distance and at the normal 40-cm near-working distance for reading. With this in mind, our example lens could be designed with a moderate amount of plus power in the distance. 

If we use +1.00 D of power in the upper portion of the lens, we can gradually increase plus power until a total of +2.00 D is achieved for near. This would appear as shown in figure above. Note that the progressive zone for this type of lens is longer and wider than the normal progressive corridor found in a general wear progressive lens. This works well, and for this type of working environment, these OPLs give excellent intermediate and near vision with less peripheral distortion.

Here is why:
  • A longer progressive zone will result in less peripheral distortion.
  • In a near variable focus lens, the difference between the powers in the upper and lower halves of the lens are usually smaller. In the example, instead of having a difference of +2.00 D, this lens has a difference of only +1.00 D. In reality this is a +1.00 D add instead of a +2.00 D add. The smaller the add power, the smaller will the unwanted cylinder be.
  • When wearing a near variable focus lens, more visual work will be done with midlevel and downward viewing than with a standard progressive where clear distance vision is important. The designer has the option of moving a larger proportion of the peripheral distortion inherent in progressive lenses into the upper periphery of the lens. Increasing the area of distortion decreases its intensity.

Power Ranges

With general progressive lens, the beginning of distance power is in the upper portion and increasing plus power as we go downward. With near variable focus lenses, it begins with the near power. The reference power is the near power instead of the distance power. We start with the near power in the lower portion and decrease plus power when moving up to the distance portion. This is no longer an addition, but a decrease in power. This decrease in power is called a degression. Manufacturers often call this the power range of the lens.

This means that near variable focus lenses do not come in regular add powers like general purpose progressives. They instead come with one or more power ranges. Again the power range is the difference in power between the lower and upper areas of the near variable focus lens.


Suppose a variable focus lens made by a certain manufacturer comes in only one power range and that power range is 1.00 D. This means that there will always be 1.00 D difference (degression) between the lower and upper portions of the lens. If a person has a prescription of

OD: plano
OS: +0.25 −0.50 × 180
Add: +2.25

Since the lens has a power range, or degression of 1.00 D, the upper area of the lens will have 1.00 D less plus power than the lower area of the lens. So the upper area of the lens has a power of:

       ( total near power)
 -     ( degression)                 
=     upper power of the lens

      +2 25
    -   1 00 
   = +1 25

In a lensmeter, the upper portion of the lens reads +1.25 D, and the near portion reads +2.25 D. Same occasions will be the pattern for the left lens.

( will be continue on part II )
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