Monday, 27 January 2014

Specialty Progressives

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 multifocals, 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.


SHORT CORRIDOR PROGRESSIVE LENSES

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. Some examples of short corridor progressives are shown in the table below. 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.


NEAR VARIABLE FOCUS LENSES

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 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.





*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.






Drawing of a simplified progressive lens with plano distance and +2.00 add. "Power range" of this lens is full two diopters.





When a prescription with plano distance power and a +2.00 D add is placed in near 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 above figure.


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. 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 regular progressives we think of beginning with the distance power in the upper portion and increasing plus power as we go downward. With near variable focus lenses, we begin 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 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.

Example

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

R: plano
L: +0.25 −0.50 × 180
Add: +2.25


or
 0 00
+2. 25
----------
                                            =  +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

or
+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 will be the pattern for the left lens.



Customizing the Near Variable Focus Lens to the Needs of the Wearer

When someone has two specific distances at which they do most of their work, the examiner may decide to prescribe for those distances. In this case the type of lens should be chosen with a power range appropriate for the prescription. Here is how it is done.

Example

Suppose a person has a regular prescription of


                                                    R: +1.25 −0.50 × 090
                                                    L: +1.25 −0.50 × 090
                                                        +2.25 add


This person does most of her near work at the conventional 40-cm working distance, but uses a computer screen situated at an intermediate distance. The examiner tests for the best refractive correction for this computer screen distance.





When a near variable focus lens has a small degression (power range), the zone of optimal vision will be larger. Here are two simplifi ed drawings comparing how a lens with a small degression might compare with another with a larger degression.  Which of the two would be the most appropriate lens will depend upon the intermediate and/or near tasks for

which the lenses are intended.




This distance is found to have an intermediate add power of +1.25. If a near variable focus lens is to be used:

A. What would the prescription read in the lensmeter through the upper and lower portions of the appropriate near variable focus lens? (Assume that the power of the upper portion and mid portion of the lens will be the same.)

B. What would the correct power range be?

C. When choosing from the lens types, which lenses would have this power in the upper portion of the lens?

Solution

A. Through a lensmeter the lower portion of this lens would have the regular near power of the prescription. This would be

                                                      +1 25 - 0 50x 090
                                                      +2 25  Add
                                                      ------------------------
                                                =   +3 50 - 0 50x 090


In the top part of the lens, we want to have the prescribed intermediate power. This will be the sum of the distance power plus the intermediate add, which is


                                                     +1 25- 0.50x 090
                                                     +1 25 Add
                                                    -----------------------
                                                  = +2.50- 0.50x 090


B. The power range, or degression, is the power decrease between lower and upper parts of the lens—in other words, the power difference between intermediate and near powers. This can be found by taking the difference between +2.50 −0.50 × 090 and +3.50 −0.50 × 090, which is


                                                                          

                                                      +3. 50- 0.50x 090
                                                      +2. 50- 0. 50x 090
                                                    --------------------------
                                                   = +1.00  


Power range or degression may also be found by taking the difference between the intermediate and near add powers, which would be


                                                       (+2 25)
                                                      -(+1 25)
                                                      ------------
                                                        +1 00


Both methods result in a power range of 1.00 D.

C. In looking through the possibilities, there are several possible choices with a 1.00 D power range.

These include the Sola Continuum, Zeiss Business, and Rodenstock Cosmolit Office. There are many other types which are not specified here.

The example just given assumes that the occupational progressive lens is to be used with maximum viewing distance being the distance from the eyes to the computer screen. If the viewing distance is to go beyond the computer viewing distance, then a larger power degression might be chosen.



Fitting the Near Variable Focus Lens

Near variable focus lens fitting recommendations vary widely, depending upon the lens style. For example, the Access lens only requires a binocular near PD and does not require any measured fitting height. It is fit just like a single vision prescription for reading glasses. The reason it is possible to use a binocular PD instead of monocular PDs is because the progressive zone of the lens is much wider than in a standard progressive lens. So if the eyes do not track down the exact center of the zones, there are not the same problems encountered.

In contrast the Rodenstock Office lens is fit like a standard progressive lens using monocular distance PDs and fitting cross heights measured to the center of the pupil. The distance prescription and standard near addition would be specified. If no power range is specifically requested, the laboratory will use the recommended range for the add power of the prescription.



OCCUPATIONAL PROGRESSIVES THAT INCLUDE DISTANCE POWERS

There are occupational progressive lenses that are used for small office environments and computer viewing, but still include a small distance portion located at the very top of the lens. This requires that the wearer drop the chin and look through the upper portion to see in the distance. Yet since the lens is entirely an occupational lens, this is not necessarily a disadvantage and may be considered an expected trade-off for intermediate viewing enhancement.

The intermediate area of the lens is positioned in front of the eye, as if looking through a trifocal segment straight ahead. Because the progressive zone is longer, going almost from the top to the bottom of the edged spectacle lens, the intermediate and near zones will still be considerably wider than standard progressives, though not as wide as near variable focus lenses with smaller degressions.




The  lens shows a large functional intermediate zone area with a small distance area in the upper portion of the lens.