|
Introduction
With the advent of the
progressive add lens (PAL) you might think PALs would take care of everyone’s
intermediate vision needs. After all, the patient should be able to find a point
in the lens that provides good vision for any distance. It’s a nice thought,
and PALs do the trick for many patients. But there are those who cannot wear, or
refuse to wear PALs; and there are those patients who prefer a wider field of
focus than PALs provide.
For these patients you will need
to measure the add power required for the particular working distance and
determine what type of lens will work best. Typical requests include music
glasses, lecturing glasses, and computer glasses.
Measuring
the add power
Hopefully your patient will
come prepared with the exact distance in inches to her music stand, podium, or
computer screen. If not, you will need to ask the patient to demonstrate the
distance for you so you can measure it (a tape measure comes in handy) or
estimate it.
If the distance is 28 inches or
closer, you can use the phoropter reading rod to measure the add power needed,
just as you measure for the reading add.
Start with the binocular distance
Rx and add plus power slowly until the target becomes clear. There may be a
range of add powers, for example +1.00 to +1.50, that provide acceptable
sharpness at the particular distance. Recommend the weakest add power in the
range. This will maximize the depth of focus.
If the distance is greater than
28 inches, you can test using the trial frame and trial lenses. If the patient’s
present distance glasses have the correct prescription, or close to it, you can
use trail clips or just hold up trial lenses in front of the glasses.
A
shortcut
There is a shortcut that you can
use that works better than 90% of the time. It is the same shortcut that lens
manufacturers use when making a standard (such as a flat-top) trifocal. Simply
figure half the strength of the reading add. This will generally place the
intermediate near point at arms length, which is the most useful intermediate
focal distance for most situations.
For example:
The manifest refraction for Mr.
Tweedie comes to:
OD -4.00+2.00 x 45
OS -3.00+1.75x135
ADD +2.00
A reading glasses correction
would be written:
OD -2.00+2.00 x 45
OS -1.00+1.75 x 135
Mr. Tweedie wants intermediate
lenses for reading music. He says the music is about arms length
away. Using the short cut method, we add +1.00 (half the ADD power) to the
distance correction to arrive at:
OD -3.00+2.00 x 45
OS -2.00+1.75 x 135
When prescribing a conventional
trifocal, if you want the intermediate add to be something other than half the
reading add, then you must specify the power that you want. Otherwise the middle
seg is automatically made half the power of the reading seg.
Music glasses
If the patient does not want PALs
or a flat-top trifocal, there are two other options. These options offer the
advantage of a wider/higher field of view.
Half glasses
 If
the patient has good distance vision, the intermediate correction can be put
into half glasses. Music can be read through the glasses, and the conductor can
be seen over the top.
Bifocals
 If
the patient needs a distance correction, the intermediate correction can be
placed into a wide executive (below), or flat-top 28 (right) or 32 bifocal seg.
Computer glasses
 Most
patients who use a computer need to be able to see at reading distance as well.
The computer screen is typically at arms length away. If the patient tells you
that the screen is closer than arms length, advise them to sit farther back from
the screen (studies have shown that the maximum danger to radiation exposure is
within 24 inches directly in front of the screen). PALs work well for this
situation, but not everyone wants to use them. The conventional flat-top
trifocal is not an ideal solution because of the small intermediate seg.
 An option that works well for
many people is a wide bifocal with the intermediate correction in the top and
the reading correction in the bifocal seg. This works well for the person that
spends a lot of time at a desk. The disadvantage is the lack of a distance
correction in the glasses.
The written prescription for this
type of lens is tricky. Let’s look at an example:
Ralph is a fifty-five year old
computer programmer who spends his entire workday looking between a computer
screen and a book of specifications. He tried PALs a few years ago and they made
him dizzy. He complains that he has to tilt his head up in the air to see his
screen through the bifocal seg in his glasses. You do a refraction, which turns
out to be:
OD +1.00 SPH
OS +1.50 SPH
ADD +2.00
He tells us the distance to his
computer screen and we measure the add for the computer screen distance to be
+1.00 D.
The written prescription for
intermediate/near bifocals will read as follows:
OD +2.00 SPH
OS +2.50 SPH
ADD +1.00
Remember the "distance"
part of the bifocal is really the intermediate power. For the right eye we add
1.00 to 1.00 to get the +2.00 sphere power. The total reading spherical power
for Ralph’s right eye is +3.00 (+1.00 plus the add of +2.00). We already have
a spherical power of +2.00 in the distance segment of the bifocal, so we put
+1.00 D in the reading seg to arrive at the total reading power of +3.00.
Other factors
When considering a patient’s
intermediate vision needs, keep in mind that most younger presbyopes (under age
55) have enough accommodative reserve for comfortable intermediate vision
without the need for a trifocal. Fortunately most presbyopes tolerate the newer
progressive lens designs and these glasses take care of most presbyope's
intermediate vision needs.
Many times patients complaining
of intermediate vision problems are under plused, or over minused in the
distance correction of their current glasses. Simply updating the distance
correction may solve the intermediate vision problem.
Top of page
Get Post-Test
|
