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Measuring the Interpupillary Distance
This post provides the methodology for measuring the interpupillary distance (PD)_ Failure
to accurately determine the interpupillary distance in a misplacement of the optical center of
the lenses. "I-his induces unwanted prismatic effects, requiring the wearer to turn his eyes inward, or even outward, to keep from experiencing double vision, Over time, this effort causes visual discomfort and can result in a decreased ability of the eyes to work together in binocular vision.
DEFINITION
The PD is the distance from the center of one pupil to the center of the other pupil, measured in millimeters, Before ordering prescription glasses or even before doing a visual examination, the distance between the pupils must be determined. It can be measured in a variety Of ways.
DISTANCE PD
Binocular PD
The most common method used to measure the PD also requires the least amount Of equipment. The technique uses a simple millimeter ruler, commonly referred to as a PD rule.
When the PD is to be measured, the dispenser should be positioned at a distance of 40 cm (16 inches) directly in font Of the subject, with his Or her eyes at the same vertical level as those of the subject.
The PD rule is positioned across the subject's nose with the measuring edge tilted back so that it rests on the most recessed part of the nose.
• The dispenser holds the PD rule between thumb and forefinger and steadies the hand by placing
the remaining three fingers against the subject's head.
The dispenser closes the right eye and sights with the left (Figure 3-1). The subject is instructed to look at the dispenser's open eye while the dispenser lines up the zero mark of the rule with the center of the subject's pupil.
When the zero mark is lined up correctly, the dispenser closes the left eye and opens the right. The subject is instructed to look at the dispenser's open The
PD for the distance prescription is read Off as that mark falling in the center of the subject's left pupil (Figure 3-2).
The dispenser now closes the right eye and opens the left. • I •he subject is again instructed to look at the dis-penser's open eye. This step is primarily a recheck to make sure the zero mark is still properly aligned. (Thistechnique is summarized in Box 3-1.) When difficulty is experienced in determining the
exact center Of the pupil, the edge Of the pupil may be used as a measuring point if both pupils are the Same size. Measurement is read from the left side of one pupil to the left side of the Other. Measuring from the inside edge Of one pupil to inside edge of the other would give an artificially low reading; from the outside edge of one pupil to the outside edge of the other, an artificially high reading.
When a person has dark irises or unequally sized pupils, it may be difficult to use either the center or the edge Of the pupil. In this case, the dispenser may use the limbus edge—the sharp demarcation between white sclera and dark iris (Figure 3-3). (Because the pupil is displaced 0.3 mm nasal ward from the center Of the limbal ring,' a limbal measure will be approximately 0.5 mm greater than the measure found using pupil centers.) The same rule must be applied when using the
limbus edge as when using the pupil edge: the same sides Of the limbus (both left or both right) must be used, or an extremely large error is induced.
Common Difficulties and Their Solutions
Dispenser Cannot Close One Eye. Occasionally the person doing the measuring is unable to close one eye independent of the other. This can be remedied by occluding (covering) the eye with the free hand.
The
practice of holding the lid down with one Finger gives an
unprofessional appearance, especially when wearing glasses. Occluding the eye with the hand held flat appears to be a natural part of the test and does not reveal a person's inability to close only one eye.
Dispenser Visually Impaired in One Eye. If the dis-penser is blind in one eye, or has visual acuity too poor to allow the ruler to be read accurately, then the technique is modified. 'I'he dispenser places the good eye directly in front Of the subject's right eye and at the
nortnal distance. The I.ero mark is lined up as usual. The dispenser then moves sideways until the good eye is positioned in front of the subject's left eye and the measurement is read, Unfortunately this method can easily lead to parallax errors. The most desirable solution for someone with this difficulty is to use another type of instrument that only requires the use of one eye.
Subject Is Strabismic. The strabismicsuhject, whose eyes are in a tropic position (i.e., with one eye pointing in a different direction from the Other) presents a special problem, since the PD rule method of measurement may then give an artificially high or low reading. To determine a true reading, simply cover the subject's eye not being observed. This ensures that the subject is fixating
with the eye under observation and ensures that it is not turned unless eccentric fixation is present. Even if eccentric fixation is present, the PD measurement is still correct, since the subject never uses this eye in any other position relative to the dominant eye.
In some instances where one eye turns out constantly, the prescribing doctor may determine that the wearer is better served if the lenses are centered in front of the pupils, even for the eye that is turned. This will require that a separate measure be taken for each eye. One measurement will then be considerably larger than the other.
Subject Is an Uncooperative Child. If the subject is young or uncooperative, making normal PD measurements impossible, the dispenser may have to take a canthus-to-canthus measurement. (The canthus is the corner of the eye where the upper and lower lids meet.)
This is done by measuring from the outer canthus of one eye to the inner canthus of the other eye. Unfortunately, this measurement is not entirely exact, Since the inner canthi of the eyes encroach farther across the sclera with younger children.
Common Causes of Errors
There are several common causes of errors inherent in using a PD rule'
I. There will be an error in measurement if the measurer's PD differs significantly from the
subject's because the lines Of sight are not parallel.For example, if the measurer's PD is 16 mm larger
than the subject's, the reading will be I mm too high because Of error.
2. The above error will be increased if the PD rule is not tilted on the subject's nosc so that the sealc is in the most recessed area. The most recessed area corresponds to the approximate position Where the spectacles will be worn.
3. Just as error will be increased when the measure's PD is significantly different from the subject's, the parallactic error will also be increased even more if the dispenser is too close to the subject. Too close is closer than the normal 40 cm (16 inch) distance.
4. A significant error will be induced if the subject is strabismic (one eye turns in or out) or if the subject
4. A significant error will be induced if the subject is
strabismic (one eye turns in or out) or if the subject does not fixate binocularly* during the PD measurement.
5. An error can result if the subject's head moves. An error Can result if the person measuring
his or her head.
7. An error will result if the person measuring does not close or occlude one eye at a time to ensure
sighting from directly in front of the subject's eye under observation.
8. The subject may not look directly at the measurer's pupil during the test, as he or she should, which will result in an error.
•What does 'not fixating binocularly" mean? It rneang that one eye
may have a tendency to turn in or Cut "hen the Subject ig not Concentrating_ In simple terms, they will be using one eye to see instead of both eyes, When this does happen, one eye usually turns Outward Somewhat and the measurement is then too large.
Monocular PD
Since faces are not always symmetrical, it is Often necessary to specify the PD for each eye independently. The main goal in taking the PD is to eventually place the optical centers Of the lenses directly in front of the subject's eyes to prevent any undesired prismatic effect. If one eye is set closer to a person's nose than is the other and the optical centers Of the lenses are placed
symmetrically in the frames, the wearer's lines Of sight will not pass through the optical centers of the lenses The error is not too serious if the lenses are of the same power and are not strong. If, however, one lens is very different from the other, the centers must be placed
accurately to prevent unwanted binocular prismatic effects (Figure 3-4). Monocular PDs are also important
when using aspheric lenses or high index lenses, including polycarbonate lenses. High index lenses have more
chromatic aberration than crown glass or regular (CR-
39) plastic lenses. The nevative effect of chromatic aber
ration on vision is increased if the eye is not looking
through the optical center of the lens. (For more infor mation on high index lenses, see Chapter 23. For more Information on aspheric lenses, see Chapter 18.)
Procedure for Measuring Monocular PDs
Using a Rulev
The monocular PD is best taken using a pupillometer. When a pupillometer is not available, monocular PDs are taken by measuring from the center of the nose to
the center of the pupils. The procedure consists of the following three steps:
l. Measure the binocular PD as described earlier in the chapter. Use the center of the pupil as the
reference vu_'int.
2. Before moving the ruler, note the scale reading on the ruler at the center of the nose. This is the right monocular PD.
3. Subtract this reading from the binocular reading to obtain the left monocular PD. For example, the binocular PD is 66. The scale reading at the center of the nose is 32. The monocular PD for
the right eye is then 32. TO calculate the monocular PD
for the left eye, subtract 32 from 66, to get a reading of
34. The procedure is the same as in taking a binocular
PD measurement, except that the two readings are inde-
pendent of one another and, for purposes of measuring,
the center of the pupil is always used. (There are other
methods that are considerably more dependable than
this method in their ability ro yield consistently accurate
results.)
Procedure for Measuring Monocular PDs
Using the Frame
One error inherent in using a ruler alone appears when
a person has an asymmetrical nose. An asymmetrical
nose often occurs when a nose has been broken. In this
case, the frame positions itself somewhat to the left or
right. For the lenses to be accurately placed, this factor
must be taken into account. It is possible to use an over-
head transparency marking pen and the glazed* lenses
in the sample frame. If the sample frame does not have glazed lenses, clear tape may be placed over the lens opening of the empty frame.
The procedure for measuring monocular PDs begins by adjusting the frame. The frame should occupy the exact position it will have with the lenses in place. The dispenser should be at the same level as the wearer and approximately 40 cm away. The dispenser closes the right eye. The wearer is instructed to look at the dis- penser's open left eye. Since there is no ruler used, the dispenser uses an overhead transparency marking pen and marks a cross on the right glazed lens. If there is no
lens in the frame, the clear tape placed over the lens
opening is marked instead, directly over the center of
the wearer's right pupil (Figure 3-5).
Next the dispenser closes the left eye and opens the right eye. The subject is instructed to look at the dispenser's open eye. The dispenser then marks a cross on rhe lens or rape directly over the left pupil center. the lens or rape directly over the left pupil center.
Because Of the movement involved in marking pupil centers and the ease with which unintentional
head can occur, it is important that markings
be carefully
When tlu is that lmpil centers are accurately marked, the frames are rernc_we.d and che dis-
th t f the bridge to the of each crass measured and recorded, (Thcse steps arc sum-
in Box 3-2.)
PD Measuring Instruments Thc interpupillary distance is mast easily measurcd by
an designed for this purpose,
Readings taken using th
subject u:' parallax errors as those taken using a PD rule,
Such a device solves the "hen the
person doing the measuring is monocular or is amblyo-
eye.
Most have
n occlusion system, which
allows for individual monocular measurements, with
eye fimtmg alternately in of
A well-designed PD measuring inurnment should
rot agälnst the bridge uf subject's nose
frame would. This most accurately apprmimatcs the way
the glasses will position themselves. IL should also posi-
t- 'n the plane It the p
planc,
The Subject will See a Of white or colored hgbt
Iren:nd 1 dark,
n the instrument.
dispenser will See subject's a scale appearing
it, which a
is Alternately, in
some instruments, a split image of the pupil may be seen
Instruments Using Corneal Reflexes
Although some instruments use a method of taking the
PD where the reference point is the geometric center of
the pupil itself, the popular alternate corneal-reflex
method is used in instruments such as the Essilor pupil-
lometer (Figure 3-6) or the Topcon PD-S, PD Meter.
The instruments are supported by means Of pads posi-
tioned so as to cause the instrument to rest on the nose
where the average frame would rest. This is superior to
a forehead supporr system used alone.
The dispenser asks the subject to hold his or her end
of the pupillometer so that the pads rest on the nose
(Figure 3-7). The forehead support should be against the
forehead. The dispenser uses one eye to look into the
instrument. (A real advantage for dispensers with good
vision in only one eye.)
An internal light produces an image by reflection on
each cornea, and the hairline within the device is moved
until coincident with this corneal reflection (Figure 3-8).
The measurement is assumed to correspond with the
subject's line of sight, but is an objective measurement
of the position of the corneal reflection rather than the
position of the line of sight. In addition to a distance PD,
,
near PD may be measured for near points from 30 or
35 cms to infinity.
The line of sight is defined as a line passing from the center Of the pupil to the Obiect Of regard. This is the line that desirably passes through the optical center of the lenses and is the basis upon which the measurement
of interpupillary distance rests.
Corneal reflections are observed along a line which
intersects perpendicularly the center Of curvature Of the
anterior surface of the cornea. (Technically this line is
referred to as the pupillary axis.) This line intersects the
line of sight at the entrance pupil of the eye. It varies in
its orientation by an angle,* which for the average eye
is approximately 1.6 degrees.' This places the corneal
reflection somewhat toward the nose. Thus a PD deter-
mined on the basis of corneal reflections will vary slightly
from that determined by the centers of the pupils.
It is possible to use a corneal-reflection-style instru-
ment to measure a PD based on pupil center distances.
To do this, rhe hairline within rhe device is moved to
the center Of the pupil rather than the center Of the
corneal reflection. The corneal reflection method is
definitely the method of choice when measuring a PD
for someone with pupils dilated from a recent eye
examination.
•This angle is angle lambda, but is often commonly designated as
angle ka Using Corneal Reflections to Measure the PD without
a Pupillometer
It is possible to use corneal reflections to measure inter-
pupillary distance With even a PD ruler, or by using the
frame with glazed lenses. Procedures need only be
slightly modified. The dispenser should be positioned at
the near working distance. The dispenser holds a pen
light directly below his or her eye and shines it into the
eye of the subject. The subject looks either at the pen
light or the dispenser's eye. The reflection of the pen
light on the cornea is used as the reference point instead
of the geometric center of the pupil. The sequence of
measurements is followed exactly as outlined in Boxes
3-1 and 3-2, except that rhe dispenser must position the
pen light directly below his or her "open eye" through-
Out the sequence.
Photographic Instruments for Measuring PD
There are instruments available for taking a wearer's
interpupillary distance that make use ofa photograph of
the wearer's eyes with the frame in place. The frames
are adjusted as they are to be worn. The wearer fixates
a light in the instrument, and the photo is taken. PD and
the wearer's eyes with the frame in place. The frames are adjusted as they are to be worn. The wearer fixates a light in the instrument, and the photo is taken. and segment height measurements are determined using the picture. Up to this point, no photography-based PD measuring system has successfully penetrated the U.S. ophthalmic market.
NEAR PD
The near is required for single vision reading glasses or for multifocals.
For single vision reading glasses, the lenses are set so that their optical centers will be in the lines of sight of the eyes when the eyes are converged for reading.
For multifocals, the distance portion is ground to cor- respond to the distance PD, while the bifocal or trifocal portion IS decentered inward to be properly situated for
near vision. The near PD can be either measured or calculated.
Measuring Near PD With a PD Rule calculate
Measuring Near PD With a PD Rule
To measure the near PD with the PD rule, the dispenser
is positioned at the subject's working distance; that is, at
the distance for which the reading portion is
prescribed.
Closing his or her poorer eye, the dispenser aligns his
or her better eye directly before the subject's nose and
instructs the subject to look into that open eye.
The PD rule is lined up with rhe zero point corre-
sponding to the center of the subject's right pupil. It
should also be held in the same place that the subject's
new frames will rest because this will also affect the
reading.
The dispenser then notes the mark corresponding to
the center of the subject's left pupil. This is the near PD
(Figure 3-9). The subject is not required to shift gaze,
and the dispenser is not required to change eyes during
the procedure. (See Box 3-3 for a summary of this
technique.)
It should be added that it is also possible to use the
edge of the pupil or the limbus for reference points in
taking the near I-SD, as long as only the right or only the
left edges are used, and not both outer or both inner
edges.
In practice, many who use a PD rule to measure the
binocular distance PD, measure the near PD ar the same
time. This is done as follows:
(rhe first three steps are how binocular distance PD
measures begin.)
l. Dispenser is positioned at 40 cm.
2. The dispenser closes his or her right eye and the
subject, using both eyes, fixates on dispenser's left
3. Dispenser lines up zero point of ruler on center of
subject's right pupil. (This next step allows for the
near PD measurement.)
3A. The dispenser looks over at the subject's left eye
and reads the scale on the ruler at the location of
the left pupil center. This is a measure of the near
PD for the distance from the subject to the
dispenser.
dispenser now continues the Steps for finding the
binocular distance as listed in BOX 3—1.
Taking Near PD Using a Pupillometer
Usually a PD measuring instrument will allow both dis-
tance and near PD to be measured. This is done through
the use of a movable internal lens that changes the image
distance and convergence for the subject. The near read
ings are carried out in the same manner as the distance
readings.
USING THE NEAR PD FOR BIFOCAL INSET
For the near "reading" area ofa pair of glasses to be used
most comfortably, it must be positioned accurately in the
lens. Horizontal placement Of the near segment viewing
area is determined by the near PD. (Vertical placement
depends on frame depth and the individual's visual need
and will be covered extensively in Chapter 5.)
The horizontal position of bifocal segments is speci-
The horizontal position of bifocal segments is speci-
fied as the distance from the farpoint PD that the seg
ments are set in toward rhe bridge.
Because of the possibility of unequal monocular PDs,
segment inset is usually specified individually for each
eye. Ordinarily segment inset is the difference between
the distance PD and the near PD, divided by 2:
(distance PD) — (near PD) Segment Inset — for each eye is 2 mm.
Where inequality of the monocular PDs exists, this rule may result in errors, since both eyes may not be required to converge equal angular amounts for near fixation. The actual amount Of error is usually so slight, however, that it is usually ignored. The exceptions would
be eases of very marked differences in monocular PD or very strong lenses.
If there is a large difference in monocular PDs, inset Where inequality of the monocular PDs exists, this rule may result in errors, since both eyes may not be required to converge equal angular amounts for near fixation. The actual amount Of error is usually so slight, however, that it is usually ignored. The exceptions would be cases of very marked differences in monocular PD or
very strong lenses.
If there is a large difference in monocular PDs, insetting the bifocal segments accordingly may result in a rather unusual-looking pair of glasses (Figure 3-10).
This effect can be made less noticeable by using a bifocal with a wider segment.
Calculating the Near PDThere are several other factors to be considered when
calculating the near interpupillary distance, most notably those that cause differences in segment inset.
Calculation
The most logical way to calculate the interpupillary distance is to draw a triangle with the center of rotation of the eyes being two points of the triangle and the near
point of fixation being rhe rhird. A similar triangle is
then constructed by drawing a line corresponding to the
spectacle plane.
By similar triangles, the monocular near PD can be
calculated from the monocular distance PD (Figure
3-11).
When using a prewritten prescription, the working
distance will normally never exceed the reciprocal of the
power Of the near addition. For example, a +2.00 diopter
near addition Will indicate a working distance no further
than 50 cm. 1 = 0.50 meters = 50 cms +2.00
Unless the professional situation or physical build of
the wearer indicates otherwise, the customary near
working distance can be assumed to be 40 cm. If,
however, the power of the near addition (add power) is
will be the reciprocal of the add power. For example,
a +3.00 diopter add indicates a working distance of
331/3 cm.
1
= 0.33— meters = 33— cms
+3.00 D
3
Gerstman has simplified the calculation of the near inset with a rule which he calls the three-quarter rule. The three-quarter rule states that for every diopter of dioptric demand the optical center Of each reading lens, or the geometric center of each bifocal addition, should
be inset 0.75 (three quarters) mm. Dioptric demand is the inverse of the reading distance in meters and is independent of the actual bifocal addition power.
Example 3•1
For a reading distance of 40 cm, and an add power of +1.00 D, what is the inset per lens?
Solution
To find the answer. we first need to know the dioptric demand.
For a reading distance of 40 cm, and an add power of +1.00
D, what is the inset per lens?
Solution
To find the answer, we first need to know the dioptric demand.
The dioptric demand is the inverse of the working distance,
not the inverse of the +1.00 add power. Therefore since
the working distance is 40 cm or 0.40 m, the dioptric
demand is 1 = 2.50D 0.40
Having found the dioptric demand. we can find the inset
per lens by multiplying by three-quarters, as the rule name
implies. Therefore the inset per lens is
2.50x3/4 =1.9 mm
The Influence Of Distance Lens Power on
Segment Inset
The power Of the distance prescription has an effect on bifocal inset. When a person looks at a near object, the eyes turn inward and are no longer looking through the optical centers of the lenses. Negative power, or minus lenses, keep the eyes from converging as much as
they normally would because of the Base In prismatic effect at this point on the lens. Positive power, or plus lenses, cause the eyes to converge slightly more than they normally would because of their Base Out prismatic effect.
For positive lenses then both the measured or the Gerstman-calculated near PD would need ro be reduced (i.e., the segment inset Of the bifocal increased). For minus lenses, the near PD would need to be increased (i.e., the inset of the segment reduced).
The position of the near reading area becomes more important when the reading area is small. This means that for progressive addition lenses, the position of the intermediate and near readings areas is very important.
Progressive addition lens designers are now taking dis- important when the reading area is small. This means
that for progressive addition lenses, the position of the intermediate and near readings areas is very important.
Progressive addition lens designers are now taking dis-tance power into consideration when determining how much inset the near viewing area should have. Segment Inset Formula. There have been several factors listed as having an effect on segment inset. These
• The distance the lenses are from the eyes
• The distance PD
• The near working distance
• The power of the distance lens
Taking all these factors into consideration, Eller-brock4 derived the following formula for segment inset.
where P is one half the distance PD, Ois the distance of the lens from the working nearpoint, s is the distance from the lens to the center Of rotation Of the eye, and f is the focal length Of the lens in the 180-degree meridian. All measurements are expressed in millimeters.
Example 3-2
What would the segment inset be for a person with a 70 mmdistance PD who is wearing a prescription of +6.500?
Assume they are wearing a +2.50 add, but are working at a near working distance of 20 cm. The spectacle lenses are near working distance of 20 cm. The spectacle lenses are
25 mm from the center of rotation of the eye to the back of
the lens.
Solution
We are using Ellerbrock's formula.
is half the distance PD, SO
70
In Ellerbrock's formula P
= 35 mm.
The value of is the distance from the lens to the near
working point in millimeters. This distance is given as 20 cm,
which is the same as 200 mm.
The focal length of the lens is the reciprocal of the power of the lens. This is
= 0.1538 Meters
6.50
= 153.5 mm
Since the lens is a sphere, the power in the 180-degree meridian is the same as the power in any other meridian.
The distance from the lens to the center of rotation of the eye is given as 25 mm. so s — 25 mm.
Since the lens is a sphere, the power in the 180•degree meridian is the same as the power in any other meridian.
The distance from the lens to the center of rotation of the eye is given as 25 mm. so s = 25 mm.
Inserting all of this into Ellerbrock's formula results in 1+200.
= 4.5 mm
1
35
1
25
153.5
So the inset per lens for this wearing situation is 4.5 mm per eye.
Summary of Factors
Fortunately the variations in segment inset caused by all these factors are not radically different from that found using the measured near PD. This assumes, Of course, that the near PD is measured at the appropriate working distance.
Table 3-2 summarizes the effect of distance lens power on segment inset for the normal working distance
(40 cm or 16 in). s
Recommendations For Finding The Near PD After all of these possibilities, what is the most appropriate way to determine segment inset? Here are some recommendarions for different situations. The idea is to provide the best accuracy without making it too difficult.
Keep in mind that just using a PD ruler may not be the most reliable method.
Recommendations for Finding the Correct Segment Inset • When the working distance is normal (40 cm)
1. Measure the near PD with a pupillometer or a Segment Inset
When the working distance is normal (40 cm) l. Measure the near PD With a pupillometer or a
PD ruler.
2. If the distance lens powers are high, use Table 3-2. When the working distance is less than 40 cm
. Again, measure the near PD with a pupillometer or PD ruler. Be certain to set the correct
working distance in the pupillometer before measuring. When measuring with a PD ruler,
the dispenser must be at the shorter working distance.
If the working distance is less than that allowed for in the pupillometer, use Gerstman's three-
quarter rule (assuming adult PDs between 62 and 68 mm), or use Table 3-1.
• When the distance lens powers are especially high If the working distance is normal (40 cm), use
Table 3-2.
. If the working distance is closer than 40 cm, use Ellerbrock's formula. (Ellerbrock's formula could
When the distance lens powers are especially high l. If the working distance is normal (40 cm), use
Table 3-2.
2. If the working distance is closer than 40 cm, use Ellerbrock's formula. (Ellerbrock's formula could
actually be used in any of the above
circumstances, but it is unhandy to work with.) Examples for Finding the Near PD
Here are some examples. Both the power of the prescription and the distance p D are known. Use the most
appropriate method to find the segment inset and then
the near PD.
Exampje 3-3
A spectacle lens wearer has the following prescription
R: —1.00 D sphere
L: -1.00 D sphere
add: +2.00
The distance PD is measured as 64 mm. For a 40 cm working distance, what is the expected near PD?
Solution
Since the working distance is 40 cm, simply measure the near PD with a pupillometer (or PD ruler). If a pupillometer is not available, use Table 3-2. In the table, we find the inset for a 32 mm monocular distance PD with a —1.00 D power
to be 2 mm. Therefore the binocular near PD would be 4 mm less than the distance PD. Since 64 — 4 = 60 mm, then the
near PD equals 60 mm.
Example 3•4
Suppose an individual has a distance PD of 64 mm, a distance prescription of —1.00 D sphere for both eyes, and a
bifocal add of +2.00. (These are the same lens powers as
given in the previous example.) What would the near PD be if the near working distance was 25 cm instead of 40 cm?
Solution
Since the working distance is less than ao cm, we would find the near PD either by a direct measurement using a pupillorneter (or PD ruler) or by using the three-quarter rule.
To find the near PD by measurement, the best option would be to use a pupillometer. unfortunately, most pupillometers only measure up to 33 However, it is possible to use a PD ruler. If a ruler is used, the dispenser's face must be at the subject's near working distance.
If the near PD is to be calculated, it is possible to do these calculations with Gerstman's three-quarter rule. To use the three-quarter rule, begin by the finding the dioptric demand.
Dioptric demand is the reciprocal of the working distance in meters. In this example, the working distance is 25 centimeters or 0.25 meters. Therefore
1
Dioptric demand
0.25 meters
Next, to find the inset per eye, the dioptric demand is
multiplied by 3/4.
3
— x 4 3 mm per eye
4
Thus the near PO will be
Distance PD — (segment inset x 2)
or
64 — 6 or 58 mm.
This means that a prescription for a person with a bifocal
add and a 25 cm working distance should have the distance
optical centers set for a far PD of 64 mm and the segments
set for a near PD Of 58 mm.
Using Table 3-1 would have yielded an inset Of 3.3 mm per
eye and a near PD of 57.4 mm. Remember that the three-
quarter rule is a close approximation and because the table
does not list every PD and working distance, it may also be
a close approximation.
Example 3-5
Example 3-5
A prescription reads as follows:
R: +1.50 - 1.00 x 180
L: +1.50 - 1.00 x 180
add +3.50
The distance PD is found to be 61 mm. What should the near
PD be?
Solution
A near addition with a pot,ver greater than +2.50 D should
be a red flag to the dispenser. An add greater than +2.50 D
means that the working distance will be less than 40
The near PD is best found by direct measurement with a
pupillometer or PD ruler. The three-quarter rule is not as
accurate because the PD is smaller than the normal 62 to
68 mm range. The next best thing is to use Table 3-1.
TO measure directly With pupillometer or PD ruler, the
working distance must be known. When an add power is
greater than +2.50 D, unless another distance is specified
the working distance is found by taking the reciprocal Of the
add power.
Working Distance —
No
1
= 0.29 M or 29 cms
The other way to find the near PD would be to use the
three-quarter rule. This is done by multiplying the dioptric
demand (3.5 D) by 0.75.
Which is:
(0.75)x (3.5) = 2.625 mm per lens
Now the near PD is
Near PD — dis tance PD — (2 x seg inset)
-61 (2 x 2.625)
= 61 - 5.25
= 55.75 The other way to find the near PD would be to use the
three-quarter rule. This is done by multiplying the dioptric
demand (3.5 D) by 0.75.
Which is:
(0.75)x (3.5) = 2.625 mm per lens
Now the near PD is
Near PD — dis tance PD — (2 x seg inset)
-61 (2x2.625)
= 61 - 5.25
= 55.75
Either inset number will yield a similar answer and both
answers will round to 56 mm for the near PD.
Example 3-6
A prescription reads as follows:
R: -8.50 D sphere
L: —8.50 D sphere
add = +1.50
For this prescription, we will assume that the distance mon.
ocular PDs have been measured. The right monocular dis-
For this prescription, we will assume that the distance mon.
ocular PDs have been measured. The right monocular dis-
tance PD was measured as 28 mm and the left as 31 mm.
For a 40 cm working distance, find the near monocular RDS.
Solution
For high•powered lenses, calculated near pos Will be more
accurate than measured near PDs. This is because measured
near PDs do not take the prismatic effects of high plus or
high minus spectacle lenses into consideration. (Looking
nasally through high minus lenses causes a base-in prismatic
effect and reduces the amount the eyes converge for near
viewing.) However, there is one situation for high-powered
lens prescriptions where measured near POs are as accurate
as calculated near This occurs if the near PDs are
measured while the prescribed distance lenses are being
worn. So if the wearer has the same existing prescription in
single vision lenses or multifocals, a PD ruler may be used
while the person is wearine their existing frame and lenses.
This prescription is for the normal working distance. The
easiest method to find the near PD for this high-powered
prescription is to consult Table 3-2. For the right and the left
lens, Table 3-2 shows segment inset to be 1.5 mm per lens.
Therefore the monocular near PDs are
prescription is to consult Table 3-2. For the right and the left
lens, Table 3-2 shows segment inset to be 1.5 mm per lens.
Therefore the monocular near PDs are
R: 28 mm — 1.5 mm or 26.5 mm
L: 31 mm — 1.5 mm or 29.5 mm
Measuring the Interpupillary Distance
This post provides the methodology for measuring the interpupillary distance (PD)_ Failure
to accurately determine the interpupillary distance in a misplacement of the optical center of
the lenses. "I-his induces unwanted prismatic effects, requiring the wearer to turn his eyes inward, or even outward, to keep from experiencing double vision, Over time, this effort causes visual discomfort and can result in a decreased ability of the eyes to work together in binocular vision.
DEFINITION
The PD is the distance from the center of one pupil to the center of the other pupil, measured in millimeters, Before ordering prescription glasses or even before doing a visual examination, the distance between the pupils must be determined. It can be measured in a variety Of ways.
DISTANCE PD
Binocular PD
The most common method used to measure the PD also requires the least amount Of equipment. The technique uses a simple millimeter ruler, commonly referred to as a PD rule.
When the PD is to be measured, the dispenser should be positioned at a distance of 40 cm (16 inches) directly in font Of the subject, with his Or her eyes at the same vertical level as those of the subject.
The PD rule is positioned across the subject's nose with the measuring edge tilted back so that it rests on the most recessed part of the nose.
• The dispenser holds the PD rule between thumb and forefinger and steadies the hand by placing
the remaining three fingers against the subject's head.
The dispenser closes the right eye and sights with the left (Figure 3-1). The subject is instructed to look at the dispenser's open eye while the dispenser lines up the zero mark of the rule with the center of the subject's pupil.
When the zero mark is lined up correctly, the dispenser closes the left eye and opens the right. The subject is instructed to look at the dispenser's open The
PD for the distance prescription is read Off as that mark falling in the center of the subject's left pupil (Figure 3-2).
The dispenser now closes the right eye and opens the left. • I •he subject is again instructed to look at the dis-penser's open eye. This step is primarily a recheck to make sure the zero mark is still properly aligned. (Thistechnique is summarized in Box 3-1.) When difficulty is experienced in determining the
exact center Of the pupil, the edge Of the pupil may be used as a measuring point if both pupils are the Same size. Measurement is read from the left side of one pupil to the left side of the Other. Measuring from the inside edge Of one pupil to inside edge of the other would give an artificially low reading; from the outside edge of one pupil to the outside edge of the other, an artificially high reading.
When a person has dark irises or unequally sized pupils, it may be difficult to use either the center or the edge Of the pupil. In this case, the dispenser may use the limbus edge—the sharp demarcation between white sclera and dark iris (Figure 3-3). (Because the pupil is displaced 0.3 mm nasal ward from the center Of the limbal ring,' a limbal measure will be approximately 0.5 mm greater than the measure found using pupil centers.) The same rule must be applied when using the
limbus edge as when using the pupil edge: the same sides Of the limbus (both left or both right) must be used, or an extremely large error is induced.
Common Difficulties and Their Solutions
Dispenser Cannot Close One Eye. Occasionally the person doing the measuring is unable to close one eye independent of the other. This can be remedied by occluding (covering) the eye with the free hand.
The
practice of holding the lid down with one Finger gives an
unprofessional appearance, especially when wearing glasses. Occluding the eye with the hand held flat appears to be a natural part of the test and does not reveal a person's inability to close only one eye.
Dispenser Visually Impaired in One Eye. If the dis-penser is blind in one eye, or has visual acuity too poor to allow the ruler to be read accurately, then the technique is modified. 'I'he dispenser places the good eye directly in front Of the subject's right eye and at the
nortnal distance. The I.ero mark is lined up as usual. The dispenser then moves sideways until the good eye is positioned in front of the subject's left eye and the measurement is read, Unfortunately this method can easily lead to parallax errors. The most desirable solution for someone with this difficulty is to use another type of instrument that only requires the use of one eye.
Subject Is Strabismic. The strabismicsuhject, whose eyes are in a tropic position (i.e., with one eye pointing in a different direction from the Other) presents a special problem, since the PD rule method of measurement may then give an artificially high or low reading. To determine a true reading, simply cover the subject's eye not being observed. This ensures that the subject is fixating
with the eye under observation and ensures that it is not turned unless eccentric fixation is present. Even if eccentric fixation is present, the PD measurement is still correct, since the subject never uses this eye in any other position relative to the dominant eye.
In some instances where one eye turns out constantly, the prescribing doctor may determine that the wearer is better served if the lenses are centered in front of the pupils, even for the eye that is turned. This will require that a separate measure be taken for each eye. One measurement will then be considerably larger than the other.
Subject Is an Uncooperative Child. If the subject is young or uncooperative, making normal PD measurements impossible, the dispenser may have to take a canthus-to-canthus measurement. (The canthus is the corner of the eye where the upper and lower lids meet.)
This is done by measuring from the outer canthus of one eye to the inner canthus of the other eye. Unfortunately, this measurement is not entirely exact, Since the inner canthi of the eyes encroach farther across the sclera with younger children.
Common Causes of Errors
There are several common causes of errors inherent in using a PD rule'
I. There will be an error in measurement if the measurer's PD differs significantly from the
subject's because the lines Of sight are not parallel.For example, if the measurer's PD is 16 mm larger
than the subject's, the reading will be I mm too high because Of error.
2. The above error will be increased if the PD rule is not tilted on the subject's nosc so that the sealc is in the most recessed area. The most recessed area corresponds to the approximate position Where the spectacles will be worn.
3. Just as error will be increased when the measure's PD is significantly different from the subject's, the parallactic error will also be increased even more if the dispenser is too close to the subject. Too close is closer than the normal 40 cm (16 inch) distance.
4. A significant error will be induced if the subject is strabismic (one eye turns in or out) or if the subject
4. A significant error will be induced if the subject is
strabismic (one eye turns in or out) or if the subject does not fixate binocularly* during the PD measurement.
5. An error can result if the subject's head moves. An error Can result if the person measuring
his or her head.
7. An error will result if the person measuring does not close or occlude one eye at a time to ensure
sighting from directly in front of the subject's eye under observation.
8. The subject may not look directly at the measurer's pupil during the test, as he or she should, which will result in an error.
•What does 'not fixating binocularly" mean? It rneang that one eye
may have a tendency to turn in or Cut "hen the Subject ig not Concentrating_ In simple terms, they will be using one eye to see instead of both eyes, When this does happen, one eye usually turns Outward Somewhat and the measurement is then too large.
Monocular PD
Since faces are not always symmetrical, it is Often necessary to specify the PD for each eye independently. The main goal in taking the PD is to eventually place the optical centers Of the lenses directly in front of the subject's eyes to prevent any undesired prismatic effect. If one eye is set closer to a person's nose than is the other and the optical centers Of the lenses are placed
symmetrically in the frames, the wearer's lines Of sight will not pass through the optical centers of the lenses The error is not too serious if the lenses are of the same power and are not strong. If, however, one lens is very different from the other, the centers must be placed
accurately to prevent unwanted binocular prismatic effects (Figure 3-4). Monocular PDs are also important
when using aspheric lenses or high index lenses, including polycarbonate lenses. High index lenses have more
chromatic aberration than crown glass or regular (CR-
39) plastic lenses. The nevative effect of chromatic aber
ration on vision is increased if the eye is not looking
through the optical center of the lens. (For more infor mation on high index lenses, see Chapter 23. For more Information on aspheric lenses, see Chapter 18.)
Procedure for Measuring Monocular PDs
Using a Rulev
The monocular PD is best taken using a pupillometer. When a pupillometer is not available, monocular PDs are taken by measuring from the center of the nose to
the center of the pupils. The procedure consists of the following three steps:
l. Measure the binocular PD as described earlier in the chapter. Use the center of the pupil as the
reference vu_'int.
2. Before moving the ruler, note the scale reading on the ruler at the center of the nose. This is the right monocular PD.
3. Subtract this reading from the binocular reading to obtain the left monocular PD. For example, the binocular PD is 66. The scale reading at the center of the nose is 32. The monocular PD for
the right eye is then 32. TO calculate the monocular PD
for the left eye, subtract 32 from 66, to get a reading of
34. The procedure is the same as in taking a binocular
PD measurement, except that the two readings are inde-
pendent of one another and, for purposes of measuring,
the center of the pupil is always used. (There are other
methods that are considerably more dependable than
this method in their ability ro yield consistently accurate
results.)
Procedure for Measuring Monocular PDs
Using the Frame
One error inherent in using a ruler alone appears when
a person has an asymmetrical nose. An asymmetrical
nose often occurs when a nose has been broken. In this
case, the frame positions itself somewhat to the left or
right. For the lenses to be accurately placed, this factor
must be taken into account. It is possible to use an over-
head transparency marking pen and the glazed* lenses
in the sample frame. If the sample frame does not have glazed lenses, clear tape may be placed over the lens opening of the empty frame.
The procedure for measuring monocular PDs begins by adjusting the frame. The frame should occupy the exact position it will have with the lenses in place. The dispenser should be at the same level as the wearer and approximately 40 cm away. The dispenser closes the right eye. The wearer is instructed to look at the dis- penser's open left eye. Since there is no ruler used, the dispenser uses an overhead transparency marking pen and marks a cross on the right glazed lens. If there is no
lens in the frame, the clear tape placed over the lens
opening is marked instead, directly over the center of
the wearer's right pupil (Figure 3-5).
Next the dispenser closes the left eye and opens the right eye. The subject is instructed to look at the dispenser's open eye. The dispenser then marks a cross on rhe lens or rape directly over the left pupil center. the lens or rape directly over the left pupil center.
Because Of the movement involved in marking pupil centers and the ease with which unintentional
head can occur, it is important that markings
be carefully
When tlu is that lmpil centers are accurately marked, the frames are rernc_we.d and che dis-
th t f the bridge to the of each crass measured and recorded, (Thcse steps arc sum-
in Box 3-2.)
PD Measuring Instruments Thc interpupillary distance is mast easily measurcd by
an designed for this purpose,
Readings taken using th
subject u:' parallax errors as those taken using a PD rule,
Such a device solves the "hen the
person doing the measuring is monocular or is amblyo-
eye.
Most have
n occlusion system, which
allows for individual monocular measurements, with
eye fimtmg alternately in of
A well-designed PD measuring inurnment should
rot agälnst the bridge uf subject's nose
frame would. This most accurately apprmimatcs the way
the glasses will position themselves. IL should also posi-
t- 'n the plane It the p
planc,
The Subject will See a Of white or colored hgbt
Iren:nd 1 dark,
n the instrument.
dispenser will See subject's a scale appearing
it, which a
is Alternately, in
some instruments, a split image of the pupil may be seen
Instruments Using Corneal Reflexes
Although some instruments use a method of taking the
PD where the reference point is the geometric center of
the pupil itself, the popular alternate corneal-reflex
method is used in instruments such as the Essilor pupil-
lometer (Figure 3-6) or the Topcon PD-S, PD Meter.
The instruments are supported by means Of pads posi-
tioned so as to cause the instrument to rest on the nose
where the average frame would rest. This is superior to
a forehead supporr system used alone.
The dispenser asks the subject to hold his or her end
of the pupillometer so that the pads rest on the nose
(Figure 3-7). The forehead support should be against the
forehead. The dispenser uses one eye to look into the
instrument. (A real advantage for dispensers with good
vision in only one eye.)
An internal light produces an image by reflection on
each cornea, and the hairline within the device is moved
until coincident with this corneal reflection (Figure 3-8).
The measurement is assumed to correspond with the
subject's line of sight, but is an objective measurement
of the position of the corneal reflection rather than the
position of the line of sight. In addition to a distance PD,
,
near PD may be measured for near points from 30 or
35 cms to infinity.
The line of sight is defined as a line passing from the center Of the pupil to the Obiect Of regard. This is the line that desirably passes through the optical center of the lenses and is the basis upon which the measurement
of interpupillary distance rests.
Corneal reflections are observed along a line which
intersects perpendicularly the center Of curvature Of the
anterior surface of the cornea. (Technically this line is
referred to as the pupillary axis.) This line intersects the
line of sight at the entrance pupil of the eye. It varies in
its orientation by an angle,* which for the average eye
is approximately 1.6 degrees.' This places the corneal
reflection somewhat toward the nose. Thus a PD deter-
mined on the basis of corneal reflections will vary slightly
from that determined by the centers of the pupils.
It is possible to use a corneal-reflection-style instru-
ment to measure a PD based on pupil center distances.
To do this, rhe hairline within rhe device is moved to
the center Of the pupil rather than the center Of the
corneal reflection. The corneal reflection method is
definitely the method of choice when measuring a PD
for someone with pupils dilated from a recent eye
examination.
•This angle is angle lambda, but is often commonly designated as
angle ka Using Corneal Reflections to Measure the PD without
a Pupillometer
It is possible to use corneal reflections to measure inter-
pupillary distance With even a PD ruler, or by using the
frame with glazed lenses. Procedures need only be
slightly modified. The dispenser should be positioned at
the near working distance. The dispenser holds a pen
light directly below his or her eye and shines it into the
eye of the subject. The subject looks either at the pen
light or the dispenser's eye. The reflection of the pen
light on the cornea is used as the reference point instead
of the geometric center of the pupil. The sequence of
measurements is followed exactly as outlined in Boxes
3-1 and 3-2, except that rhe dispenser must position the
pen light directly below his or her "open eye" through-
Out the sequence.
Photographic Instruments for Measuring PD
There are instruments available for taking a wearer's
interpupillary distance that make use ofa photograph of
the wearer's eyes with the frame in place. The frames
are adjusted as they are to be worn. The wearer fixates
a light in the instrument, and the photo is taken. PD and
the wearer's eyes with the frame in place. The frames are adjusted as they are to be worn. The wearer fixates a light in the instrument, and the photo is taken. and segment height measurements are determined using the picture. Up to this point, no photography-based PD measuring system has successfully penetrated the U.S. ophthalmic market.
NEAR PD
The near is required for single vision reading glasses or for multifocals.
For single vision reading glasses, the lenses are set so that their optical centers will be in the lines of sight of the eyes when the eyes are converged for reading.
For multifocals, the distance portion is ground to cor- respond to the distance PD, while the bifocal or trifocal portion IS decentered inward to be properly situated for
near vision. The near PD can be either measured or calculated.
Measuring Near PD With a PD Rule calculate
Measuring Near PD With a PD Rule
To measure the near PD with the PD rule, the dispenser
is positioned at the subject's working distance; that is, at
the distance for which the reading portion is
prescribed.
Closing his or her poorer eye, the dispenser aligns his
or her better eye directly before the subject's nose and
instructs the subject to look into that open eye.
The PD rule is lined up with rhe zero point corre-
sponding to the center of the subject's right pupil. It
should also be held in the same place that the subject's
new frames will rest because this will also affect the
reading.
The dispenser then notes the mark corresponding to
the center of the subject's left pupil. This is the near PD
(Figure 3-9). The subject is not required to shift gaze,
and the dispenser is not required to change eyes during
the procedure. (See Box 3-3 for a summary of this
technique.)
It should be added that it is also possible to use the
edge of the pupil or the limbus for reference points in
taking the near I-SD, as long as only the right or only the
left edges are used, and not both outer or both inner
edges.
In practice, many who use a PD rule to measure the
binocular distance PD, measure the near PD ar the same
time. This is done as follows:
(rhe first three steps are how binocular distance PD
measures begin.)
l. Dispenser is positioned at 40 cm.
2. The dispenser closes his or her right eye and the
subject, using both eyes, fixates on dispenser's left
3. Dispenser lines up zero point of ruler on center of
subject's right pupil. (This next step allows for the
near PD measurement.)
3A. The dispenser looks over at the subject's left eye
and reads the scale on the ruler at the location of
the left pupil center. This is a measure of the near
PD for the distance from the subject to the
dispenser.
dispenser now continues the Steps for finding the
binocular distance as listed in BOX 3—1.
Taking Near PD Using a Pupillometer
Usually a PD measuring instrument will allow both dis-
tance and near PD to be measured. This is done through
the use of a movable internal lens that changes the image
distance and convergence for the subject. The near read
ings are carried out in the same manner as the distance
readings.
USING THE NEAR PD FOR BIFOCAL INSET
For the near "reading" area ofa pair of glasses to be used
most comfortably, it must be positioned accurately in the
lens. Horizontal placement Of the near segment viewing
area is determined by the near PD. (Vertical placement
depends on frame depth and the individual's visual need
and will be covered extensively in Chapter 5.)
The horizontal position of bifocal segments is speci-
The horizontal position of bifocal segments is speci-
fied as the distance from the farpoint PD that the seg
ments are set in toward rhe bridge.
Because of the possibility of unequal monocular PDs,
segment inset is usually specified individually for each
eye. Ordinarily segment inset is the difference between
the distance PD and the near PD, divided by 2:
(distance PD) — (near PD) Segment Inset — for each eye is 2 mm.
Where inequality of the monocular PDs exists, this rule may result in errors, since both eyes may not be required to converge equal angular amounts for near fixation. The actual amount Of error is usually so slight, however, that it is usually ignored. The exceptions would
be eases of very marked differences in monocular PD or very strong lenses.
If there is a large difference in monocular PDs, inset Where inequality of the monocular PDs exists, this rule may result in errors, since both eyes may not be required to converge equal angular amounts for near fixation. The actual amount Of error is usually so slight, however, that it is usually ignored. The exceptions would be cases of very marked differences in monocular PD or
very strong lenses.
If there is a large difference in monocular PDs, insetting the bifocal segments accordingly may result in a rather unusual-looking pair of glasses (Figure 3-10).
This effect can be made less noticeable by using a bifocal with a wider segment.
Calculating the Near PDThere are several other factors to be considered when
calculating the near interpupillary distance, most notably those that cause differences in segment inset.
Calculation
The most logical way to calculate the interpupillary distance is to draw a triangle with the center of rotation of the eyes being two points of the triangle and the near
point of fixation being rhe rhird. A similar triangle is
then constructed by drawing a line corresponding to the
spectacle plane.
By similar triangles, the monocular near PD can be
calculated from the monocular distance PD (Figure
3-11).
When using a prewritten prescription, the working
distance will normally never exceed the reciprocal of the
power Of the near addition. For example, a +2.00 diopter
near addition Will indicate a working distance no further
than 50 cm. 1 = 0.50 meters = 50 cms +2.00
Unless the professional situation or physical build of
the wearer indicates otherwise, the customary near
working distance can be assumed to be 40 cm. If,
however, the power of the near addition (add power) is
will be the reciprocal of the add power. For example,
a +3.00 diopter add indicates a working distance of
331/3 cm.
1
= 0.33— meters = 33— cms
+3.00 D
3
Gerstman has simplified the calculation of the near inset with a rule which he calls the three-quarter rule. The three-quarter rule states that for every diopter of dioptric demand the optical center Of each reading lens, or the geometric center of each bifocal addition, should
be inset 0.75 (three quarters) mm. Dioptric demand is the inverse of the reading distance in meters and is independent of the actual bifocal addition power.
Example 3•1
For a reading distance of 40 cm, and an add power of +1.00 D, what is the inset per lens?
Solution
To find the answer. we first need to know the dioptric demand.
For a reading distance of 40 cm, and an add power of +1.00
D, what is the inset per lens?
Solution
To find the answer, we first need to know the dioptric demand.
The dioptric demand is the inverse of the working distance,
not the inverse of the +1.00 add power. Therefore since
the working distance is 40 cm or 0.40 m, the dioptric
demand is 1 = 2.50D 0.40
Having found the dioptric demand. we can find the inset
per lens by multiplying by three-quarters, as the rule name
implies. Therefore the inset per lens is
2.50x3/4 =1.9 mm
The Influence Of Distance Lens Power on
Segment Inset
The power Of the distance prescription has an effect on bifocal inset. When a person looks at a near object, the eyes turn inward and are no longer looking through the optical centers of the lenses. Negative power, or minus lenses, keep the eyes from converging as much as
they normally would because of the Base In prismatic effect at this point on the lens. Positive power, or plus lenses, cause the eyes to converge slightly more than they normally would because of their Base Out prismatic effect.
For positive lenses then both the measured or the Gerstman-calculated near PD would need ro be reduced (i.e., the segment inset Of the bifocal increased). For minus lenses, the near PD would need to be increased (i.e., the inset of the segment reduced).
The position of the near reading area becomes more important when the reading area is small. This means that for progressive addition lenses, the position of the intermediate and near readings areas is very important.
Progressive addition lens designers are now taking dis- important when the reading area is small. This means
that for progressive addition lenses, the position of the intermediate and near readings areas is very important.
Progressive addition lens designers are now taking dis-tance power into consideration when determining how much inset the near viewing area should have. Segment Inset Formula. There have been several factors listed as having an effect on segment inset. These
• The distance the lenses are from the eyes
• The distance PD
• The near working distance
• The power of the distance lens
Taking all these factors into consideration, Eller-brock4 derived the following formula for segment inset.
where P is one half the distance PD, Ois the distance of the lens from the working nearpoint, s is the distance from the lens to the center Of rotation Of the eye, and f is the focal length Of the lens in the 180-degree meridian. All measurements are expressed in millimeters.
Example 3-2
What would the segment inset be for a person with a 70 mmdistance PD who is wearing a prescription of +6.500?
Assume they are wearing a +2.50 add, but are working at a near working distance of 20 cm. The spectacle lenses are near working distance of 20 cm. The spectacle lenses are
25 mm from the center of rotation of the eye to the back of
the lens.
Solution
We are using Ellerbrock's formula.
is half the distance PD, SO
70
In Ellerbrock's formula P
= 35 mm.
The value of is the distance from the lens to the near
working point in millimeters. This distance is given as 20 cm,
which is the same as 200 mm.
The focal length of the lens is the reciprocal of the power of the lens. This is
= 0.1538 Meters
6.50
= 153.5 mm
Since the lens is a sphere, the power in the 180-degree meridian is the same as the power in any other meridian.
The distance from the lens to the center of rotation of the eye is given as 25 mm. so s — 25 mm.
Since the lens is a sphere, the power in the 180•degree meridian is the same as the power in any other meridian.
The distance from the lens to the center of rotation of the eye is given as 25 mm. so s = 25 mm.
Inserting all of this into Ellerbrock's formula results in 1+200.
= 4.5 mm
1
35
1
25
153.5
So the inset per lens for this wearing situation is 4.5 mm per eye.
Summary of Factors
Fortunately the variations in segment inset caused by all these factors are not radically different from that found using the measured near PD. This assumes, Of course, that the near PD is measured at the appropriate working distance.
Table 3-2 summarizes the effect of distance lens power on segment inset for the normal working distance
(40 cm or 16 in). s
Recommendations For Finding The Near PD After all of these possibilities, what is the most appropriate way to determine segment inset? Here are some recommendarions for different situations. The idea is to provide the best accuracy without making it too difficult.
Keep in mind that just using a PD ruler may not be the most reliable method.
Recommendations for Finding the Correct Segment Inset • When the working distance is normal (40 cm)
1. Measure the near PD with a pupillometer or a Segment Inset
When the working distance is normal (40 cm) l. Measure the near PD With a pupillometer or a
PD ruler.
2. If the distance lens powers are high, use Table 3-2. When the working distance is less than 40 cm
. Again, measure the near PD with a pupillometer or PD ruler. Be certain to set the correct
working distance in the pupillometer before measuring. When measuring with a PD ruler,
the dispenser must be at the shorter working distance.
If the working distance is less than that allowed for in the pupillometer, use Gerstman's three-
quarter rule (assuming adult PDs between 62 and 68 mm), or use Table 3-1.
• When the distance lens powers are especially high If the working distance is normal (40 cm), use
Table 3-2.
. If the working distance is closer than 40 cm, use Ellerbrock's formula. (Ellerbrock's formula could
When the distance lens powers are especially high l. If the working distance is normal (40 cm), use
Table 3-2.
2. If the working distance is closer than 40 cm, use Ellerbrock's formula. (Ellerbrock's formula could
actually be used in any of the above
circumstances, but it is unhandy to work with.) Examples for Finding the Near PD
Here are some examples. Both the power of the prescription and the distance p D are known. Use the most
appropriate method to find the segment inset and then
the near PD.
Exampje 3-3
A spectacle lens wearer has the following prescription
R: —1.00 D sphere
L: -1.00 D sphere
add: +2.00
The distance PD is measured as 64 mm. For a 40 cm working distance, what is the expected near PD?
Solution
Since the working distance is 40 cm, simply measure the near PD with a pupillometer (or PD ruler). If a pupillometer is not available, use Table 3-2. In the table, we find the inset for a 32 mm monocular distance PD with a —1.00 D power
to be 2 mm. Therefore the binocular near PD would be 4 mm less than the distance PD. Since 64 — 4 = 60 mm, then the
near PD equals 60 mm.
Example 3•4
Suppose an individual has a distance PD of 64 mm, a distance prescription of —1.00 D sphere for both eyes, and a
bifocal add of +2.00. (These are the same lens powers as
given in the previous example.) What would the near PD be if the near working distance was 25 cm instead of 40 cm?
Solution
Since the working distance is less than ao cm, we would find the near PD either by a direct measurement using a pupillorneter (or PD ruler) or by using the three-quarter rule.
To find the near PD by measurement, the best option would be to use a pupillometer. unfortunately, most pupillometers only measure up to 33 However, it is possible to use a PD ruler. If a ruler is used, the dispenser's face must be at the subject's near working distance.
If the near PD is to be calculated, it is possible to do these calculations with Gerstman's three-quarter rule. To use the three-quarter rule, begin by the finding the dioptric demand.
Dioptric demand is the reciprocal of the working distance in meters. In this example, the working distance is 25 centimeters or 0.25 meters. Therefore
1
Dioptric demand
0.25 meters
Next, to find the inset per eye, the dioptric demand is
multiplied by 3/4.
3
— x 4 3 mm per eye
4
Thus the near PO will be
Distance PD — (segment inset x 2)
or
64 — 6 or 58 mm.
This means that a prescription for a person with a bifocal
add and a 25 cm working distance should have the distance
optical centers set for a far PD of 64 mm and the segments
set for a near PD Of 58 mm.
Using Table 3-1 would have yielded an inset Of 3.3 mm per
eye and a near PD of 57.4 mm. Remember that the three-
quarter rule is a close approximation and because the table
does not list every PD and working distance, it may also be
a close approximation.
Example 3-5
Example 3-5
A prescription reads as follows:
R: +1.50 - 1.00 x 180
L: +1.50 - 1.00 x 180
add +3.50
The distance PD is found to be 61 mm. What should the near
PD be?
Solution
A near addition with a pot,ver greater than +2.50 D should
be a red flag to the dispenser. An add greater than +2.50 D
means that the working distance will be less than 40
The near PD is best found by direct measurement with a
pupillometer or PD ruler. The three-quarter rule is not as
accurate because the PD is smaller than the normal 62 to
68 mm range. The next best thing is to use Table 3-1.
TO measure directly With pupillometer or PD ruler, the
working distance must be known. When an add power is
greater than +2.50 D, unless another distance is specified
the working distance is found by taking the reciprocal Of the
add power.
Working Distance —
No
1
= 0.29 M or 29 cms
The other way to find the near PD would be to use the
three-quarter rule. This is done by multiplying the dioptric
demand (3.5 D) by 0.75.
Which is:
(0.75)x (3.5) = 2.625 mm per lens
Now the near PD is
Near PD — dis tance PD — (2 x seg inset)
-61 (2 x 2.625)
= 61 - 5.25
= 55.75 The other way to find the near PD would be to use the
three-quarter rule. This is done by multiplying the dioptric
demand (3.5 D) by 0.75.
Which is:
(0.75)x (3.5) = 2.625 mm per lens
Now the near PD is
Near PD — dis tance PD — (2 x seg inset)
-61 (2x2.625)
= 61 - 5.25
= 55.75
Either inset number will yield a similar answer and both
answers will round to 56 mm for the near PD.
Example 3-6
A prescription reads as follows:
R: -8.50 D sphere
L: —8.50 D sphere
add = +1.50
For this prescription, we will assume that the distance mon.
ocular PDs have been measured. The right monocular dis-
For this prescription, we will assume that the distance mon.
ocular PDs have been measured. The right monocular dis-
tance PD was measured as 28 mm and the left as 31 mm.
For a 40 cm working distance, find the near monocular RDS.
Solution
For high•powered lenses, calculated near pos Will be more
accurate than measured near PDs. This is because measured
near PDs do not take the prismatic effects of high plus or
high minus spectacle lenses into consideration. (Looking
nasally through high minus lenses causes a base-in prismatic
effect and reduces the amount the eyes converge for near
viewing.) However, there is one situation for high-powered
lens prescriptions where measured near POs are as accurate
as calculated near This occurs if the near PDs are
measured while the prescribed distance lenses are being
worn. So if the wearer has the same existing prescription in
single vision lenses or multifocals, a PD ruler may be used
while the person is wearine their existing frame and lenses.
This prescription is for the normal working distance. The
easiest method to find the near PD for this high-powered
prescription is to consult Table 3-2. For the right and the left
lens, Table 3-2 shows segment inset to be 1.5 mm per lens.
Therefore the monocular near PDs are
prescription is to consult Table 3-2. For the right and the left
lens, Table 3-2 shows segment inset to be 1.5 mm per lens.
Therefore the monocular near PDs are
R: 28 mm — 1.5 mm or 26.5 mm
L: 31 mm — 1.5 mm or 29.5 mm
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