PRISM
THINNING
One
slight drawback to progressive addition lenses in certain power ranges is
thickness. Increased thickness is especially evident when the distance powers
are either plus or low minus. Progressive lenses in plus or low minus power
ranges will be thicker than a fl at-top multifocal lens of equal power. This
increased thickness is a result of the steepening front curve in the lower half
of the lens. (This same problem also occurs in “Executive” multifocals and can
be solved in the same way.) As the lower progressive portion of the lens
increases in plus power, the surface curvature steepens. This thins the bottom
edge. To keep the lower lens edge from becoming too thin, the whole lens must
be thickened.
To
overcome the problem, the lower edge must somehow be thickened without
thickening the upper edge. This can be done by adding base-down prism to the
whole lens. When this is done properly, overall lens thickness will actually
decrease. The technique, known as yoked base-down prism, is illustrated in
Figure 1. Naturally, both right and left lenses must receive the same
amount of base-down prism; otherwise the wearer will experience double vision
as a result of unwanted vertical prism differences.
Figure 1 shows the use of base-down prism to thin a progressive addition lens. A, The progressive addition lens as ground without prism thinning. The dotted lines indicate how the lens would be curved if it were a single vision lens instead of a progressive lens. B, Adding base-down prism thickens the bottom of the lens only. C, The line between prism and original lens has been removed. It is now possible to see how this lens with newly added base-down prism could be further thinned because both top and bottom are thick. D, The hatched area shows how much lens thickness may be removed now that both edges are equally thick. E, Excess lens thickness has been removed and progressive lens prism thinning achieved.
The
exact amount of prism needed to thin the lens effectively varies according to
the strength of the addition, the size and shape of the lens after edging, and
the design of the lens. As a rule of thumb, Varilux suggests adding prism power
amounting to approximately two thirds of the power of the add. (The use of
yoked basedown prism for Varilux lenses has been referred to by the name
Equithin.)
Prism Thinning Causes
Prism at the PRP
It
should be mentioned that base-down prism used to thin the lens will show up at
the PRP of the lens. This is particularly important to note when only one lens
is to be replaced since both right and left lenses must have the same amount of
vertical prism. Thus vertical prism found at the PRP of the lens is acceptable
when both left and right lenses have the same amount of vertical prism.
THE EFFECT OF PRESCRIBED
PRISM ON PROGRESSIVE LENS FITTING
Success
in fitting progressive addition lenses depends on accurate horizontal placement
of the monocular PDs. If monocular PDs are incorrect, the eyes do not track
down the progressive corridor. This reduces intermediate vision. Incorrect PDs
also displace the reading zone, reducing its usable size.
Success
in fitting progressive addition lenses is also influenced by the accuracy of fitting
cross heights. An inaccurate fitting cross height will cause one eye to track
down the corridor ahead of the other. This means that the add power is not
increasing equally for the two eyes. The eye farther down one corridor is
looking through more plus power than the partner eye following a few steps
behind. An inaccurate fitting cross height also causes the eye to track down
the progressive corridor off-center, narrowing the effective width of the
intermediate viewing.
When
prism is placed before the eye, it causes the image of an object to be displaced
in the direction of the prism apex. The eye must turn toward the apex to view
the displaced image. For example, if base-down prism is placed before one eye,
that eye turns upward toward the apex to fixate the displaced image.
Vertical Rx Prism
Changes Fitting Cross (and Bifocal) Heights
When
vertical prism is present in a prescription, it causes one of the wearer’s eyes
to turn slightly up or down. But when fitting cross height measurements are
taken, the prism is not present. When the wearer is able to keep the eyes
working together without the prism the eyes are looking straight ahead. One eye
will not likely be turned upward or downward in relationship to the other.
However,
once the prescription lenses are in the frame, the eye must turn in the
direction of the apex of its prescribed prism. The amount of displacement in
the spectacle plane will be 0.3 mm for every 1 prism diopter of prescribed prism.
When
vertical prism is present, the fitting cross should be raised 0.3 mm for every
diopter of base-down prism or lowered 0.3 mm for every diopter of base-up
prism.
If the
entire amount of vertical prism is prescribed before one eye, the vertical
displacement of the fitting cross should be carried out on one lens. But if the
vertical prism is split, the displacement of the fitting crosses should also be
split in the same proportion.
To be
certain of vertical fitting cross positioning with perscription prism, cover
the wearer’s left eye when measuring the fitting cross for the right eye. Then
when measuring fitting cross height for the left eye, cover the wearer’s right
eye.
Example
A
prescription reads as follows:
R: +2.75 −1.00x180
“3 prism base up”
L: +2.75 −1.00x180 “3 prism base down”
The
frame of choice is adjusted to fit as it should when being worn. Next fitting
cross heights are marked on the glazed lenses to correspond to pupil center
location. Heights are measured to be as follows:
R:
27 mm
L:
27 mm
What
fitting cross heights should be ordered?
Solution
Vertical
prism for the right lens is noted. The amount of vertical compensation is
calculated as follows:
Vertical prism amount x 0.3 =
change in fitting cross height in millimeters.
Or
in this case
3 x0.3 = 0.9 mm.
This is rounded off to 1 mm.
Because
prescribed prism causes the pupil of the right eye to be displaced 1 mm
downward, the fitting cross must be moved 1 mm downward as well. The left lens
has an equal but opposite amount of vertical prism. Therefore the prism in the
left lens necessitates moving the left fitting cross 1 mm upward. The end
result is that the two fitting cross heights are modified and should be ordered
as
R: 26 mm
L: 28 mm
Horizontal Rx Prism
Changes PD Measurements
When
horizontal prism is prescribed, failure to horizontally compensate the MRP
placement will cause the eyes to track along the inside or outside edge of the
progressive corridor. This greatly reduces the usefulness of the intermediate
zone and narrows the field of view for near work.
Example
Suppose
a prescription reads as follows:
R:
−2.25 −0.50 x180 “5 prism base in”
L:
−2.25 −0.50 x180 “5
prism base in”
Using
a pupillometer, the monocular PDs are measured as follows:
R:
29.5 mm
L:
30.0 mm
What
monocular PDs should be ordered to compensate for the prescribed horizontal
prism?
Solution
Noting
horizontal prism, the amount of pupil displacement is calculated as follows:
5
x0.3 = 1.5 mm.
Base-in
prism will cause the eye to move outward by an amount equal to 0.3 mm for every
diopter of horizontal prism. In this case 5 prism diopter of base-in prism will cause each
pupil to be displaced outward by 1.5 mm. The resulting monocular PDs are modified
to
R:
31.0 mm
L:
31.5 mm
When Might the Amount of
Horizontal Prism Be Modified?
When
prism is prescribed in conventional, nonprogressive, multifocal lenses, the PD
is not modified to allow for a change in pupil location. This is quite
acceptable because the widths of nonprogressive multifocals are so wide in
comparison with the corridors of progressive addition lenses that there is
little need for modification.
When
an Optometrist tests for prism, the measuring prism on the phoropter is in
front of the sphero-cylinder lens combination. As the measuring prism is
increased in power, the eye responds by turning, leaving its location behind
the OC of the refractive lenses. When the eye moves away from the OC of the
lens combination, a second prismatic effect, caused by lens “decentration,” is
induced (Figure 3). Practically speaking this second prismatic effect is of
no consequence since the measuring prism is taking it into account. But what
happens if the refractive MRP location is altered during fitting?
Fig 3 A measuring prism in front of the refractive lens will cause the eye to turn outward. As it turns, it leaves its previous location directly behind the optical center of the lens.
When
the fitting cross is changed to correspond to the prismatically altered eye
position, the decentration prism that was present
during refraction disappears. Without decentration prism, the net prismatic
effect that was present during refraction has changed. When prescription sphere
and cylinder powers are small, this is of minimal consequence. As the
refractive power increases, however, the prismatic amount becomes more evident.
Example
Suppose
a person is wearing or needs a prescription as follows:
R:
−3.50 sphere
L:
−3.50 sphere with 6Δ base-in prism
+2.25 add
(Although
it may not be advisable to place all prism in front of one eye, we will use
this example for simplicity.)
Before
refraction the monocular PDs are measured using a pupillometer. There are no
refractive lenses in place. The PD measures as follows (Figure 4):
R
monocular PD = 31 mm
L
monocular PD = 31 mm
How
should the monocular PDs and prescribed prism amounts be modified to allow the
eyes to accurately track down the progressive corridor and still maintain the
same net prismatic corrective effect?
Solution
Placing
6Δ of base-in prism before the left eye will cause the eye to deviate outward
by
6
× 0.3 mm = 1.8 mm,
which
will be rounded off to 2 mm.
During
phoria testing, the eye was looking 2 mm temporally through the −3.50 D
refracting lens (see Figure 3).
Using
Prentice’s rule, we see that prism caused by the eye being decentered in
relation to the lens is
Δ = cF
= 0.2 × 3.5
= 0.7Δ
Since the
lens is minus, prism caused by the eye moving in relationship to the refractive
lens is base out. Therefore the net prismatic effect for the eye is
(Prescribed Δ) + (Decentration Δ) = (Total Δ).
Or in this
case
6 base in + 0.7 base out =
5.3 base in.
To
position the progressive zone in front of the eye, the MRP must be moved 2 mm
outward. (When the position of the MRP moves, so does the fitting cross
location. The fitting cross is directly above the MRP.) When the MRP moves outward,
the finished spectacle lens prescription will no longer duplicate the
refractive situation. This is because the 0.7Δ of decentration prism caused by
the −3.50 D lens no longer exists (Figure 5). To maintain the same total prismatic
effect, the prescribed prism must be reduced from 6Δ base in to 5.3Δ base in. The
PDs are ordered as follows:
R monocular PD = 31 mm
L monocular PD = 33 mm
It is
helpful to note that when the MRP is moved in the direction of eye deviation,
there will always be a reduction of prescribed prism for minus lenses and an increase
in the amount of prescribed prism for plus lenses. In other words:
·
For minus lens: reduce the Rx prism by
an amount equal to the calculated decentration prism.
Fig 4 A pupillometer normally measures the interpupillary distance with no lens correction in place
and with the eyes in a straight-ahead position.
Fig 5 If the monocular interpupillary distance were to be altered to compensate for prismatically induced eye movement and correct progressive corridor placement, the net effect would be to change the amount of prism in the prescription. The decentration prism resulting from eye movement caused by the measuring prism will no longer be present.
·
For plus lenses: increase the
Rx prism by an amount equal to the calculated decentration prism.
When filling
an existing prescription, it should be noted that a modification to the Rx
prism amount that is done to maintain the prescribed optical effect is no
different than changing sphere and cylinder power in response to a change in
lens vertex distance. Changing the amount of “Rx prism” to compensate
for decentration prism does not change the prescription.
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Compensating Fitting
Cross Height or Monocular PDs for Prescribed Prism
A. How to Compensate Fitting Cross
Height for Prescribed Vertical Prism
1. Measure the fitting cross
heights.
2. Multiply the amount(s) of
prescribed vertical prism by 0.3.
3. If the prism is base down,
raise the fitting cross height by the calculated
amount. If the prism is base up, lower the fitting
cross height by the
calculated amount.
B. How to Compensate Monocular PDs for
Prescribed Horizontal Prism
1. Measure monocular PDs using a
pupillometer.
2. Multiply the amount(s) of
prescribed horizontal prism by 0.3.
3. Modify the monocular PD(s) by
the calculated amount, increasing the PD
for base in prism and decreasing
the PD for base out prism.
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Compensation Used if
Modifying the Monocular Interpupillary Distances Produces Clinically Significant
Changes in Rx Prism
It
is suggested that compensation be considered clinically significant if moving the MRP will cause a change in prismatic
effect of 0.50 Δ or more. A change of at least 0.50 Δ will occur if the prescribed prism totals ≥6.00 Δ and refractive power in the
meridian of movement is ±2.50 D or greater.
1.
When horizontal prism is present, find the power of the lens in the
horizontal meridian. When vertical prism is present, find the power of the
lens in the vertical meridian.
2.
Multiply the power in the meridian of eye movement by the change in monocular
PD or change in fitting cross height. That is,
Δ = cF
where
Δ = change in prescribed prism power, c =change in PRP (prism reference point) location in cm, and
F = power in the meridian of PRP movement.
3.
For minus lenses, subtract this amount from the prescribed prism. For plus
lenses, add this amount to the prescribed prism.
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SUMMARY
Prescribed
vertical prism in progressive add lenses requires that the fitting cross be
moved up or down by an amount equal to 0.3 times the prism amount. The direction
of movement is opposite from the base direction of the prism.
Prescribed
horizontal prism in progressive add lenses requires that the monocular PDs be
increased or decreased by an amount equal to 0.3 times the prism amount. The
direction of eye and MRP movement is opposite to the base direction of the
prescribed prism. Steps to take when modifying fitting cross height are found
in above box , A. Steps to take when modifying monocular PD amounts are
summarized in box B.
Changing
the prism amounts should only be done if there would be clinically significant
changes to the prescribed prism. This does not happen unless the prescribed prism
is greater than or equal to 6.00Δ and the refractive power in the prism
meridian is greater than plus or minus 2.50 D. If this is the case, then
prescribed prism may be altered according to the summary found in above last box.
