When progressive addition lenses were introduced in the early 1970s,
non-adaptation was a commonly encountered problem. Advancements in the understanding of the physiology of vision and progressive design have virtually eliminated non-adapts. Assuming a progressive lens is fit and fabricated properly, rare cases of the non-adapt may present a mystery to the eyecare practitioner—particularly when the patient has successfully worn progressive lenses in
the past. Here we will attempt to discuss one of the rare (and least
understood) causes of non-adapts among previously successful PAL
wearers: prism thinning.
Although prism thinning is rarely prescribed specifically i.e., lenses
ordered with base down prism O.U., it is present in many of the progressive lenses produced and dispensed to patients. Prism thinning
will usually be present in any pair of progressive lenses with plus distance power. Progressive lenses with minus power may have prism
thinning as well—if
the amount of minus
distance power is
less than the amount
of plus add power.
Since prism thinning
is considered a cosmetic element of the
lens, there is typically
no indication on the
order (or returning
invoice) to indicate
whether prism thinning has been applied
(or, if it has, the
amount of prism thinning present in the
Ideally, the effects
of prism thinning
are transparent (pun
intended). However, prism thinning does impact the optical properties of the lens. Therefore, changes in the amount of prism thinning between pairs of PALs may cause visual changes, which are
noticeable to the PAL wearer. Usually, patients who notice changes
in prism thinning are those who have successfully worn progressives
in the past. Since most patients who have worn PALs in the past
adapt very quickly to subsequent PAL eyewear, patients who react
to changes in prism thinning often present a mystery to the practitioner (particularly since the amount of prism thinning is not a
parameter often measured in day-to-day practice).
The calculation of prism thinning is usually performed in the laboratory and is a function of lens power and frame size. Changes in
these parameters may result in changes to the amount of prism thinning applied to a patient’s progressive lenses. Understanding prism
thinning—and the method used to calculate prism thinning—will
allow eyecare practitioners to better meet the needs of their patients.
WHAT IS PRISM THINNING?
Prism thinning is yoked base-down prism that is applied to progressive addition lenses to reduce overall lens thickness and
weight. In this case, the term “yoked” means the prism deviates
images equally and in the same direction for both the right and left
eye. For this reason, the presence (or absence) of prism thinning
is usually not noticeable to the PAL wearer (i.e., the optical effect
is “cancelled out” because it is the same in both eyes).
Prism thinning is applied to progressives to reduce the thickness and weight of a PAL as illustrated in Figure 1. Figure 1a
illustrates a PAL with no prism thinning. To create add power
(i.e., additional plus power), the curvature of a progressive lens
increases at the bottom of the lens. In order to be fit into a
frame, the lens must have some thickness over a certain diameter
(i.e., the lens must be thick enough to provide a lens diameter
large enough to be edged down to the frame shape). For many
prescriptions, the end result is a lens in which the top is thicker
than the bottom. Typically, progressive powered lenses will have
a positive combined near power and will tend to be thicker at the top edge. Example: A lens with a distance power of -1.00D
sphere and an add of +2.00 will have a combined near power of
+1.00. Without prism thinning, this lens will likely be thicker at
the top when edged.
The lens (Fig. 1a) cannot be ground any thinner, because any
reduction in thickness will decrease the lens area (diameter) at
the bottom of the lens and the lens will not be large enough to
support the frame size.
By adding base down prism to the lens (Fig.1b), thickness
between the top and bottom is equalized- which is why prism thinning is sometimes referred to as “equi-thinning.” Generally speaking, the amount of prism used to thin the lens is usually equal to
about 2/3 of the power of the addition. Therefore, a lens with a
+1.50 add would have around 1.00 diopter of base down prism to
thin the lens. This general rule also means prism thinning should
rarely, if ever—exceed 2.00 diopters of base down prism.
Figure 1c illustrates the combination of the base down prism and
the original lens shape. Because the top and bottom of the lens are
of equal thickness, the entire lens may be ground thinner without
reducing the diameter (size) of the lens. The result is a thinner/lighter
lens that is more cosmetically appealing and comfortable (Fig.1d).
Prism thinning is generally considered a cosmetic element of the
finished ophthalmic lens—it is rarely (if ever) prescribed. Customarily, the decision to apply prism thinning to a lens is made by the
Rx calculation program at the laboratory, which computes the
edge thicknesses and determines if the addition of base down
prism will reduce the overall thickness of the lens. Nevertheless,
prism thinning does affect the optical characteristics of the lens-and may therefore affect the wearer’s visual perception.
WHAT IS THE VISUAL EFFECT
CREATED BY PRISM THINNING?
Light traveling through a prism bends in the direction of the
prism’s base (Fig. 2). However, the image produced by a prism is
displaced towards the apex. As the illustration shows, light from
the object (a star) impacts the front surface of the prism and is
refracted toward the base. When the light emerges from the other
side of the prism, it is again refracted towards the base. Refraction
occurs because the speed of light, as it travels within the prism, is
slower than the speed of light in air. When a lens slows light, the
path of the light will bend towards a path that is perpendicular to
the lens surface. When light emerges from the lens, it regains speed,
and bends away from a path that is perpendicular to the surface.
In the illustration, the eye perceives light that has passed through
a base down prism. Therefore, the eye perceives the object (a star)
in the direction from which the light is coming (i.e., towards the
apex of the prism). Even though the object is basically in front of
the eye, when viewing through a base down prism, the eye will
rotate slightly upward (because the image of the
object is displaced upwards towards the apex). An eye viewing through a prismatic lens will
therefore naturally rotate towards the apex
aligning the visual axis with the incoming
image. The eye rotates in the direction of
the apex to align itself with the direction
from which the light appears to originate.
The amount of rotation is minimal; 1mm
of rotation requires approximately 3.00
diopters of prismatic power.
As previously mentioned, prism thinning
is yoked (equal between the eyes) base-down prism. Prism thinning therefore
causes an equal upward displacement of
images in both the right and left lenses.
This slightly decreases the downward rotation of the eye necessary to view through
the near zone of the progressive powered
lens, because the image produced by the near zone is slightly raised.
Because the amount of image displacement is small, and equal
between the two eyes, most PAL wearers have no awareness of the
visual effect caused by prism thinning. However, a small percentage
of patients do seem to notice even the smallest changes in their eye-wear. Consider first the patient who is sensitive to the addition of
prism thinning. Perhaps this patient is new to progressive lenses, or
(more likely) has always worn PALs that did not have prism thinning. This patient may report that the floor seems to “tilt up”
towards them (i.e., the image of the floor is displaced upwards). This
patient will sometimes make a comment such as: “I feel like I’m constantly walking uphill.” Finally, the patient may even feel the need
to tilt his head downward to view through the distance area.
By contrast, a patient who is sensitive to a decrease in prism thinning (i.e., a patient who has worn prism thinned PALs in the past
who receives PALs that do not have prism thinning) may perceive
the reading area to be lower in the frame or may have trouble
finding the reading area.
Again, these reactions are not common, but do occur in a small
percentage of patients. In most cases, the cause for non-adaption
to a PAL will be related to the refractive power of the lens or to
non-optimal placement of the lens in front of the eye. However,
once the refraction and fit have been verified, a comparison of the
amount of prism thinning present in the previous and current
lenses should be among the lens parameters evaluated.
HOW IS PRISM THINNING MEASURED?
Prism thinning is measured at the PRP (primary reference point) of
a PAL (Fig. 3). The PRP is the optical center of a progressive powered lens and is always found at the midpoint between the engraved
circles on the surface of the PAL (the circles are always 34mm
apart). Depending on the manufacturer,
the PRP may be at or anywhere from 2 to
6mm below the fitting cross, so re-mark the
PRP and fitting cross using a manufacturer-supplied template (sometimes referred
to as a cut-out or fitting chart).
Once the PRP has been marked on the
lens, situate the lens in the lensometer so
the center pin falls upon the PRP. Adjust
the power drum to make the lines as crisp
as possible (note: it is impossible to read
the power at the PRP, simply adjust the
power drum to make the sphere and cylinder lines as crisp as possible). Record the
amount of prism in the lens at the PRP
and repeat the process for the other lens.
The amount of prism in each lens should
ALWAYS be equal at the PRP—unless the refractionist has
prescribed prism. Repeat the process for both the previous and
current lenses and note any differences. In cases where a previously successful PAL wearer is non-adapting to new eyewear, if
there is noticeable difference in the amount of prism thinning
between the previous and new eyewear, consider matching the
previous amount of prism thinning when reordering lenses.
HOW IS PRISM THINNING
CALCULATED IN THE LABORATORY?
Every laboratory has an LMS or “Laboratory Management System.”
Among other things, the LMS calculates the processing parameters necessary to create lenses of appropriate Rx, shape and
size. Different systems use different methods to calculate prism
thinning; however, most calculate prism thinning based on the
- Is the total power of the lens in the vertical meridian plus or
minus in power?
- Is the PRP of the PAL above the datum line? The datum line
is a horizontal line that passes through the geometric center of the
lens. For example, if the “B” (or vertical) measurement of the lens
is 30mm, the datum line will be a horizontal line 15mm above the
bottom of the frame.
- Will prism thinning reduce overall lens thickness?
Many LMS programs rely solely on the third parameter. That is,
prism thinning is applied to any lens when the LMS determines a
reduction of weight and thickness will result.
OD -1.00 sph OU ADD +2.50
OS -0.75 sph
Frame’s “B” measurement = 30 mm
Fitting Height = 20mm
The order placed in ’05 received prism thinning (fig. 4a).
The total power is plus (distance + ADD = +1.50).
The PRP is above the frame midline.
- PRP is 4mm below fitting height @16mm
- Datum height is 1/2 the “B” measurement @15mm
The amount of prism thinning was 1.67D yoked base
down (prism thinning is usually 2/3 the value of the
ADD power: +2.50 x 2/3 = 1.67).
OD -1.00 sph OU ADD +2.75
OS -1.00 sph
Frame’s “B” measurement = 30mm
Fitting Height = 17mm
The order placed in ’07 did not receive prism thinning (fig. 4b).
The total power is still plus (distance + ADD = +1.75)…
…but the PRP is now below the datum line.
- PRP is 4mm below fitting height @13mm
- Datum height is 1/2 the “B” measurement @15mm
In some LMS programs, the decision to apply prism thinning
may be based upon a combination of the first two parameters.
Depending on the settings selected by the individual laboratory,
the program will apply prism thinning only under certain conditions. For example, the program may be set to apply prism thinning only if the total power in the vertical meridian is plus and the
PRP falls above the datum line. If either of these conditions is not
met, the program will not apply prism thinning to the order.
Since subsequent pairs of eyewear are sometimes ordered from
different laboratories (or, even if the same laboratory fabricates
subsequent pairs, perhaps there have been changes in the calculation parameters or the LMS program itself), the amount of prism
thinning the patient receives may vary from eyewear to eyewear.
Usually, a patient’s Rx remains fairly stable from year to year; therefore, the parameter most likely to impact the calculation of prism
thinning is the location of the PRP relative to the datum line. The
PRP/datum line relationship is influenced by: 1.) The fitting height,
and 2.) The “B” measurement of the frame. In Figure 4, the PRP is
4mm below the fitting reference point (aka, the “fitting cross”).
Although the PRP is 4mm below the fitting cross on many PAL
designs, the separation between fitting cross and the PRP varies
from as little as 2mm to as much as 6mm; one manufacturer places
the PRP at the fitting cross. All Varilux designs use
a 4mm spacing between the fitting cross and the PRP, for other
progressives, consult your manufacturer-supplied layout chart.
The PRP will be positioned above the datum line whenever the
fitting height is greater than 1/2 the “B” measurement of the frame
(e.g., if the “B” measurement is 35 and the fitting height is 22, the
PRP will fall 0.5mm above the datum line).
To demonstrate how prism thinning can be applied to one spectacle order- but not the next, consider the two orders above on a
system that has been set to only apply prism thinning to lenses in
which the vertical meridian has plus power and a PRP which falls
above the datum line.
In most cases, the change in prism thinning will go unnoticed by
the patient. However, a certain percentage will notice a difference
(understandably, it may be hard for them to describe the difference—
leading to various comments and complaints which may be vague).
Prism thinning rarely affects the wearer’s visual experience with a
PAL. However, changes in the amount of prism thinning may have
undesirable side effects on
PAL performance for a small
percentage of patients. If a
previously successful PAL
wearer fails to adapt to new
eyewear—and all other parameters (fit, Rx accuracy, etc.)
have been eliminated, consider the following steps:
- Measure the amount of prism thinning present in the patient’s
previous PAL spectacles, and
- Specify the same amount of
prism thinning to be applied
in new PAL orders.