Prism Thinning and Progressive Addition Lenses

By Pete Hanlin

Approved for Ohio Credit by the Ohio Optical Dispensers Board

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 lenses).

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 following parameters:

1. Is the total power of the lens in the vertical meridian plus or minus in power?
2. 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.
3. 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.

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).

CONCLUSIONS

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:

1. Measure the amount of prism thinning present in the patient’s previous PAL spectacles, and
2. Specify the same amount of prism thinning to be applied in new PAL orders.