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.
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?
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.
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?
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.
Suppose a person is wearing or needs a prescription as follows:
R: −3.50 sphere
L: −3.50 sphere with 6Δ base-in prism
(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?
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
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.
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
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.
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.
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.