Prentice's Rule and Total Power in any Meridian

Question 1

What is the formula for Prentice’s rule?

Answer 1

P=hD, where P = prismatic effect, h = distance from optical center in centimeters, and D = power in meridian of prism base

Question 2

What is the simple number sequence to find total power in any meridian, and how can you use it to find the percentage of the cylinder power applied in that meridian?

Answer 2

The number sequence is 0124567889988765421. Each number corresponds to how many degrees from axis, in increments of 5. To find the percentage of cyl power, add the next number in the sequence to the previous one.

Question 3

What is the completed Percentage Cylinder Power Chart, using the simple number sequence?

Answer 3

The left column represents degrees from axis. The middle column is the number sequence. The right column is the percentage of cylinder power in this meridian.
0 0 0
5 1 1
10 2 3
15 4 7
20 5 12
25 6 18
30 7 25
35 8 33
40 8 41
45 9 50
50 9 59
55 8 67
60 8 75
65 7 82
70 6 88
75 5 93
80 4 97
85 2 99
90 1 100

Question 4

What is the total power in the 90th meridian and the 180th meridian?
+3.50 -2.00 x045

Answer 4

90th: +2.50
180th: +2.50

Question 5

What is the total power in the 90th meridian and the 180th meridian?
+0.50 -2.00 x030

Answer 5

90th: -1.00
180th: plano

Question 6

What is the total power in the 90th meridian and the 180th meridian?
-6.00 -3.00 x150

Answer 6

90th: -8.25
180th: -6.75

Question 7

+4.50 +0.50 x180
+2.00 +0.25 x090
ADD +1.75
What is the vertical imbalance at a 10mm reading level?

Answer 7

Because ADD power is OU, that value may be eliminated from the equation.
TP = Total Power
TP in OD at 90th meridian = +4.50 + 0.50 = +5.00
TP in OS at 90th meridian = +2.00
Difference between total powers is +3.00 D
Using Prentice’s Rule: P = hd = 1cm x +3.00 = 3.00 D of vertical imbalance

Question 8

-4.50 -1.50 x180
-1.00 -1.00 x030
ADD +1.75
What is the vertical imbalance at a 10mm reading level?

Answer 8

Because ADD power is OU, that value may be eliminated from the equation.
TP = Total Power
TP in OD at 90th meridian = -4.50 + -1.50 = -6.00
TP in OS at 90th meridian: cyl is 60 degrees from 90 so use 75% of cyl power, so: TP = -1.00 + -0.75 = -1.75
Difference between total powers is -4.25 D
Using Prentice’s Rule: P = hd = 1cm x -4.25 = -4.25 D of vertical imbalance

Question 9

-2.50 -0.50 x180
-4.50
PD = 67 (33.5 monocularly)
During final inspection, it was determined that the optical centers of the lenses were placed 63mm (31.5 OU monocularly). How much prism will this produce, and in which direction in the base for each eye?

Answer 9

The axis is at 180 and it’s a horizontal measurement, so we don’t need to consider cyl.
Using Prentice’s rule:
ODP = hd = 0.2cm x -2.50 = 0.5. It is base OUT because the optical center of this lens is narrower than the patient’s PD, and it is a minus lens.
OSP = hd = 0.2 x -4.50 = 0.9. It is base OUT because the optical center of this lens is narrower than the patient’s PD, and it is a minus lens.

Question 10

If the Rx +3.00 +0.50 x090 is decentered 5mm out too much, how much prism will be created, and in which direction is the base?

Answer 10

100% of the cylinder power will be applied, since it’s a horizontal measurement.
Using Prentice’s rule
P = hd = 0.5 x +3.50 = 1.75. It is base OUT because the optical center of this lens is wider than the patient’s PD, and it is a plus lens.