Prism

Question 1

If there is no prism then how do you find the optical center?

Answer 1

The MRP and optical center are at the same point

Question 2

What does MRP stand for?

Answer 2

• Major Reference Point
• Where prism is equal to the amount prescribed

Question 3

What happens to the vision when an excessive base-in or base-out prism exists?

Answer 3

• Causes horizontal objects to appear tilted
• The side that appears too high will be aligned towards the apex

Question 4

When does base-up prism occur in plus lenses and minus lenses?

Answer 4

• In plus lenses when optical centers are higher than the pupil
• In minus lenses when optical centers are lower than the pupil.

Question 5

When does base-down prism occur in plus lenses and minus lenses?

Answer 5

• In plus lenses when optical centers are below the pupil
• In minus lenses when optical centers are above the pupil

Question 6

What does an excessive base-down prism look like through the lens?

Answer 6

Base-down makes it seem like the floor is concave or walking uphill

Question 7

What does an excessive base-up prism look like through the lens?

Answer 7

Base up makes it seem like the floor is convex or walking downhill.

Question 8

What is the formula for Prentice’s Rule?

Answer 8

• /\ = c x F
• /\ = prism diopters
• c= amount of decentration in cm
• F = dioptric power in the meridian being determined

Question 9

Solve this problem using the prentice’s Rule.
Rx: -3.00D OU
Patient’s PD: 60mm
Optical centers: 66 mm

Answer 9

Answer: Decentration 6mm/2=3mm
/= 0.3cm x 3.00D
0,9 /=0.3cm x 3.00D
0.9 /\ base in OU
1.8/\ total BI

Question 10

What are the ANSI Standards?

Answer 10

The total horizontal prism should not exceed 0.67 prism diopters total
OR
2.5mm of difference between PD and optical centers if the Rx IS +/- 2.75D or higher

Question 11

What is the overall effect when Prism are canceling?

Answer 11

• Base in and base out
• Base up and base up
• Base down and base down

Question 12

Is this example canceling or Compounding?
OD 2D base in OS 4D base out = 2D base Out OS

Answer 12

Canceling

Question 13

What is the overall effect when prism is compounding?

Answer 13

• Base in and base in
• Base out and base out
• Base up and base down

Question 14

Is this example canceling or compounded?
OD 2D base out OS 4D base out = 6D base out total

Answer 14

compounding

Question 15

When the optical centers of the plus lens are wider than the patient’s PD, which type of prism is induced?

Answer 15

base out

Question 16

When the optical centers of the minus lens are wider than the patient’s PD, Which type of Prism is induced?

Answer 16

base in

Question 17

When the optical centers of a minus lens are narrower than the patient’s PD, which type of prism is induced?

Answer 17

base out

Question 18

When the optical centers of a Plus lens are narrower than the patient’s PD, which type of prism is induced?

Answer 18

base in

Question 19

Solve this problem using the prentice’s Rule.
Rx: -1.00D OU
Patient’s PD: 60mm
Optical centers: 66 mm

Answer 19

Answer: Decentration 6mm/2=3mm
/= 0.3cm x 1.00D
/=0.3
0.3 /\ base in OU
0.3x2=0.6
0.6/\ total BI

Question 20

Calculate the amount of prism induced in the following situation?
Patient PD 60mm
Optical Centers 64mm
Lens power -2.50D OU

Answer 20

1.00D total BI

Question 21

Calculate the amount of prism induced in the following situation?
Patient PD 66mm
Optical Centers 58mm
Lens power -2.50D OU

Answer 21

2.00D total BO

Question 22

Calculate the amount of prism induced in the following situation?
Patient PD 58mm
Optical Centers 62mm
Lens power +3.00D OU

Answer 22

1.20D total BO

Question 23

what is the average degree of Pantoscopic tilt?

Answer 23

Most frames average 10 to 12 degrees of tilt

Question 24

How far should the optical center should be below the pupil?

Answer 24

5-6 millimeters