8 - Prism Displacement

Question 1

Where is the image displaced? 2

Answer 1

Towards the apex
prism as being an arrow with its apex pointing in the direction of the displaced image. “The eye turns in the direction the prism points.”

Question 2

What is the Prism Diopter?

Answer 2

The amount that an image is displaced

Question 3

What is the relationship between displacement, distance, prism and light?

Answer 3

If a ray of light is displaced 1
cm at a distance of 1 m from
the prism, the image of an
object in front of the prism will
be correspondingly displaced 1
cm for each meter the object is
from the prism.

Question 4

What is SOH?

Answer 4

SIN A = Opposite/Hypotenuse

Question 5

What is CAH?

Answer 5

COS A = Adjacent/Hypotenuse

Question 6

What is TOA?

Answer 6

TAN A = Opposite/Adjacent

Question 7

Review page 8+9 of powerpoint.

Question 8

What are the 2 methods to specifying prism base directions?

Answer 8

Prescriber’s Method
360-Degree Laboratory Reference System

Question 9

During vertical prism how do you know if it is Base Up or Base Down?

Answer 9

‘right side up’ base downward and the apex pointing upward = base-down prism.
If it is “upside down,” the prism = base up

Question 10

During horizontal prism how do know if it is Base In or Base Out?

Answer 10

Prism oriented with its base toward the nose is said to be base in
Prism turned with its base away from the nose is referred to as being base out

Question 11

In the Prescriber’s Method of measuring Base prism was is used as a reference?

Answer 11

The patients face is used as a reference for the prism direction

Question 12

In the 360 lab reference system method of measuring Base prism was is used as a reference?

Answer 12

360 degree axis to avoid errors.

Question 13

In the 360 lab method what degree would the base be facing in all 4 directions?

Answer 13

Base facing right/left = 0 degrees
Base facing left
/right = 180 degrees
Base facing up = 90 degrees
Base facing down = 270 degrees

Question 14

How is right and left eye measured differently in the 360 degree method (degrees)?

Answer 14

Right Eye = Base Out = 180 degrees
Right Eye = Base In = 0 degrees
Left Eye = Base Out = 0 degrees
Left Eye = Base In = 180 degrees

Question 15

What is compounding prism?

Answer 15

When two prisms are combined
in power and base orientation
to form one prism that is the
equivalent of both. (adding)

Question 16

What is cancelling prism? 2

Answer 16

cancels out any optical effects of prism
for a patient (subtracting)
often used in progressive as the lower portion will have prism.

Question 17

What is image displacement?

Answer 17

The light ray comes from a specific object, the object itself appears to be displaced. There is no actual displacement, what the eye sees is a displaced image of that object.

Question 18

What are the conditions for compounding vertical prism?

Answer 18

If the prisms are opposite direction then you compound (+)

Question 19

What are the conditions for cancelling vertical prism?

Answer 19

If the prisms are in the same direction then you cancel (-)

Question 20

What are the conditions for compounding horizontal prism?

Answer 20

If the prisms are in the same direction then you compound (+)

Question 21

What are the conditions for cancelling horizontal prism?

Answer 21

If the prisms are in opposite direction then you cancel (-)

Question 22

What is the Compounding table for Prism?

Answer 22

Right Eye Left Eye
Base out - Base out
Base In - Base in
Base Up - Base Down
Base Down - Base Up

Question 23

What is the Cancelling table for Prism?

Answer 23

Right Eye Left Eye
Base out - Base in
Base In - Base Out
Base Up - Base Up
Base Down - Base Down

Question 24

Practice splitting prism (compounding/ cancelling)