8 - Prism Displacement Flashcards
Where is the image displaced? 2
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.”
What is the Prism Diopter?
The amount that an image is displaced
What is the relationship between displacement, distance, prism and light?
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.
What is SOH?
SIN A = Opposite/Hypotenuse
What is CAH?
COS A = Adjacent/Hypotenuse
What is TOA?
TAN A = Opposite/Adjacent
Review page 8+9 of powerpoint.
What are the 2 methods to specifying prism base directions?
Prescriber’s Method
360-Degree Laboratory Reference System
During vertical prism how do you know if it is Base Up or Base Down?
‘right side up’ base downward and the apex pointing upward = base-down prism.
If it is “upside down,” the prism = base up
During horizontal prism how do know if it is Base In or Base Out?
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
In the Prescriber’s Method of measuring Base prism was is used as a reference?
The patients face is used as a reference for the prism direction
In the 360 lab reference system method of measuring Base prism was is used as a reference?
360 degree axis to avoid errors.
In the 360 lab method what degree would the base be facing in all 4 directions?
Base facing right/left = 0 degrees
Base facing left
/right = 180 degrees
Base facing up = 90 degrees
Base facing down = 270 degrees
How is right and left eye measured differently in the 360 degree method (degrees)?
Right Eye = Base Out = 180 degrees
Right Eye = Base In = 0 degrees
Left Eye = Base Out = 0 degrees
Left Eye = Base In = 180 degrees
What is compounding prism?
When two prisms are combined
in power and base orientation
to form one prism that is the
equivalent of both. (adding)
What is cancelling prism? 2
cancels out any optical effects of prism
for a patient (subtracting)
often used in progressive as the lower portion will have prism.
What is image displacement?
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.
What are the conditions for compounding vertical prism?
If the prisms are opposite direction then you compound (+)
What are the conditions for cancelling vertical prism?
If the prisms are in the same direction then you cancel (-)
What are the conditions for compounding horizontal prism?
If the prisms are in the same direction then you compound (+)
What are the conditions for cancelling horizontal prism?
If the prisms are in opposite direction then you cancel (-)
What is the Compounding table for Prism?
Right Eye Left Eye
Base out - Base out
Base In - Base in
Base Up - Base Down
Base Down - Base Up
What is the Cancelling table for Prism?
Right Eye Left Eye
Base out - Base in
Base In - Base Out
Base Up - Base Up
Base Down - Base Down
Practice splitting prism (compounding/ cancelling)
