prisms and centration Flashcards
What is centration?
is the positioning of the optical centre of the lens directly in front of the patients pupil.
- The optical centre should match where the patients PD.
- Want the patient to look through the optical centre.
What are plus and minus lenses thought of?
-As prisms with their bases either pointing towards each other or against one another.
What happens when light passes through the optical centre? of the lens?
So if eye is positioned at optical centre what does this mean?
it has no prismatic effect
No prism is experienced -
What happens if patient eye is looking away from the optical lens?
they will experience some prism
And which prism or how much they experience will depend on where they are looking.
What is the thickest point of a minus lens?
- when patient looking at lower part
- give base down prism
- thickest point of lens facing downwards
What is the thickest point of a positive lens?
-When patient is looking at lower part
-But it gives base up prism
thickest point of the lens is facing upwards
What is the prentice rule?
the relationship between the power of the lens, the distance of the eye away from the optical centre and prismatic effect it gives.
What is the prentice rule equation?
P=cF
P= prismatic effect
c= distance from optical centre of the eye (cm)
F= back vertex power of lens in Dioptres
We ignore + and -
This equations shows the importance of correct centration
What do you need to do when looking at the amount of prism the patient experiences?
-look at magnitude (amount of prism created ) and base direction.- draws lens out to find base direction.
What happens if the lens is toric?
we need to know the power in the correct meridian
+3.00/-1.00x90
If find power of lens vertically F= vertical = +3.00 - Sph
F horizontal = +2.00- cyl
How can we create prism by decantation?
by moving the lens so the patient looks through the correct amount of prism
-Dont want the patient to look through the optical centre.
How do we find the calculation of prism created?
rearrange prentice rule
c= P/F
What is the rule for positive lenses and negative ?
- For POSITIVE lenses the direction of the OC movement to induce prism is the SAME as the direction the prism base is required
- For NEGATIVE lenses the direction of the OC movement to induce prism is the OPPOSITE to the direction of the prism base required.
for example if you have a. negative lens and need base up.
-need to move lens down.so patient looks through top half of lens
What is unwanted prism?
- when moving eyes away from optical centre.
- If the patient is not looking through the optical centre, then they will experience unwanted prism.
- important PD measurement is done properly
What is differential prismatic. effect?
If the prism encountered by each eye is different then the patient will experience
-The F in our equation is difference in prescription in Each eye
