Prisms Flashcards
What is prisms?
Transparent block of material that deviate light away from the apex towards the base. It helps with the ocular muscles imbalance.
Why do we use prisms?
To allow binocular fusion when the eye has diplopia (double vision) or the patients eye or both has strabismus. This is a result of the eyes muscles has a defect which cause imbalance rotation.
What is the power of prisms?
Prisms has no power as it is only deviate light, it doesn’t refract it.
What is the base of the prisms?
It is the thickest part of the prism and it is what it is used to tell the prism direction.
what is the apex?
is the point where the surfaces meet, the thinnest part of the prism.
what is the direction of the base in terms of th apex?
the base is always opposite to the apex.
in which direction is the image displaced?
the image is displaced in the direction of the apex
how to we measure prisms?
by the direction of the base
how do we prescribe prisms?
separtely or in combination with a normal Rx. this is can be done by surfacing the lens or asking the Px to look at the certain part through the lens which contains that prism naturally (inducing by decantation)
state another way to measure prisms.
by measuring the amount of deviation they produce by using the equation d=(n-1)a [a is the angle of the apex and d is the angle of deviation]
is it possible to measure the apical angle?
no so we use the prism diopters instead (the little triangle)
1 prism diopters displace an image 1 uint when viewed at a distance of 100 units.
what we should give when prescribing prisms?
magnitude and direction of prisms
what are the directions can be used when prescriben prisms? state the 2 ways of notaions
up, down, in and out
in nasty and out temporally. or we can use 360 notation.
how to express prisms in one prism direction if they are horizontal and vertical forms in the same direction base?
if the prism base directions are the same or parallel in one lens then they can be added together.
how to express prisms in one prism direction if they are horizontal and vertical forms in different direction base?
if the base directions are not parallel it is necessary to find the single resultant prismatic power and oblique base direction
what is compounding prisms rules?
if prisms is given in horizontal and vertical form into a resultant prism 360 degree notation.
find the compound prism for the following eye:
L 4 base in 3 base up
R^2 = PV^2 + PH^2 R= 5 prism diopters
angle of prism:
θ=tan^-1 (opp/adj) = 36.87
direction 360 notation:
= 180 - 36.87= 143.13 degrees
resulting prism= 5 base 143
rules to find the resultant directions of prisms in 360 notation.
if the graph between 0-90 –> keep θ
between 90-180 –> 180 - θ
between 180-270 –> 180+θ
between 270 -360 –> 360 - θ
how we resolve prisms and why?
resolve prisms to have it in the form go for and ver direction from an oblique resultant prism.
we do:
PV: sinθ= opp/H
PH: cosθ= adj/H
what is Centration ?
is the positioning of the optical centre of the lens directly in front of the patients pupil. the optical centre must match the patients PD.
what is the Prentice’s rule?
the relationship between the power of the lens , the distance of the eye from the optical centre and the prismatic effect
what is the Prentice’s rule equation and what does it count?
is P=cF and count the prismatic effect. P=prismatic effect C: distance from the optical centre from the eye in cm F=the back vertex power we ignore the sign
how to find the powers if the lens is Toric?
you use the meridians 90=vertical 180=horizontal for example: \+3.00/-1.00 x90 F vertical = +3.00 F horizontal = +2.00
Calculate the prismatic effect induced when the patient is looking 4mm below and 5mm inwards. RE +2.00 / +2.00 x 180
Fv = +4.00 P= cF FH = +2.00 Pv=0.4x4=1.60 base UP PH=0.5x2=1 base OUT
