prisms Flashcards
how is the rotation of the eye controlled ?
rotation of the eye is brought about by the 6 extrinsic ocular muscles attached to the globe of the eye.
what does defects in the innovation of the muscle or in the muscle them selves cause ?
can bring about an imbalanced rotation in the eyes this will cause a failure in the fusion of the two images and result in diplopia for the patient and the appearance of strabismus
what is strabismus ?
eye not looking straight ahead so it might appear that the eyes are turning in or out or up or down and can affect both eyes or one eye
what do we do when a patient suffers from diplopia or strabismus ?
we must place a lens which is going to deviate the light displacing the image and allowing binocular fusion
what is property of a prism ?
. may be with or without focal power
. it deviates the light from its original path it doesn’t refract
. prisms may be incorporated into an ophthalmic lens by surfacing or induced by decentration
what is the base of the prism ?
the base is the thickest part and thats the place we use to tell people the prism direction
what do we refer to prisms when prescribing them ?
we refer to them in the direction of the base
what is the structure of prism ?
. the point where the surfaces meet is the apex , the thinnest edge of the prism
. opposite the apex is the base , the thickest edge of the prism
where is the image displaced ?
in the direction of the apex
where do we measure prism ?
by direction of the base
what happens when surfacing prism ?
when surfacing prism, tools are tilted during manufacture to produce specified amount of prismatic effect
how would you write prescription for
+2.00DS + 5 △ down plano prism
+2.00DS 5 △ DN lens
what is the function of prism ?
. deviate light from its originial path
how can we measure prisms ?
by the amount of deviation they produce
d=(n-1) a
a=angle of apex
d= angle of deviation
n= refractive index of prism material
why is it difficult to measure apical angles
due to ophthalmic prims having very small apical angles therefore we can’t calculate deviation
what do we use instead of apical angles ?
unit of the prism diopter
how is the unit of prism diopter represented ?
.1 prism diopter displaces an image 1 unit when viewed at a distance of 100 units
. 1 prism displaces an image 1cm when viewed at 100 cm
. 2 prism displaces an image 2cm when viewed at 100 cm
when prescribing what do we need to give ?
. we need to give the magnitude and direction of the prism
. we state if base direction is pointing up , down , in or out
what are the prism direction ?
base up
base down
base out - temporal towards the ears
base in - towards the nose
what is other way you can describe prism direction ?
using 360 notation up - 90 down -270 right eye nasal - 360 right eye temporal - 180 left eye nasal - 180 left eye temporal - 360
what happens if prism base directions are the same or parallel in one lens if expressed in vertical and horizontal component ?
if the prism base direction are the same or parallel in one lens they can be added together
what happens if prism or base direction are not parallel if oblique prism is expressed in vertical and horizontal component ?
then it is necessary to find the single resultant prismatic power and oblique base direction
when is compounding prisms required ?
. compounding prisms is required if prism is given in horizontal and vertical form into a resultant prism in 360 degree notation
-apply Pythagorus theorem.
what is to do if resultant prism is between 0 and 90 ?
answer would be θ
what if resultant prism is between 90 and 180 ?
180- θ
-in and up
what if resultant prism is between 180 and 270 ?
180 + θ
-down and in
what if resultant prism is between 270 and 360 ?
360 - θ
-down and out to left eye
when is resolving prism required ?
required if prism is given as a single oblique resultant prism in 360 deg notation and it is requires in horizontal and vertical component form
