Reason for needing Prisms
- rotation of the eye is brought about by the 6 EOM attached to the globe of the eye
- defects will bring about imabalanced rotation bet the eyes
- if innervation of EOM is disrupted then you’ll have a binocular vision problems
- pathology is also a cause eg diabetes
What happens to the image in the eye?
Image in either eye doesnt fall in the same place on the retina, hence you get double vision, eye strain and headaches
- causes a failure in the fusion of the two images and result in diplopia for the patient and the appearance of strabismus
- we need a lens which is going to deviate the light, displacing the image and allowing binocular vision
Prism
- point where the surfaces meet is the apex
- apex will be the thinnest edge of the prism
- opp the apex is the base
- the base will be the thickest edge of the prism
- image is displaced on the direction of the apex
Prism Lens
- May be with/without focal power
- deviates the light from its original path
- prism may be incorporated into an opthalmic lens by surfacing or induced by decentration
- when surfacing prism, tools are tilted during manufacture to produce a specified amount of prismatic effect
Prism Units
- 1 prism dioptre displaces and image 1 unit when viewed at a distance of 100 units
eg 1 prism dioptre displaces an image 1cm when viewed at 100cm
Compounding and Resolving Prisms
- if the prism base directions are the same or parallel in one lens then they can be added together
- if the base directions are not parallel it is necessary to find the single resultant prismatic power and oblique base direction
- this would be the case if the oblique prism is expressed in vertical and horizontal components
- compounding prism is required if prism is given in horizontal and vertical from into a resultant prism in 360 degree rotation
Resolving Prism
required if prism is given as a single oblique resultant prism in 360 degree notation and it is required in horizontal and vertical component form
Centration
Positionin of the optical centre of the lens directly in front of the patients pupil
- want the px to look through the oc
Optical Centre
- where vision is best, where rx is most accurate and where px receives the least amount of distortion or aberrations
- also where theres no prismatic effect
- if no prism is prescribed, you want oc to be in front of the pupil
Incorrect Centration and Prism
- if the eye isnt positioned at the oc then prism will be experienced - eye has been moved
Prentice’s rule
P=cF
c= decentration (cm) and f= power
- relationship bet the power of the lens, the distance of the eye from the oc and the prismatic effect is derived from this rule
Prism by Decentration
- for positive lenses the direction of the oc movement to induce prism is the same as the direction the prism 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
Unprescribed Prism
- if px isnt looking through the oc then they will experience unwanted prism
- if prism encountered by each eye is diff then the patient will experience differential prismatic effect - more problematic
Anisometropia
A diff in prescription between the two eyes of more than 1.50D
Splitting Prisms
- as prism is described to allow the two eyes to work together, the prism can be shared bet the 2 eyes to balance these effects - results in thinner lenses
- as long as the resulting difference bet the 2 eyes is still the amount of prism that is prescribed, the correct prism will be induced and the effect on the image displacement will still be the same
Vertical Prisms
can add opp bases together
- up and down are additive
Horizontal Prisms
- in and in are additive
- out and out are additive
