W9 Prisms

What is a prism?

2 flat polished refracting surfaces inclined at an angle relative to each other

What are examples of prism forms?

Flat Plano prism, meniscus Plano prism- Plano means neither type has dioptric power

How do light rays move in a prism?

They deviate towards the prism base

What is the image placement in a prism?

The image is displaced towards the prism apex (pointy bit)

How does single binocular vision occur?

When the image forms on the fovea of both eyes

What essentially happens if you put a prism in front of the eye?

A prism can cause double vision (diplopia) when it displaces the image in one eye away from the fovea

How can single binocular vision be restored after diplopia?

If the eye behind the prism rotates towards the prism apex so the image is once again, on the fovea

How can we assess the flexibility of someone’s eyes?

Via PRISMS- can assess how far the eyes can rotate to maintain single binocular vision- increase prism to see if eye compensates and maintains single binocular vision (fusional reserves)

How can we use prism as treatment?

Treatment of heterophobia- extraocular muscle imbalances

Prisms reduce the amount of eye rotation needed to maintain comfortable single binocular vision

How can prisms be used to treat squints?

Strabismus can be treated by using prisms to move the image to the fovea of a deviated eye (only if it’s sudden/onset)

What are varifocals?

Progressive power lenses (prism thinning)- positive charge increases to equalise the thickness of the lens for cosmetic purposes

How can prisms be used to combat large differences in eye power (anisometropia)?

Prism is required to compensate for a differential prismatic effect when looking away from the optical centre

What happens if you increase the apical angle of a prism?

Angle of deviation increases, prismatic power increases and base thickness

What is the difference between base in and base out?

Base in means towards the nose, base out means towards the temporal side of the px

How do you calculate differential prism?

Same base horizontally is an additive (in/out), same base vertically you subtract (up/down), different base vertically you add to highest base (up/down) and different base horizontally you subtract (in/out)