L6 Flashcards
(29 cards)
What is the typical apical angle of ophthalmic prisms?
Less than 10 degrees.
How is the position of a prism indicated?
By the direction of its base.
In a trial lens, what does the marking indicate?
The base of the prism.
In a prism, where is the image of an object displaced?
Towards the apex.
What are some clinical uses of prisms in optometric practice?
Management of heterophoria, heterotropia (strabismus, squint), nystagmus control, head posture problems, and to improve the cosmetic appearance of progressive power lenses (prism thinning).
What are some prismatic effects that should be avoided in practice?
Prismatic effects due to viewing off-centre, poor centration, bifocal construction, differential prismatic effects in anisometropia, and colour dispersion.
What happens when a patient with strabismus is not looking directly at an object?
They see double (diplopia) because both eyes are not aligned on the object.
How can a prism help a patient with strabismus?
A prism can divert light so that the image appears aligned, allowing both foveae to fixate on the same object.
What is a thin prism in ophthalmic practice?
A prism with a small apical angle (less than 10 degrees), generally considered to have nearly normal incidence of light and a fixed angle of deviation.
What is the effect of combining thin prisms with the same base direction?
The effects are additive (summative).
What is the effect of combining thin prisms with opposite base directions?
The effects are subtractive.
What is a differential prism?
The net prismatic power when prisms are placed in front of both eyes and the viewing is binocular.
What is the effect of combining horizontal prisms (Base IN + Base IN or Base OUT + Base OUT)?
They have a summative effect.
What is the effect of combining horizontal prisms with opposite bases (Base IN + Base OUT)?
They have a subtractive effect (may cancel out).
What is the effect of combining vertical prisms with the same base direction (Base UP + Base UP or Base DOWN + Base DOWN)?
They have a subtractive net effect.
What is the effect of combining vertical prisms with opposite bases (Base UP + Base DOWN)?
They have a summative effect.
How do positive spectacle lenses act like prisms?
A positive lens is like two prisms touching at the base.
How do negative spectacle lenses act like prisms?
A negative lens is like two prisms touching at the apex.
In which direction is light deviated by a prism?
Towards the base.
What is centration in spectacle lenses?
Ensuring that the eyes are viewing through the optical centre of the lens, in the absence of prescribed prism.
What is the centration distance?
The distance between the two viewing points (usually corresponding to the patient’s pupillary distance for distance corrections).
What is hand neutralisation?
A method to assess the power of a lens by observing prismatic movement when the lens is moved and neutralizing it with a lens of equal but opposite power.
What is the prismatic effect when looking through the periphery of a lens?
Light is deviated towards the centre (base), and the image is displaced towards the apex.
What happens when a positive lens is moved down during hand neutralisation?
The image is displaced upwards (against movement).
