3. opthalmic prisms

what happens to the speed of light as it moves through a surface with parallel sides

slows down

which way does the light bend through an unparallel surface

towards the bottom

define the angle of deviation

the angle the emerging ray is deviated from its original course

prismatic effect

when light is refracted through eyewear, causing images to appear differently than they are in real life

prentices rule

p= c x f

p = amount of prism correction

c= decentration ( distance between the pupil centre and lens optical centre cm)

F= lens power (in diopters)

Base in

The thickest edge of the prism is on the inner edge of the lens ( plus power lens)

Base out

The thickest edge of the prism is on the outer edge of the lens ( minus power lens)

vertical imbalance

Define polychromatic light

Polychromatic light refers to light that contains multiple wavelengths or colors

The higher the refractive index of a lens material the _________

lower the abbe number ( V value)

Define a prism

a three-dimensional shape, triangular in cross-section, that, when made from a transparent refracting material will deviate monochromatic light without focusing it

what is an angle of incident

The angle between the incoming ray and the normal to the surface

what is the angle of reflection

The angle between the reflected ray and the normal. According to the law of reflection, the angle of incidence is equal to the angle of reflection

how do you calculate prism diopters (power)

deviation in cm / distance in m

what is a compounding prism

Prisms that are split between the two eyes to help reduce weight and to improve cosmetic appearance of the lenses

vertical split =

up and down split

horizontal split

in & in and out & out

what are the two types of prisms

1. compounding

2. canceling

vertical direction : RE- down and LE - down

compounding

vertical direction : RE- and up LE - down

canceling

Horizontal direction same direction

cancelling

Horizontal direction opposite direction

compounding

what are some solutions for vertical imbalance

• The patient could be asked to turn their head to ensure they view through the optical centers.

• This may work with single vision lenses but won’t work for patients with multifocal lenses such as bifocals or progressive lenses.

• Two pairs of glasses may be prescribed – one for reading and one for distance.

• This option is not always appreciated by your patients.

• Other options include ◦ Split bifocals where the lenses have 2 separate elements bonded together.

• Bi-prism lenses (where prism is removed or added) by surfacing the lens.

which condition may require intentional decentration

esophoria

what is a fresnels prism

Made from a thin transparent plastic sheet consisting of multiple angular grooves on one side

used to treat conditions such as squints, amblyopia, nerve palsies, and nystagmus

what are yoked prisms

prisms prescribed with their bases in the same direction. ◦ E.g. base out in the left eye and base in for the right eye

what do yoked prisms help treat

• Motion sickness/tight neck muscles.

• Strabismus (eye deviation). Vestibular disorders/nystagmus (a rapid involuntary moving of the eyeballs back and forth).

• Ptosis (a drooping of the upper eyelid).

• Vision related learning disabilities.

• Adaptation of progressive lenses.

• Head trauma patient