Prepared by

Loknath Goswami

B.Sc Optom 2nd year

Prism A prism is an object made up of a transparent material like

glass or plastic that has at least two flat surfaces that form

an acute angle (less than 90 degrees)

A traditional prism has two flat, nonparallel surfaces

Parallel light entering the prism is bent towards the base

of the prism and leaves the back surface at an angle

A prism is thicker at the base than at the apex. The larger

the prism, the thicker the base of the prism will be

PrismAPEX

BASE

DEVIATION TOWARDS THE BASE

Properties of prism Dispersion

Deviation occurs towards the base

Total internal reflection occurs in some specific prisms.

Prism never change the vergence of light rays. A prism

does not magnify or minify the image.

Recumbent prisms Prisms which deviate the light by 90˚

Right angle prisms and prisms used in recumbent

spectacles

History In 1935, Mr. Andrew McKie

Reid designed the ‘recumbent

spectacles’, as a visual aid for

patients suffering from spinal

lesions and other diseases who

were confined to the bed in a

supine position for weeks or

months

History The spectacle was made by

Messrs Hamblin ofWigmore Street, Londonand was shown for the firsttime at the OxfordOphthalmologicalCongress in July 1935

Recumbent spectacles Recumbent spectacles are equipped with reflecting prism

that enable a person lying flat on his or her back to seestraight ahead while looking straight up

The spectacle consists of a pair of prisms mounted in aframe which could either be worn over the patient’s ownglasses or the refractive prescription could be incorporatedin the frame

These can be useful for those wishing to read while beingrequired to remain on their backs for long periods of time

The prisms must be perfectly aligned, or diplopia occurs,making wear impossible

Recumbent spectacles

Principle The prisms had an apical angle of 35 degrees and basal

angles of 70 and 75 degrees

These angles were calculated such that a book held in the

normal position on the chest could be read by a person in

the supine position even though apparently looking at the

ceiling

So, with the base of the prism facing the patient, the line

of vision through refraction and double internal reflection

within the prisms was bent almost 90 degrees

Optics

a 2 = 52˚

Optics

Recumbent spectaclesAdvantage Disadvantage

Help people to read in supine

position

It is heavy and not much comfortable

History In 1970, the optical sciences group of San Rafael,

California, developed a series of Fresnel Press-on prisms –

thin plastic membrane prisms ranging in powers from 0.5

to 30 prism diopters

History These prisms are an

adaptation of the hand-

ground lenses designed by

the French engineer and

physicist Augustine Fresnel

in 1821

Fresnel’s lenses were

originally used for

lighthouse beacons.

Fresnel lens and Fresnel prism

Fresnel lens and Fresnel prism

Fresnel prism A Fresnel prism attempts to avoid thickness by building a

tower of small prism

A Fresnel prism is only 1 mm thick

The refraction depends on the apical angle

The angle between the two surfaces remains constant

across the prism.

E.g. 3M Press-on

3M Press-On Manufacturer: 3M

Location: USA

Material: PVC (Polyvinyl chloride)

Thickness: 1mm

Usable area: 65mm diameter

Base is noted in one corner of the prism, if it is cut then

base may be found by running finger nail along prism side

Press-on prism availability1∆, 2∆, 3∆, 4∆, 5∆, 6∆, 7∆, 8∆, 9∆, 10∆, 12∆,

15∆, 20∆, 25∆, 30∆, 35∆, 40∆

Optics

Optics

Fresnel prism trial sets Fresnel prisms trial optics are similar to traditional prism

trial optics(ring) found in trial ring sets.

Thin and light weight (particularly in the higher diopter

powers)

Raised edges around the Fresnel prism trial optic prevents

scratching of prism features when placed on flat surfaces

Allows for accurate testing of large oculomotor

imbalances and allows patient opportunity to experience a

Fresnel prism

Recommend for prism cover test

Available in two, 7 ring sets: TS4000 & TS5000

TS4000: 2∆, 3∆, 4∆, 5∆, 6∆, 8∆, 10∆

TS5000: 12∆, 15∆, 20∆, 25∆, 30∆, 35∆, 40∆

TS4000 TS5000

A coin has two faces

Advantages It is very thin and extremely lightweight

It is flexible and can be applied to an existing spectacle

lens

It can be cut to any shape with scissors or a razor blade.

This means that it can be cut and applied to one sector of a

lens

It can be applied or removed at anytime

Disadvantages Fresnel prisms look different

than conventional lenses. It

gives striated appearance (to

observers) of the grooves. The

striated appearance can be

improved to some extent by

use of a lightly tinted or

photochromic lens

Fresnel prisms have a number

of small ledges, they are

harder to clean than

conventional lenses

High-powered prisms will cause a slight decrease in visualacuity

Visual acuity loss is due to scattering of light, causedby the grooves. The scattering of light causes both a loss ofcontrast and the presence of bothersome reflections. Theminimal acuity decrease through Fresnel prisms is less thanone line on a Snellen chart at a 90% contrast level comparedwith acuity through conventional prism.

The flexible static vinyl material that gives these lensesthe ability to be easily cut to custom shapes, unfortunately,reduces the contrast of objects viewed through the lens

Chart photographed with and without a

40 diopter Fresnel press-on

When to prescibe When a prismatic component is to be included in a

patient’s prescription, the advantages and disadvantages of

press-on prisms should be carefully weighed to determine

the suitability of using press-on prism.

When are Fresnel prism used? High amount of prism

Fresnel prism is very

thin comparatively to the

conventional prism. So it is

useful for high amount of

prism Conventional prism

Fresnel prism

Use and reuse

Fresnel prism lenses are

easy to apply and remove. They

may be used and reused. This is

helpful when determining how a

given prism amount will work

long term or for use during

visual training.

Sectorial application

A partially paralyzed

extraocular muscle may result

in a different amount of prism

needed for different directions

of gaze. A Fresnel prism can be

cut to fit that particular lens

area. Prism is present only

where it is needed.

Visual field defects

A person may have a

visual field defect

For example, the case of

bilateral hemianopia, the

right half of the visual

field is blind for both right

and left eyes.

Fresnel prism can be applied to the right side of the both

lenses.

In this instance, prism base direction would be base right.

With prism in place, the wearer looks to the right but does

not have to turn the eyes as far to see an object in the right-

hand field of view

Slowing of nystagmus

Nystagmus is a defined as regular and rhythmic to-and-froinvoluntary oscillatory movements. Such movement isinvoluntary and reduces vision.

In some cases, nystagmus may slow when the person looks toone side or the other

For example, if the examiner sees that movement slows whenthe person looks to the right, equal amounts of prism may beapplied to both lenses. The correct base direction would bebase left. Because the eyes turn toward the apex, prism baseleft will keep the head pointed straight while the eyes turn tothe right. Since the eyes are turned to the right, nystagmusslows.

How to apply a Fresnel prism to a spectacle lens Firstly we have to determine correct base direction

After that we have to take the carrier lens out of the

spectacle frame

We have to place the Fresnel prism on the back of the

carrier lens with its smooth side against the carrier.

Making sure base direction is properly oriented and then

we have to mark the boundary

With a razor blade, trim the Fresnel prism along the

marking

How to apply a Fresnel prism to a spectacle lens Reinsert the carrier lens into the frame

Wash both carrier and Fresnel prism with a weak solution

of lotion-free liquid detergent

In a bowl of warm water, or under a stream of warm

water, apply the smooth side of the Fresnel to the carrier.

Work out any air bubbles that may be trapped between the

two surfaces

Then we can give the lenses to the wearer, but have to

instruct the wearer to handle the lenses with care for 24

hours until drying is complete.

How to clean Fresnel prisms The manufacturer’s recommended method of cleaning

these lenses is to rinse under warm running water. If the

lenses have dirt in grooves, use a soft brush. Blot dry with

a soft, lint-free cloth. Hard contact lens cleaning solutions

have also been used to clean Fresnel optics.

How to clean Fresnel prismsFinally, the right tool to clean your

Fresnel Optics. The secret is the

cloth, a lint-free micro fiber cloth

that can reach into the small deep

grooves of a Press-On Lens.

Use with the alcohol free lens

cleaner to remove finger prints,

oils, make-up and the daily dirt and

grime that can get onto your lens.

Costs $5.00

REFERENCES Troy E. Fannin, Clinical optics 1st edition, Pg No. 106-107

System For Ophthalmic Dispensing-2nd edition, Clifford

W. Brooks , Irvin M. Borish, Pg No. 516-519, 536

http://www.sciencephoto.com/media/117959/view

http://study.com/academy/lesson/what-is-a-prism-

definition-refraction-quiz.html

http://www.hemianopsia.net/the-fresnel-prism

http://www.gcsescience.com/pwav32.htm

http://bjo.bmj.com/content/94/5/535.full

For letting me speak here