Recumbent prisms and fresnel prisms
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Transcript of Recumbent prisms and fresnel prisms
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
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