About the history, optics, and availability of Fresnel prisms and recumbent prism

1. Prepared by
Loknath Goswami
B.Sc Optom 2nd year

2. 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

3. Prism
APEX
BASE
DEVIATION
TOWARDS
THE BASE

4. 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.

6. Recumbent prisms
 Prisms which deviate the light by 90˚
 Right angle prisms and prisms used in recumbent
spectacles

8. 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

9. History
 The spectacle was made by
Messrs Hamblin of
Wigmore Street, London
and was shown for the first
time at the Oxford
Ophthalmological
Congress in July 1935

10. Recumbent spectacles
 Recumbent spectacles are equipped with reflecting prism
that enable a person lying flat on his or her back to see
straight ahead while looking straight up
 The spectacle consists of a pair of prisms mounted in a
frame which could either be worn over the patient’s own
glasses or the refractive prescription could be incorporated
in the frame
 These can be useful for those wishing to read while being
required to remain on their backs for long periods of time
 The prisms must be perfectly aligned, or diplopia occurs,
making wear impossible

11. Recumbent spectacles

12. 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

13. Optics
a 2 = 52˚

14. Optics

15. Recumbent spectacles
Advantage Disadvantage
 Help people to read in supine
position
 It is heavy and not much
comfortable

17. 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

18. 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.

19. Fresnel lens and Fresnel prism

20. Fresnel lens and Fresnel prism

21. 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

22. 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

23. Press-on prism availability
1∆, 2∆, 3∆, 4∆, 5∆, 6∆, 7∆, 8∆, 9∆, 10∆, 12∆,
15∆, 20∆, 25∆, 30∆, 35∆, 40∆

24. Optics

25. Optics

26. 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

27.  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

28. A coin has two faces

29. 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

30. 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

31.  High-powered prisms will cause a slight decrease in visual
acuity
Visual acuity loss is due to scattering of light, caused
by the grooves. The scattering of light causes both a loss of
contrast and the presence of bothersome reflections. The
minimal acuity decrease through Fresnel prisms is less than
one line on a Snellen chart at a 90% contrast level compared
with acuity through conventional prism.
 The flexible static vinyl material that gives these lenses
the ability to be easily cut to custom shapes, unfortunately,
reduces the contrast of objects viewed through the lens

32. Chart photographed with and without a
40 diopter Fresnel press-on

33. 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.

34. 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

35.  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.

36.  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.

37.  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.

38.  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

39.  Slowing of nystagmus
 Nystagmus is a defined as regular and rhythmic to-and-fro
involuntary oscillatory movements. Such movement is
involuntary and reduces vision.
 In some cases, nystagmus may slow when the person looks to
one side or the other
 For example, if the examiner sees that movement slows when
the person looks to the right, equal amounts of prism may be
applied to both lenses. The correct base direction would be
base left. Because the eyes turn toward the apex, prism base
left will keep the head pointed straight while the eyes turn to
the right. Since the eyes are turned to the right, nystagmus
slows.

40. 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

41. 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.

43. 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.

44. How to clean Fresnel prisms
Finally, 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

45. 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

46. For letting me speak here