1. Light transmission through lenses is determined by calculating the percentage of light lost to reflection off the front and back surfaces and light absorbed by the lens material. 2. The human eye absorbs different wavelengths of light depending on the ocular tissue. The cornea absorbs UV light while the lens absorbs more UV as we age. 3. Sunglasses protect the eyes by reducing the amount of UV and infrared radiation that reaches the eyes, preventing damage from prolonged sun exposure like cataracts.

1. Transmission, Absorptive Lens
and Sunglass Standards
Mohammad Arman Bin Aziz
Optometry Faculty
ICO, CU
May 11, 2014

2. Absorption & Transmission
• Light may be defined as a form of electromagnetic radiant energy that is
capable of stimulating our retinal photoreceptors and causing the
sensation of vision.
• When light strikes a lens, it results -
• Reflection
• Absorption
• Transmission
• Light transmission through a lens is determined by calculating
• percentage of light lost by reflection-front surface
• percentage of light lost by absorption
• percentage of light lost by reflection-back surface

3. Reflection
•Amount of light reflected by a lens
surface
IR = ( I )
(n' – n)2
(n' + n)2

4. Absorption
•Lambert’s law of absorption
•For an absorptive material, layers of equal
thickness absorb equal quantities
(percentages) of light regardless of the
intensity of the light
•Transmittance factor (q)

5. •If light passes through a number of lenses, one
after another, the ultimate opacity is found by
multiplying the separate opacities of each of
the lenses.
OU = (O1)(O2)(O3)…
Absorption

6. • If light passes through a number of lenses, one
after another, the ultimate transmission is found
by multiplying the separate transmission of each
of the lenses.
TU = (T1)(T2)(T3)…
Absorption

7. • Additional terms used in connection with absorption are
• Opacity = reciprocal of transmission
• Density = stated for a given thickness
Absorption
O =
1
T
density = - log T

8. EM Spectrum

9. The optical spectrum
•We are regularly exposed to some UV radiation, the
visible spectrum, and the IR portion of the electro-
magnetic spectrum.
•Although exposure to radiation bordering on the
visible spectrum does not cause the sensation of
vision, these bands of radiation can exert harmful
effects on the eyes.

10. •UV radiation extends approximately from 100 to
380 nm.
•The certain bands of UV radiation are associated
with particular biological effects, the UV spectrum
is arbitrarily subdivided into three bands:
• UV-A extends from 380 to 320 nm.
• UV-B extends from 320-290 nm.
• UV-C extends from 290-200 nm.

11. The visible spectrum
• The visible spectrum, extending from approximately 380 to
760 nm.
• The range varies with the level of illumination, the clarity of
the crystalline lens of the eye, and other factors relative to
the observer.
• Within the specified boundaries, radiation reaching the
retina acts as a physical stimulus to produced electrical
impulses that are conducted via the optic nerve to the
occipital cortex of the brain, which provides the sensation of
vision.

12. The IR spectrum
•The IR spectrum extends from 760 to 106 nm.
•It is divided into three portions:
•IR-A extends from 760-1400nm.
•IR-B extends from 1400-3000nm.
•IR-C extends from 3000 nm – 1 mm.

13. Classification of radiation effects
• Draper’s law states that for radiation to have an effect on a
substance through which it travels, it must be absorbed by the
substance.
• Radiation has no effect (beneficial or deleterious) on a
substance through which is completely transmitted or by which
it is completely reflected.
• Radiation in the region of the visible spectrum causes the
sensation of vision because it is absorbed by the
phpotopigments of the retina.
• Ionizing radiation
• Non-ionizing radiation

14. Ionizing radiation
• Most ionizing radiation pass through the eye, but small
amount is absorbed.
• The damage depends on the exposure time, concentration,
and the type of radiation.
• Ionizing radiation may have direct or indirect effect on
ocular tissue.
• A direct effect may produce cellular anomalies or death.
• Indirect effect can result in damage to the blood vessels
and thus restrict the blood supply to the tissue.

15. • Ionizing radiation can affect nearly all ocular tissue. Of the
ocular tissue, the conjunctiva, cornea, and lens are the most
vulnerable.
• At low level, the conjunctiva vessels become engorged and the
cornea loses its normal luster.
• Heavier doses result in exfoliation of the epithelium cells,
cornea ulcer, and keratitis.
• The most common effect of ionizing radiation is the formation
of cataract.
• High level of ionizing radiation can result in retinal damage
and degeneration; extremely high levels can result is sudden
blindness.

16. Nonionizing radiation
•When radiation is absorbed by an ocular tissue,
various effects are produced by the transfer of
radiant energy to the molecules and atoms of the
absorbing tissue.
•The absorbed energy can affect the visual apparatus
in the following ways:
• The thermal effect
• The photochemical effect
• Photoluminescence (fluorescence)

17. Nonionizing radiation
• The thermal effect
• Heating effect
• Solar retinopathy, cause by looking directly at a solar eclipse.
• The photochemical effect
• In the visible spectrum, produces a chemical reaction in the retina
initiating the sensation of vision.
• Harmful photochemical effects can occur with other ocular tissues, such
as photokeratitis produced by excessive absorption of UV radiation by the
cornea.
• Photoluminescence (fluorescence)
• The lens is capable of visible fluorescence when illuminated by UV light.

18. •As radiant energy passes through the eye, it is
attenuated in a number of ways:
•Absorption by the ocular media
•Scattering within the eye
•Reflection by the various optical interfaces
•Loss caused by the aberrations of the eye’s optical
system
Concentration of radiant energy by the eye

19. • The concentration of radiant energy within the eye
also depends on the size of the pupil and the angular
extent of the source.
• For a point source of high intensity, refraction by the
eye’s optical system concentrates the energy of the
retina (A) and cause tissue damage, but has little
effect on the cornea and the lens.
•Solar retinopathy: occur after exposure to a solar
eclipse.

20. A small source of low-intensity radiation is usually harmless to the
retina, an extended source of the same intensity may provide a
dangerous concentration of radiant energy in the lens.
A
B
Concentration of energy in the eye. A, point source; B. extended source.

21. Absorption of radiation by the ocular tissue
• The tear layer absorbs only a small amount of radiation.
• absorbs below 290 nm and IR radiation above about 3000 nm.
• transmits radiation from approximately 290 to 3000 nm.
• The cornea absorbs UV radiation.
• absorbs below 290 nm and IR radiation above about 3000 nm.
• transmits for UV in the range 290 to 315 nm and for IR in the range of
1000 to 3000 nm.
• High transmission in the range extending from 315 to 1000 nm, which
includes the long UV wavelengths, all the visible spectrum, and the
shorter IR wavelengths.
• The transmission of the cornea particularly for the shorter wavelengths
decreases markedly with age.

22. •The aqueous humor absorbs very little radiation, with
the result that any radiation that is transmitted by the
cornea is also transmitted by the aqueous humor, and
passes to the iris and the lens.
•In the iris, the uveal pigment absorbs radiation and
converts in the heat.
• This conversion can be accompanied by a marked
contraction of the pupil, probably because of the release of
histamine.

23. •The lens, like the cornea, has variable absorption
properties, depending on age.
• The child absorbs UV radiation below about 310 nm and IR
radiation beyond 2500 nm, and thus transmits UV radiation
between 310 – 380 nm.
• Old adult absorbs almost all radiation below about 375 nm
and therefore transmits very little UV radiation.
• There is no change in the absorption of IR radiation with
increasing age.

24. •The vitreous mainly absorbs radiation below 290 nm
and above 1600 nm and therefore transmits to the
retina radiation in the range from 290 to 1600 nm.
• As the lens absorbs more UV radiation with increasing
age, the amount of UV radiation available to the vitreous
gradually decreases.

25. •The radiation received by the retina is the
radiation transmitted by vitreous.
•Although UV radiation received by the retina
decreases in amount with age, IR radiation does not
decrease in amount -94% of IR radiation of 770 nm
reaches the retina, then falls to 90% at 900 nm to a
very low level beyond 1500 nm.

26. Transmission of radiation by the ocular media
Ultraviolet (nm) Visible (nm) Infrared (nm)
Tear layer 290 -380 380-760 760-3000
Cornea 290-380 380-760 760-3000
Aqueous 290-380 380-760 760-3000
Lens (child) 310-380 380-760 760-2500
Lens (older adult) 375-380 380-760 760-2500
Vitreous 290-380 380-760 760-1600

27. 1. RADIATION CAN DAMAGE EYES
1. Short Term
• Excessive blinking, swelling or difficulty looking at strong light.
• Exposure also causes acute photokeratopathy (Sunburn of the
cornea)
2. Long Term
• cataracts (cloudiness of the lens)
• Pterygium (an overgrowth of the conjunctiva on to the cornea)
• Solar keratopathy (cloudiness of the cornea)
• Cancer of the conjunctiva
• Skin cancer of the eyelids and around the eyes

28. How sunglasses protect the eyes
• Sunglasses reduce
• The amount UV/ Infrared radiation that reaches the eyes.
• Close fitting, wrap around styles prevent UV rays from entering through the
sides and top of sunglasses.

29. 2. Illumination??? Brightness
• High levels of illumination can cause discomfort
• Comfortable level of illumination is 400 ftL (summer day under a shade
tree)
• Uncomfortable level of illumination (sunny Texas day in July--10,000
ftL)

30. 3. Glare
•Defined as one or more areas in the field of vision that
are of sufficient brightness to cause
• an unpleasant sensation
• a temporary blurring of vision
• a feeling of ocular fatigue
• Remember ? Types
• Disability glare
• Discomfort glare
• Specular reflection glare

31. Reduces the amount of transmitted light or radiant energy
Acts as a filter
May be uniform or neutral, absorbing light of all wavelengths
equally
May be selective, absorbing light of certain wavelengths more
than others
1. Absorptive Lenses

32. Major forms of absorptive lenses
1. Tinted solid glass lenses
2. Glass lenses with surface coatings
3. Tinted plastic lenses
4. Photochromatic lenses
5. Polarizing lenses

33. 1. Tinted solid glass lenses
 Introduction of metals or metallic oxides during
manufacturing by mixing it in the batch
 Spectral transmission characteristics are
controlled by the quantities of metals used
 Color imparted is of no significance other than
cosmetic

34. 34
Tinted solid glass lenses
• Elements used and the colors they produce are
 Manganese – pink
 Cobalt – blue
 Cerium – pinkish brown
 Nickel – brown
 Uranium – yellow
 Chromium – green
 Gold – red
 Silver – yellow
 Didynium – pink
 Vanadium – pale green

35. 2. Glass lenses with surface coatings
• Thin metallic oxide is deposited on the surface of
the lens
• Requires high temperatures
• AR coated lenses
• Ray diagram ? Principle???????
nC = nG

36. 3. Tinted plastic lenses
Tinted by dipping into a dye
Dye penetrates the lens surface to a uniform
depth;
 therefore, no change in density with changes in lens
thickness from lens center to edge
Over-tinting a lens can be reversed by dipping the
lens into a bleaching solution

37. 4. Photochromic Lenses (Glass)
• Developed by Corning in 1964
• Glass contains silver halide crystals ? Plastic
• Lenses darken when exposed to long-wavelength UV
radiation
• UV transforms silver halide crystals into silver and
halogen atoms
• Darkening rate is temperature dependent

38. •“In-Mass” technology
Corning SunSensors
Rodenstock
•Imbibed technology
Transitions
Photochromic Lenses Plastic

39. •In-Mass
•Molecules never “wear out”
or fatigue
•Darkens up to 50%
•Scratches do not affect
performance
•Exterior 1.5 mm of lens
activated preventing
uneven darkening

40. •Imbibed
Darkens consistently across
across the lens, regardless
of prescription
Available in wide range of
materials and designs
Photochromic Lenses Plastic

41. Photochromic Lenses Glass

42. Photochromic Lenses Plastic
Corning SunSensors

43. Photochromatic Lenses Plastic
Imbibed
In-mass

44.  Splitz (Sola)– changeable fashion tint
 Blue lenses darken to green
 Rose lenses darken to purple
 Yellow lenses darken to orange
 Solera (Invicta) – darken into a darker fashion tint outdoors
 Yellow, orange, rose, violet, blue & teal
Photochromatic Lenses Plastic Fashion Tints

45. 5. Polarizing Lenses
• Invented in 1929 by Dr. Edwin C. Land, founder of the Polaroid
Corporation.
• Originally manufactured with 2 sheets of glass laminated with adhesive
to either side of the polarized film—over time, delaminating became a
problem.
• In 1990’s, manufacturing changed to suspending the polarized film
within the lens mold and casting the lens with the film in place.

46. • Eliminates specularly reflected horizontally polarized light
• Improves visual acuity and restores the natural balance of light
intensities.
• Helpful for -
• fisherman by reducing glare from the water;
• motorists for reducing glare from the road;
• skiers for reducing glare from snow.

47. • Materials that best polarize light by reflectance are generally
nonconductors, called dielectrics: such as glass, pavement,
sand and snow.
• Light reflected from a dielectric is completely polarized at a
specific angle of incidence (Brewster’s Angle).

48. Sunlight
Light reflected off a
horizontal surface is
mostly horizontally
polarized

49. • Brewster’s Angle occurs when the angle between the
associated refracted and reflected rays at the surface is 90°.
• Brewster’s Angle
tan i = n
Here,
n = index of the medium to
which the light is incident,
i = angle of incidence

50. Specular Reflecting Glare With polarized lenses
Polaroid lenses admit only the
vertically vibrating light wavesReflected horizontally
polarized light

51. Categories of absorptive lenses
oLenses for general wear
oAbsorbs spectrum evenly and little more UV radiation ,visible light and IR
radiation than clear ophthalmic crown glass
oSelectively absorb UV radiation and transmit visible spectrum evenly
oOutdoor use
oSelectively absorb portions of the visible spectrum in non-uniform
manner
oWhose absorption characteristics vary with level and type of
illumination (photochromic)
oMiscellaneous
oOccupational use

52. Specification of transmission
• Transmission properties
• Spectral transmission curve.
• Shows percentage of transmission for all visible spectrum and
portion of UV and IR radiations.
• Depicts the amount of radiant energy transmitted as a percentage of
the radiant energy entering the lens as a function of wavelength.
• Manufacturers transmission data based on lens thickness of 2mm.

53. General wear lenses absorbing spectrum evenly
• Lightly tinted
• Absorbs little or no more UV, visible or IR radiation than OCG
• Indoor/cosmetic tints
• Cerium oxide
• Useful
• Lightly pigmented individuals
• High refractive errors
• Not in good general health
• Poorly designed artificial illumination

54. Lenses selectively absorb UV radiation but
transmit visible radiation evenly
• CR-39 lenses
• UV absorbers
• UV- 380(optical radiation group)
• Absorb all radiation below 400nm
• NoIR lenses, absorbs all radiation below 400nm
• Glass lenses
• Transmit < 5% of UV radiation below 400nm
• American optical hazelmaster ,Vision ease yellow
• Shooting and night driving lenses

55. •Spectra shield human II
•Unique glass lens
• 28 layers of dielectric material on the concave surface.
• Reflect all UV radiation below 400nm and IR >700nm.
•Corning photochromic lenses.

56. Lens that selectively absorb portion of visible spectrum
• Not common.
• Yellow, blue and bluish green.
• Absorb all illumination below 500nm
and greatly reduce scattering of light
• Blue light most scattered
• Hunting and shooting purposes.
• Blue for cosmetic.

57. Lenses for occupational use
Glass blowers lens
Filters out the yellow band of the spectrum
Clearly see what is happening to the glass than the flame
Didymium filter lens
Rose color in incandescent lightning
Aqua –fluorescent lightning
Welding lens
Same as above
Luminance transmission –shade number

58. Standards for absorptive lenses
• ANSI Z 80.3-1995 ophthalmic non prescription sunglasses and
fashion wear
• 1. General purpose:
• 8-40% of transmission in the visible range
• UVA: not greater than transmittance of visible range
• UVB: not greater than one half of the transmittance of visible spectrum
or 5%
• 2. Cosmetic use;
• More than 40% transmittance in visible spectrum
• UVA: not greater than transmittance of visible range
• UVB: not greater than one half of the transmittance of visible spectrum
or 30%

59. Standards for absorptive lenses
•Special purpose;
•Minimal transmittance for visible spectrum 3%
•UVA one half of the amount of visible spectrum
•UVB not greater than 1%

60. Sunglasses
Sunglasses or sun glasses are a form of protective eyewear
designed primarily to prevent bright sunlight and high-energy
visible light from damaging or discomforting the eyes.
In the early 20th century they were also known as sun cheaters.

61. History of Sunglasses
In prehistoric and historic time, Inuit
people wore flattened walrus ivory
"glasses," that came with narrow slits
that block harmful rays of the sun.
First Sunglass in History Which
protect the eyes from Snow.
Inuit snow goggles function by
reducing exposure to sunlight, not by
reducing its intensity.

62. Modern developments
In the early 1900s, the use of sunglasses
started to become more widespread,
especially among movie stars. It is
commonly believed that this was to avoid
recognition by fans, but an alternative
reason sometimes given is that they often
had red eyes from the powerful arc lamps
that were needed due to the extremely
slow speed film stocks used
Foster in 1929. Foster found a ready
market on the beaches of Atlantic City, New
Jersey, where he began selling sunglasses
under the name Foster Grant from a
Woolworth on the Boardwalk

63.  Visual clarity and comfort
 Sunglasses can improve visual comfort and visual clarity by protecting the eye from
glare.
 Various types of disposable sunglasses are dispensed to patients after receiving
mydriatic eye drops during eye examinations.
 The lenses of polarized sunglasses reduce glare reflected at some angles off shiny non-
metallic surfaces such as water. They are popular among fishermen because they allow
wearers to see into water when normally only glare would be seen.
 The glare is neutralized by blocking the vertical (magnetic) components of light.
Functions of Sunglasses

64. Protection of Eyes from many Factors
• Sunglasses offer protection against excessive exposure to light, including
its visible and invisible components.
• The most widespread protection is against ultraviolet radiation, which can
cause short-term and long-term ocular problems.
• High-energy visible light (HEV) has been implicated as a cause of age-
related macular degeneration. before, debates had already existed as to
whether "blue blocking" or amber tinted lenses may have a protective
effect.
• Sunglasses are especially important for children, as their ocular lenses are
thought to transmit far more HEV light than adults.

65. Assessing the protection of sunglasses
• The only way to assess the protection of sunglasses is to
have the lenses measured, either by the manufacturer or
by a properly equipped optician .
• Several standards for sunglasses allow a general
classification of the UV protection .
• Manufacturers often indicate simply that the sunglasses
meet the requirements of a specific standard rather than
publish the exact figures .

66. Further functions of sunglasses
• Sunglasses can also be used to hide emotions; this can range
from hiding blinking to hiding weeping and its resulting red eyes
.
• Fashion trends can be another reason for wearing sunglasses,
particularly designer sunglasses
• People may also wear sunglasses to hide an abnormal
appearance of their eyes.
• Fashion trends can also draw on the "cool" image of sunglasses.
• Some lawbreakers have also been known to wear sunglasses
during or after committing a crime as an aid to hiding their
identities.

67. Special use sunglasses
Sunglasses in Sports - Sunglasses have to meet
special requirements when worn for sports .
Strap or other fixing is typically used to keep
glasses in place during sporting activities, and
they have a nose cushion . Mountain climbing or
traveling across glaciers or snowfields requires
above-average eye protection, because sunlight
(including ultraviolet radiation) is more intense
in higher altitudes, and snow and ice reflect
additional light. Popular glasses for this use are
a type called glacier glasses or glacier goggles.
They typically have very dark round lenses and
leather blinders at the sides, which protect the
eyes by blocking the sun's rays around the edges
of the lenses.

68. Special-use sunglasses
Sunglasses in space - Special protection is
required for space travel because the sunlight is
far more intense and harmful than on Earth,
where it is always filtered through the
atmosphere. Sun protection is needed against
much higher UV radiation and even against
harmful infrared radiation, both within and
outside the spacecraft. Within the spacecraft,
astronauts wear sunglasses with darker lenses
and a thin protective gold coating.
During space walks, the visor of the astronauts'
helmets, which also has a thin gold coating for
extra protection, functions as strong sunglasses.

69. Aviator sunglasses - are a style of sunglasses
that were developed by Bausch & Lomb and
branded as Ray-Ban. They are characterized by
dark, often reflective lenses having an area two
or three times the area of the eyeball, and very
thin metal frames with double or triple bridge
and bayonet earpieces or flexible cable temples
that hook behind the ears.
 The original design featured G-15 tempered
glass lenses, i.e., neutral gray, transmitting 20%
of incoming light. The large lenses are not flat
but slightly convex. The design attempts to cover
the entire range of the human eye and prevent
as much light as possible from entering the eye
from any angle
Aviator sunglasses, or "pilot's glasses", were
originally developed in 1936 by Ray-Ban for
pilots to protect their eyes while flying.

70. Mirrored sunglasses - Mirrored lenses, having a metallic,
partially reflective coating on the outer surface,
combined with a tinted glass lens, are an alternative to
polarization for UV protection, improving contrast when
depth perception is important such as seeing moguls and
ice while skiing or snowboarding.
The mirrored lens reflects glare to protect the eyes, but
improves the ability to see contrasts, and mirrored lenses
of different colors can expand the range of fashion styles.

71. Oversized sunglasses - Oversized
sunglasses, which were fashionable in the
1980s, are now often used for humorous
purposes. They usually come in bright
colors with colored lenses and can be
purchased cheaply.
Singer Elton John sometimes wore
oversized sunglasses on stage in the mid-
1970s as part of his Captain Fantastic act.
In the early 21st century moderately
oversized sunglasses had become a
fashion trend. There are many variations,
such as the "Onassis", discussed below,
and Dior white sunglasses.

72. Teashades - "Teashades" (sometimes also
called "John Lennon glasses", "Round
Metal", or, occasionally, "Granny Glasses")
were a type of psychedelic art wire-rim
sunglasses that were often worn, usually for
purely aesthetic reasons, by members of the
1960s counterculture, as well as by
opponents of segregation.
Pop icons such as Mick Jagger, Roger Daltrey,
John Lennon, Jerry Garcia, Boy George, Liam
Gallagher and all wore teashades. The
original teashade design was made up of
medium-sized, perfectly round lenses,
supported by pads on the bridge of the nose
and a thin wire frame. When teashades
became popular in the late 1960s.

73. Wayfarers - The Ray-Ban Wayfarer is a
plastic-framed design for sunglasses
produced by the Ray-Ban company.
Introduced in 1952, the trapezoidal lenses
are wider at the top than the bottom and
were famously worn by James Dean, Roy
Orbison and other actors and singers. The
original frames were black; frames in many
different colors were later introduced.
There is always a silver piece on the
corners as well.

74. Wrap-around sunglasses - Wrap-arounds
(sometimes also called "Yoko Ono glasses") are a
specific design of sunglasses. They are
characterized by a single, smooth, semi-circular
lens that covers both eyes and much of the same
area of the face covered by protective goggles.
The lens is usually combined with a minimal
plastic frame and single piece of plastic serving
as a nosepiece. As an alternative, the glasses can
have two lenses, but the design evokes the same
semicircle.

75. There are three major sunglass standards, which are popularly known
mostly as a reference for sunglass protection from UV radiation.
• The Australian Standard is AS/NZS 1067:2003 Sunglasses and fashion spectacles.
The five ratings for transmittance (filter) under this standard are based on the
amount of absorbed light.
• The European standard EN 1836:2005 has four transmittance ratings: "0" for
insufficient UV protection, "2" for sufficient UHV protection, "6" for good UHV
protection.
• The U.S. standard is ANSI Z80.3-2001, which includes three transmittance
categories. According to the ANSI Z80.3-2001 standard.
Standards for sunglasses

76. Sunglasses Standard
• The new Lenses categories are:
• Australia sunglass standard AS/NZS 1067:2003: 'Sunglasses and
Fashion Spectacles'
• 0 Fashion spectacles:
• providing some protection from UV radiation but no reduction in sun glare.
• 1 Fashion spectacles:
• providing protection from UV radiation and limited reduction of sun glare – not
suitable for driving at night.
• 2 Sunglasses for general use:
• providing good protection from UV radiation and sun glare.
• 3 Sunglasses providing extra protection from UV radiation and sun glare.
• 4 Sunglasses providing a high level of protection from UV radiation and
sun glare – must not be used when driving.

77. • AUSTRALIAN STANDARD FOR SUNGLASSES AND
FASHION SPECTACLES AS/NZS 1067:2003
• Sunglasses prevent at least 95% of UV radiation from
reaching the eyes.
• An Eye Protection Factor (EPF) rating from 1-10.
• Sunglasses with EPF 9-10 transmit almost no UV rays.
• Choose close fitting, wrap-around style sunglasses to
stop UV radiation entering around the sides and tops of
the lenses.
Note: Coloured glasses are less effective at blocking
UV radiation, and mirror finishes by themselves
don't significantly reduce UV radiation absorption.

78. References
• System for ophthalmic Dispensing by Theodore Grosvenor
• Lecture notes of Mr. Ashutosh Jnawali, Former Optometry Faculty,
ICO, CU
• Clinical Optics by Troy E Fennin
• Internet