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PURPOSE
The purpose of this experiment was to determine what spectacle lens
material and/or tints and coatings best absorbs ultraviolet (UV) light.
I became interested in this idea because I have an interest in optical
studies. My uncle is an eye doctor and I think his job is fascinating.
I know ultraviolet light can damage the human body, especially the eye.
The information gained from this experiment will benefit society by
reducing the chance of ocular damage due to ultraviolet light over exposure.
HYPOTHESIS
My hypothesis is that the polarized lens will best absorb ultraviolet
light of all lens materials with and without tints and coatings tested.
I base my hypothesis on the fact that polarized lenses filter plane-polarized
light, which is reflected light, as well as natural scattered UV light.
EXPERIMENT DESIGN
The constants in this study were:
* The light source of ultraviolet light
* The light meters used to measure ultraviolet light
* The intensity of ultraviolet light shining on the lens
* The distance of the source to the lens and the distance from the
lens to the meter.
* Photometer
The manipulated variable was the type of lens materials, tints, and
coatings.
The responding variable was the intensity of ultraviolet light the lens
transmits.
To measure the responding variable I used a meter to determine the intensity
of ultraviolet light that was transmitted through the lens and a photometer.
MATERIALS
| QUANTITY |
ITEM DESCRIPTION |
| 1 |
UV light meter |
| 1 |
*Reflecting box |
| 1 |
UV lamp arc (to produce UV) |
| 23 |
*Assorted types of spectacle lenses |
| 1 |
Notepad to record data |
| 1 |
Stop watch |
| 1 |
Photometer |
Reflecting box is a cardboard box 32 inches long, 10.5 inches wide and
7 inches deep. There is a 1.5-inch hole (made with cardboard tube) in the
center of the long side of the box. The lower 1/3 of the hole is covered
by duct tape, thus allowing light to be emitted through the upper 2/3s
of the hole. A lens holder was constructed on the interior surface of the
box that allows the lenses to fit centrally and snuggly over the hole.
The exterior side of the opening is constructed with 3 thumbtacks to hold
the lens meter tightly over the opening. All interior surfaces are covered
by aluminum foil accept for the opening. A 19-inch UV lamp arc is placed
behind the opening to allow the middle of the tube to be centered over
the hole. See Table #1 on lens analysis.
PROCEDURES
Test 1
1) Gather the reflecting box (that can be sealed) with the UV (ultraviolet)
lamp arc.
2) Gather the first lens that is going to be tested.
3) Center and fix the lens on the inside of the hole in the reflecting
box.
4) Turn on the UV source.
5) Gather the UV intensity meter.
6) Fix the lower part (UV scale) of the UV meter to the outside of
the hole on the reflecting box.
7) Allow UV source illuminate for 20 seconds.
8) Read the results on the meter quickly.
9) Determine whether the UV light is passing through the lens in a
low low, moderate low, high low, low moderate, moderate moderate, high
low, high moderate, high high amount.
10) Record your data.
11) Repeat procedures (2-10) for lenses 2-23.
Test 2
12) Repeat steps (2-5)
13) Fix the upper right hand corner (pass/fail scale) of the meter
to the outside hole to be tested again.
14) Repeat procedures (7 and 8.)
15) Determine if lens passes or fails the 100% UV absorption test.
16) Record the data.
17) Repeat procedures (12-16) for lenses 2-23.
Test 3
18) Gather the photometer.
19) Turn on photometer and turn on UV light source, allow it to warm
up.
20) Reset meter to 100% transmission.
21) Place lens on lens holder.
22) Read meter in percentage transmitted through the lens.
23) Record measurement.
24) Repeat procedures (20-23) for lenses 2-23.
25) Turn off UV light source and turn on visible light source and allow
to warm up.
Test 4
26) Repeat procedures (20-24) for lenses 2-23
Notes: First test should be done with no lens in place to get baseline
findings. Im testing each lens three different ways to get a more accurate
data.
Calculate UV absorption by taking the percentage of UV transmission and
subtracting it from 100. (See Chart #1).
RESULTS
The results of this experiment indicate that the polarized lens best
absorbs UV light when compared to other lens materials with different tints
and coatings tested ( See table #3 and graph #5). This may be biased
by the fact that all the tested polarized lenses were #3 grade sunglasses.
When the polarized lenses were compared to other #3 sunglass lenses, all
lenses rated above 96% UV absorption, with the plastic lenses absorbing
99.2% and polarized lenses absorbing 99.0% (See table #4 and graph #6).
When comparing crown glass, CR-39 plastic, and polycarbonate (the three
most common materials for prescription lenses), there was a profound difference
between the sunglass #3 tinted materials and the lighter tints and coatings.
The #3 tints absorbed an average of 97% UV light and the lighter tints
and coatings absorbed only 59%. Consequently, the protection from
UV light is more dependent on the darkness of the tint rather than the
lens material (See table #4 and graph #6).
There was no standard protection from the UV light from antireflective
coatings (AR) (See table #1 and #2, lenses #4, 9, and 12).
When evaluating the UV protection based on cost, there was no definite
difference between high quality lenses and those of low quality (See tables
#1 and 2). One definite fact is that all high quality sunglass lenses,
regardless of the material, absorb 100% UV light that was tested.
Plastic photochromatic lenses absorbed 100% UV light while glass only
absorbed 89% in its darken state (See tables #1 and 2).
In comparing the accumulation of the data, using the reflecting box
and lens meter to the photometer, a significant difference was apparent
in the quality of the data collected.
The photometer was considerably more sensitive, giving much better
data (See table #2 and graphs #1 and #2).
There was a general trend that with all lenses tested, the less the
visible light was transmitted; the greater the UV was absorbed (See graphs
#3 and 4).
Click here to see my graphs
CONCLUSION
My hypothesis that the polarized lens would best absorb the UV light
was proven to be true through the experiment. The polarized lens best absorbs
UV light because it was 7.5 percent greater in absorption to the plastic
lens, which was the second most efficient lens.
The results of the experiment showed that all materials in a grade #3
sunglass lens absorbed at least 96% of UV-B light.
If I were to conduct this experiment again I would use a photometer
because it gave much more precise findings then the lens meter and reflecting
box. The results of the lens meter test #1 and #2 did not provide accurate
enough results to prove or disprove the hypothesis.
I would also use equal numbers of each lens material and equal numbers
of differing levels of tints and coatings. The experiment showed that the
darker the lens the better the protection. Therefore, more data collection
and analysis is necessary.
Further experiments to enhance the knowledge of UV-B protective eyewear
could be:
1) Test lens materials of differing ages to see if aging of the material
significantly affects the UV-B protection.
2) Test similar materials and vary the grade of tinting to see if the
darkness of the tint had a profound effect on the UV-B absorption qualities.
This experiment has been very thought provoking and I believe can add
important information for the safety of peoples eyes and vision. Definitely,
more studies need to be made regarding this dangerous health risk.
Glossary
Aqueous humor The clear, watery fluid which fills the anterior chamber
of the eye.
Basal cell carcinoma A malignant tumor of epithelial (skin)
tissue.
Biological activity A reaction within living tissue.
Choroid The middle coat of the eye lying between the retina and sclera.
Ciliary body The part of the uvea anterior to the ora serrata between
the sclera outside and the vitreous and the posterior chamber inside.
Coating A thin deposit of a metallic salt, such as magnesium fluoride,
about one fourth as thick as a wavelength of light, applied to the surfaces
of a lens.
Cornea The transparent anterior portion of the fibrous coat of the eye.
Cortical cataracts A cataract in which the opacity lies in the cortex
of the crystalline lens.
Macular degeneration Degeneration of the macular area of the retina
in the aged population which progresses to pigmented scar formation.
Nanometer A unit of length equal to one millionth of a millimeter.
Optic nerve Anatomically, cranial nerve II of the peripheral nervous
system.
Photic maculopathy Macular disease caused by extended exposure to extremely
bright light.
Photochromatic Pertaining to substances which change in color and in
light transmission properties upon exposure to a change of light intensity
or to ultraviolet radiation.
Photokeratitis A superficial punctate inflammation of the cornea caused
by exposure to ultraviolet radiation or and intense electric spark.
Pinguecula A small, slightly raised, yellowish, nonfatty thickening
of the bulbar conjunctiva on either side of the cornea.
Plane-polarized light Polarized light in which the transverse wave vibrations
are parallel to a plane through the axis of the beam.
Polarizer An agent or medium which induces or effects polarization.
Pterygium A horizontal, triangular growth of the bulbar conjunctiva,
occupying the intrapalpebral fissure, with the apex extending toward the
cornea.
Pupil The aperture in the iris, normally circular and contractile, through
which the image-forming light enters the eye.
Sclera The white, opaque, fibrous, outer tunic of the eyeball, covering
it entirely excepting the segment covered anteriorly by the cornea.
Solar maculopathy Degeneration of the macula from overexposure to the
ultraviolet rays from the sun.
Spectrum The spatial arrangement or series of the dispersed components
of radiant energy, in order of their wavelengths, emitted, absorbed, or
reflected by a substance.
Ultraviolet light Radiant energy of wavelengths shorter than the violet
end of the visible spectrum and longer than the roentgen radiations (X-rays),
usually considered to be wavelengths from 400 to 20 nm.
Uveal tract The pigmented vascular coat of the eyeball, consisting of
the choroids, the ciliary body, and the iris, which are continuous with
each other.
Wavelength The distance in the line of advance of a wave from any one
point to the next point at which, at the same instant, the phase is the
same.
RESEARCH REPORT
Introduction
Ultraviolet light (ultraviolet radiation [UVR]) is invisible electromagnetic
radiation with the waveband between 4 and 400 nanometers (nm). The
human eye can only respond to light in the visible spectrum with the waveband
between 440 and 870 nm. The past 15 years have seen a growing concern
for the protection from the toxic effects of UVR. Ultraviolet lights
most prevalent source is sunlight. UVR is divided in three wavebands,
UV-A (315-400nm), UV-B (180-315) and UV-C (4-180). The lower the
wavelength, the greater the biological activity that results from overexposure.
Only UV-A and UV-B go through the earths atmosphere. UV-C, the most
harmful, is filtered out by the ozone layer. With the thinning of
the ozone layer due to pollution, this could become a significant problem
in the future.
Since UVR is invisible, its hard to judge the dosage based on the brightness
of the sunlight. Since clouds do not filter UVR, we forget to wear
adequate protection on overcastted days.
There are also, occupational sources of UVR. Of these, welding
arcs are the most prevalent source. Other occupational sources include
lasers, UV lamps to control skin conditions (i.e. Psoriasis), cure dental
resins. Tanning beds are a large recreational source.
Polarized Light
Both Christian Huygens and Isaac Newton, physicists, knew that when
light was directed through certain crystals, it would come out much dimmer.
If a second crystal of the same type were placed at a certain angle in
the path of the dimmed light, the light could pass through it. Then,
as either of the two crystals was slowly turned, the light coming from
the second crystal grew dimmer until it was entirely blacked out.
Evidently something in the structure of the first crystal allowed only
part of the light to pass. When the second crystal was lined up with
the first, it allowed the same amount of light to pass. When it was at
the wrong angle to the first crystal, it screened out the light from the
first crystal.
Thomas Young, a British physician, and Austin Jean Fresnel, a French
physicist, developed the idea that light waves were transverse, It
compares to the waves made when a rope stretched from a post in jerked
up and down, as compared to longitudinal sound waves. The rope itself
only moves up and down at right angles to the forward movement of the wave.
The motion could be in any direction between sideways and up and down,
just so long as it was at right angles to the direction of the wave.
Wave motion of this kind in one plane only is called plane-polarized light..
The Human Eye
The eyelids protect the front of the eyeball by the blink mechanism.
The blink protects the eyeball from foreign bodies as well as keeps the
eye lubricated by circulating tears. The sclera and the cornea are the
outer tissues of the eye. The white part of the eye is the sclera. The
sclera is strong and feels like soft leather. The cornea is transparent
and is located in the front of the colored part of the eye. (Iris) The
cornea allows light rays to come into the eyeball. The middle layer of
the wall of the eyeball is the uveal tract. The three parts of it from
front to back are the iris, the cilary body, and the choroid. The colored
disk that sits behind the cornea is the iris. The pupil is the round opening
in the center of the iris. The iris is the muscle that controls the size
of the pupil. The cilary body is around the iris. The cilary body produces
a transparent, watery substance called aqueous humor. This sustains and
oils the inner cornea and the lens, and it occupies the area between them.
The choroid makes up the back of the uveal tract. It looks and feels like
a blotter absorbed in black ink. The chorid supplies the blood for the
inner layers of the retina and its dark color absorbs reflected light in
the eyeball. The retina makes up the inside of the eyeball. It is light
sensitive and connected to the brain by the optic nerve, which transmits
the images to the brain and allows us to see.
Ultraviolet Radiation Impacts On the Eye
When light is absorbed, it causes heat or chemical reactions.
This can damage the eye if the amount is beyond the eyes natural capacity
to heal itself.
UVR can damage virtually every tissue layer of the eye. The various
ocular tissues absorb the wavebands of UVR to varying degrees. The
longer the wavelength the deeper the penetration and the greater the damage
to the ocular tissue. Consequently, UV-B is more detrimental to the
eye. UV-A is considered a greater risk to produce skin cancer than
damage the eye.
UVR can create both acute and chronic damage. Among the potential
acute effects:
Photokeratitis (keratitis photoelectria) This is the clinical term for
corneal sunburn. UV-B is the major cause although UV-A may contribute
also. This occurs in high elevations with snow (snowblindness).
High intensity exposure can cause damage within seconds.
Solar maculopathy If we stare directly into the sun, within 20 seconds
the UVR will burn a hole in the macula, the part of the retina responsible
for central vision. This is the damage done when we look at a solar
eclipse. This damage is irreversible.
Photic maculopathy In the past, this occurred from the surgical
lamp during expanded intraocular surgery. Todays surgical lamps
are filtered for UVR.
The potential chronic effects of UVR are:
Cortical cataracts Clinical evidence shows a link between UVR overexposure
and cortical cataracts.
Pinguecula, pterygium and other keratopathies Circumstantial evidence
suggests the UVR overexposure may lead to anterior segment degenerations.
Age-related macular degeneration Evidence suggests that UVR and visible
blue light may be causative agents.
Basal cell carcinoma Skin cancer, as it is commonly called, can be found
often on or around the eyelids. It is well documented that overexposure
to UVR can lead to this popular form of cancer.
Sunglasses
Polarized Lenses
In 1928 Dr. Edwin Land created a revolutionary film called polarizer.
This film blocked horizontal light reflected off the flat surfaces.
This enhanced the comfort of the eye in bright light.
For many years polarized lenses were difficult to make and very expensive.
With todays advanced technology, the manufacturing of polarized lenses
are much more common and less expensive.
Facts about polarized lenses:
1) Polarized lenses reduce surface glare consequently clarifying vision,
thereby improving visual comfort and enhancing contrast.
2) Polarized lenses offer eye protection. The polarizing filtering
system protects the eyes from direct and indirect bright-reflected visible
light.
3) Polarized lenses block reflected visible light thus enhancing vision
and eliminating glare.
4) Polarized lenses provide some of the best protection for your eyes.
Plastic Lenses
Plastic lenses are the best all around lens for most uses. They
are lightweight and can be tinted and/or coated for most uses. The
inherent material will absorb about 80% of UV-B light.
Polycarbonate Lenses
Polycarbonate lenses are the strongest materials for safety lenses.
The plastic was originally developed for the NASA space program.
The down side is the material is not available in all tints and coatings.
The inherent material absorbs 30% or more of the UVR.
Photochromatic Lenses
Photochromatic lenses are either glass or plastic lenses that lighten
and darken to the amount of UV light passing through the lens. Plastic
photochromatic lenses are much better at absorbing UV-B then that is glass
lenses. Both materials absorb significantly more UV light in their dark
state then in their lighter state.
Glass Lenses
Glass lenses are the best material for optical quality and clarity.
The down side is that the material can break and shatter and is not available
in all tints and coatings and only absorb 20-30% of the UVR.
The Federal Trade Commission governs ophthalmic lenses and strict
regulations are placed on their manufacturing and must absorb between 60%
to 92% of visible and UV-A light and 95% to99% of UV-B light. However,
most sunglasses are not ophthalmic and consequently are not under the FTCs
jurisdiction. In fact, the regulations are under the same limited
regulations as toys. Because of this, their marketing can be very
misleading and sometimes false.
SUMMARY
UV light can damage the eyes severely. High quality sunglasses absorb
the UV before it gets to the eyes. This project is on what type of spectacle
lens best absorbs UV light. This will benefit society by preventing UV
damage to the eyes. Polarized sunglasses are some of the high quality sunglasses
that absorb a high amount of the UV to reduce the chances of eye damage.
Polarized lenses also have an added feature to take the glare off of reflected
light.
BIBLIOGRAPHY
Bruneni, Joseph L. Polarized, Eyecare Business. vol. 15:1 pp.49-54,
January 21, 2000
Characteristics of Polarization. [Online] Available
http://infoplease.com/ce5/CEO41484.html, January 19, 2000
Diseases of the Eye, Multimedia Encyclopedia World Book, 1999
Glossary of Ophthalmologic Terms. [Online] Available
http://www.west.net/~eyecare/glossary.html, January 25, 2000
Parts of the Eye, Multimedia Encyclopedia World Book, 1999
Polarization Techniques. [Online] Available
http://www.infoplease.com/ce5/CEO41484.html, January 19, 2000
Preventing Eye Damage, Multimedia Encyclopedia World Book 1999
Rainwater, Clarence. Light and Color. New York: Golden Press, 1971.
pp.53
Sunglasses. [Online] Available
http://lensdoc.com/sunglass/polarized.htm, January 25, 2000
Sunglasses. [Online] Available
http://www.revoptom.com/ISSUE/0598F7.HTM, January 25, 2000
Ultraviolet Radiation, Multimedia Encarta Encyclopedia. 2000
Ultraviolet Radiation. [Online] Available
http://encarta.msn.com/find/Concise.asp?z=1&pg=2&ti=065D0000,
November 9, 1999
Ultraviolet Radiation. [Online] Available
http://www.encyclopedia.com/articles/13207.html, November 9, 1999
Ultraviolet Radiation, World Book. 1999. vol. 20. pp. 17-18
What is Ultraviolet Light? [Online] Available
http://snoopy.gsfc.nasa.gov/~orfeus2/ultraviolet.html, January 19,
2000
Why You Need to Protect Your Eyes from the Sun. [Online] Available
http://www.optictians.org/consumer/ultra.html, January 25, 2000
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