What Ophthalmic Lenses Best Absorb Ultraviolent Light?

Researched by Bryan G.
1999-2000




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
*Reflecting box
1 UV lamp arc (to produce UV)
23 *Assorted types of spectacle lenses
Notepad to record data
Stop watch
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 2/3’s 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. I’m 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 people’s 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 light’s 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 earth’s 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, it’s 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 eye’s 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.  Today’s 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 today’s 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 FTC’s 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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