Sam Optics

bausch and lomb

2005-10-23T07:32:00.000-07:00

bausch and lomb
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Titanium Eye Wear

2005-10-18T04:57:00.000-07:00

While many believe titanium is a new, space-age material, it is actually a lightweight dark gray metallic element that English mineralogist William Gregor discovered in 1791. It was originally utilized in such technology-driven industries as shipbuilding and sports products (e.g., golf clubs and bicycle frames), and has now found its way into medical/surgical instruments and the aerospace industry. The first titanium eyewear was introduced in 1982.Highlights of Titanium EyewearPeople’s interest in titanium frames stems from some very appealing qualities. First, it is strong and lightweight. Because titanium has superior tensile strength, it allows manufacturers to produce more resilient frames without using spring hinges, although it is considered the ultimate in durability to combine the two. Safilo USA incorporates both titanium and spring temples in its Titanium model 173. Titanium is twice as strong as aluminum and monel, which allows for the production of much thinner styles. The average titanium frame is 48% lighter than conventional metal frames. Second, titanium is 20 times more corrosion resistant than monel or nickel/silver alloys. This allows the eyecare professional to confidently recommend titanium frames regardless of climate or skin condition. Third, titanium frames retain their appearance for a much longer period of time. Fourth, frames that are pure titanium can meet the needs of patients who have an allergy to nickel.

Features and Benefits of Titanium Frames
Lighter in weight, 48% lighter than conventional metals.
Superior comfort.
Extremely strong and durable.
Hypoallergenic.
Up to 20 times more corrosion resistant than conventional metals.
What Defines a Titanium FrameOphthalmic frame manufacturers can make eyewear from pure titanium or combine it with other materials, such as nickel or copper, to produce an alloy. In order for a frame to be labeled titanium, it must have a metal content of at least 75% titanium. Silhouette Optical, Ltd.’s 7410 utilizes a flexible titanium alloy that contains nickel but uses a hard clear coating to cover the metal, thus allowing people with nickel allergies to wear them with confidence. And some frames use titanium or titanium alloys in certain areas, such as the bridge and temples.Types of Titanium FramesAccording to the Vision Council of America (VCA), a pure titanium frame must have a minimum of 90-100% titanium by weight from the ASTM (American Society for Testing and Materials) grades 1-4, excluding temple tips, hinge screws, washers, and nosepads. In addition, frames should be marked “TITANIUM –100.” Pure titanium frames contain no nickel plating or coloration and is the titanium frame of choice for those who have nickel-related allergies (about 10% of the total population). Pure titanium frames can be made very thin. In one manufacturing technique —laser titanium —frame sheets are cut out of a large sheet of titanium, resulting in a flat metal profile. Safilo’s Titanium 541 features a front and endpiece cut out of a continuous sheet of titanium. Newer technology now allows even hinges, endpieces, and special decorative pieces to be added. Marcolin USA’s Ti 22 at 189 (private label) is an example of this new technology. It will be incorporated into other brands in the future.Combination titanium is the name given to frames that use titanium for major parts, such as eyewires and temples, while trim pieces are made from nickel silver or monel. No alloys are used. These frames offer a weight savings of about 25% and are about 50% stronger than monel.Memory titanium is an alloy made with 40-50% titanium, with the balance being made of nickel. This material is remarkably flexible, as demonstrated by Marchon USA’s Flexon® line. While it is durable, its ability to remain flexible diminishes with age. It doesn’t share the same corrosion resistance as pure titanium and because of the nickel content, it cannot be considered hypoallergenic. The weight saving is about 25% that of conventional metals. Memory metals are generally restricted to the bridge and temple shank portions of the frame. The Flexon Select collection also features T-Wire technology—the eyewire is embedded in a grove in the lens, making the styles even more lightweight.Beta titanium is a new titanium alloy containing titanium, vanadium, and aluminum chrome. According to Charmant Group, Inc., USA, beta titanium is lighter and stronger than conventional metals, but maintains the corrosion-free, hypoallergenic properties of a pure titanium frame. While not a memory metal, it is that lack of memory that allows it to be adjusted with ease and hold its adjustment. Aluminum is the only metal known that is lighter than titanium, and vanadium gives hardened steel its strength and durability. Beta titanium is the material used in Charmant’s 8320 rimless.

How to Market Titanium
Present titanium as the material of choice.
Display titanium frames in their own section of the frame board. Make them look special.
Always show titanium first. In fact, you can even demonstrate the differences between titanium and other metals with display resources provided by some manufacturers.
Be educated; know your product availability, applications, features, and benefits.
Wear titanium frames in the most fashionable styles and colors. Your patients will want what you’re wearing.
Fitting Titanium EyewearTitanium frames need no special adjusting techniques, but because of the strength and flexibility of the material, the eyewear provider needs to spend a little extra time insuring the adjustment is acceptable. Pure titanium frames are best adjusted using various optical pliers. For example, bracing pliers will have to be used in conjunction with angling pliers to adjust pantoscopic tilt. Pad arms can be bent using a standard nosepad plier but care should be taken in providing extra support at the lens/eyewire edge near the pad arm. Adjusting the temple ends might be more easily achieved using a small amount of heat for the plastic and a plier to curve the temple ends to match the wearers behind the ear structure; finger pressure may not work alone.Titanium is a major material player in ophthalmic frames today. From thin threadlike designs to the high-tech appearance of laser-cut flat-metal designs, there’s a wide array of options. From 100% titanium to titanium alloys to beta-titanium, the number of models are growing. Rigid titanium fronts are being paired with beta-titanium temples for added adjustability, and titanium is also being coupled with stainless steel for added strength. With all this going for it, is there any other choice for today’s high-tech lens offerings?

Anatomy Of Eye

2005-10-18T04:15:00.000-07:00

No member of the eyecare profession is immune from complaining patients. Many times, these patients describe specific symptoms that may or may not be significant. But a firm understanding of the eye’s structure and function can help you determine the more significant symptoms and make the proper referrals. In addition, being familiar with the anatomy and pathology of the anterior segment of the eye will help you when fitting the myriad of contact lenses now available—including bifocals, torics, opaque colors and disposables.
Below we will discuss the anatomy and physiology of the anterior segment structures and attempt to identify certain problems that can negatively affect contact lens wear.
LidsThe lids function by carrying out a complex system of tear production and drainage. They also protect the orbit and globe from external objects. The eyelids blink at an average rate of 12 times per minute. Lids open when the third cranial nerve (oculomotor) innervates levator palpebrae superioris. Lids close via another mechanism, the orbicularis oculi—a round muscle located within both lids. This muscle, when innervated by the seventh cranial nerve (facial), contracts and causes closure.
Being familiar with the anatomy and pathology of the anterior segment of the eye will help you when fitting the myriad of contact lenses now available.
The lid also contains Mueller’s muscle, which is made of smooth tissue, unlike the striated variety in the levator and other extraocular muscles. A tarsal plate separates the front of the lids from the back. This structure is responsible for maintaining the shape of the lids and also serves as the structure that attaches to the levator. There is no fatty tissue in the lids.
Tears are extremely important to the health and comfort of the eye. The lids contain two glands that produce tear components. The meibomian glands are located in parallel vertical rows in the superior lid and are visible with lid eversion during slit-lamp evaluation. These glands, and the glands of Zeis found at the base of the lid follicles, produce sebum (oil), and contribute that component of tears. The glands of Wolfring and Kraus are responsible for producing the lacrimal, or watery, component. The lid also contains glands of Moll, which are sweat glands located around the base of the lashes.
As tears coat the eye, oxygen is absorbed by the cornea, causing a need for periodic replenishment by freshly oxygenated tears. The lids are instrumental in this tear exchange. Lids close in a vertical and slightly nasal direction. Each closure of the eye actively distributes a new tear film over the entire corneal surface. The older, de-oxygenated film is forced toward the drainage mechanism in the nasal portion of both lids. These drainage tubes, or canaliculi are not visible in the slit-lamp except for a small opening at their entrance called the punctum. The superior and inferior lids each have puncta that are perfectly aligned with each other. When the lids close, these two puncta come together and create a negative pressure (or suction). When the lids reopen, the suction pulls the older tear layer into the canaliculi and starts it on its way to the drainage system.
Eyelashes are important structures in keeping particulate matter out of the eye. The lashes act as brushes by capturing dust and foreign matter before they can cause possible injury to the cornea and globe.
Pre-Corneal Tear FilmThe importance of the pre-corneal tear film must not be understated. The tears cleanse the eye, supply vital nutrients to the cornea, provide a lubricant to ease lid function and remove of excess fluids from the cornea.
Let’s mentally perform a science experiment by taking a thin filter and using it to divide a large jar into two halves.
The filter is specially designed to allow water to flow through it but not salt. We then fill the jar to two-thirds of its capacity and put a few tablespoons of salt on one side of the filter. Now, there is water on both sides of the filter, but salt on only one side. This establishes a condition known as a “concentration gradient.” The solution will attempt to reach equilibrium (or evenness) between the two sides. Therefore, the salt will attempt to migrate to the unsalted side but will not be able to because the filter will block it. The only other way to achieve the equilibrium is for the water to migrate to the salted side. The filter will let this happen. So we will find a shifting of the water toward the side with the salt.
In short, the water shifted from the lower salt concentration (or hypotonic solution) to the higher salt concentration (or hypertonic solution). The cornea’s relationship with tears acts in a similar fashion. Tears are exposed to the environment and begin to evaporate while coating the cornea. The fluid portion of tears vaporizes and the salt remains behind. Therefore, after a few seconds, the tears have less water and a higher concentration of salt. The cornea hasn’t changed and is now hypotonic. Water will drift out of the cornea and into the tears. In this way, tears help to drain excess fluid from the cornea. This process of “passive diffusion” occurs without the expenditure of any energy.
The pre-corneal tear film is comprised of three layers: the sebaceous (or oily layer), the lacrimal (or watery layer) and the mucin (or protein layer). The most important layer is the lacrimal. The glands of Wolfring and Kraus, which are located in the lids, produce the lacrimal layer. It is in this middle layer where we find most of the cornea’s oxygen supply, nutrients and vital proteins such as lactoferrin and immunoglobulins. These proteins are basically in charge of cleaning up the refuse and protecting against infection. In fact, the primary function of the sebaceous and mucin layers is to prolong the contact of the lacrimal with the cornea. The viscous sebaceous layer, is secreted by the Meibomian glands and functions to slow the rate of evaporation of the lacrimal layer. The mucin layer solves a basic problem: fat and water don’t mix. Because tears are mostly water and the anterior cornea is comprised of lipid (fatty) cell membranes, the cornea repels the tears. The mucin layer actually performs like two-sided tape, attracting the cornea on one side and attaching to the tears on the other. This increases the time that the tears coat the cornea. The mucin layer is secreted by goblet cells located in the superior fornix of the conjunctiva.
ConjunctivaThe conjunctiva is effectively a continuation of the corneal epithelium. It is a clear, vascular tissue that forms an envelope-like pocket on the front part of the globe. This serves to protect the orbit from bacteria and infectious agents as well as from dust and pollutants. The conjunctiva seals off the back of the eye and is the reason why a contact lens can’t float behind it.
The conjunctiva attaches to the lid margin and lines the inner surface of the eyelids. This portion is referred to as the palpebral conjunctiva. After leaving the lid surface, the conjunctiva continues into the orbit and folds into a loop called the fornix. It then comes back and lies over the globe and is called the bulbar conjunctiva, which adjoins the globe and ends at the limbus. The goblet cells are located in the superior fornix of the conjunctiva.
The term “conjunctiva” is not a familiar to most people, but “conjunctivitis,” which refers to an inflammation of the conjunctiva, is. There are several causes of conjunctival inflammation, including bacterial, viral, seasonal allergies and fungal. The lid tissue can induce an allergic reaction when exposed to certain contact lens materials and solutions. These problems might lead to a lumpy overgrowth of the conjunctival tissue, a condition known as giant papillary conjunctivitis (GPC).
Cornea The cornea is a lens. Located at the anterior pole of the globe, it is the most powerful lens in the eye. An average cornea supplies approximately +43.00 diopters of power to the eye. With a center thickness of 0.5 mm and an edge thickness of 1.0 mm. Because the index of refraction of aqueous is approximately 1.334 and the index of the cornea is 1.375, there is a very small difference between the densities of the two media. With such a small difference, almost no refraction occurs. Thus, the back surface of the cornea is nullified and the convex front surface supplies almost all of the power.
There are five layers to the cornea, which can be easily remembered in order from anterior to posterior alphabetically: ABCDE.
A—Anterior epitheliumB—Bowman’s membraneC—Corneal StromaD—Descemet’s MembraneE—Endothelium
EpitheliumThe anterior epithelium is made up completely of cells responsible for removing fluids from the eye and generating energy for the metabolic processes of the cornea. There are multiple layers of cells in the epithelium, with each layer representing a different stage of life of the epithelial cells. The basal layer is the deepest and is comprised of the youngest of the cells. The single layer of basal cells has Velcro-like devices that attach the cells to a basement membrane. This causes the layer to be very stable. As new cells are produced, they replace older basal cells and displace them to the middle portion of the epithelium. These cells change shape and become wing cells. There are two or three layers of these wing cells sandwiched between the basal and superficial cell layers. The most anterior layer of cells in the epithelium is the superficial layer, composed of the oldest epithelial cells and the ones that are ultimately sloughed off by the lids. This intimate relationship between neighboring cells has such a tight bond that no fluid can enter the cornea between them.
Bowman’s Membrane Bowman’s Membrane is an extremely thin tissue layer (about 12 microns thick), which lies between the epithelium and the corneal stroma. The composition of Bowman’s is primarily protein. No cells are contained in the membrane. This tissue has similar components to the corneal stroma. Bowman’s is not regenerative and any trauma or disease process that causes a defect in the membrane will result in scarring.
Corneal StromaApproximately 85 to 90 percent of the corneal thickness is stroma. The arrangement of the thousands of collagen fibrils is in a parallel fashion and form a larger unit called the lamella (pl. lamellae), which allow the cornea to maintain transparency. The lamellae are shaped similar to long pieces of lumber as they stretch from limbus to limbus across the entire cornea. Imagine a pile of lumber arranged in layers where the bottom level has 30 to 40 boards lying side by side on the ground. The next level up is also made of 30 to 40 boards side by side, however this layer is aligned perpendicularly to the bottom one. The third layer has side-by-side boards that are aligned with the first layer. The trend continues until there are many levels of wood with all the odd numbered rows pointing east to west and the even numbered rows pointing north to south.
Pile of wood in alternating rows, similar to the arrangement of lamellae in the corneal stroma
The lamellae follow a similar arrangement in the corneal stroma. The difference is there is a space between each lamella (or board) in the stroma. The regular placement of each lamella combined with the consistent size of each space is what provides the clarity to the corneal tissue. Any disruption of this arrangement, even by a clear substance such as water, will result in opacity in the tissue. This is why the role of the epithelium and endothelium in the removal of fluids (deturgescence) is so important to the clarity of the cornea. The sclera, a continuation of the stroma, is also comprised of collagen and other proteins and ground substance, but does not have the ordered arrangement of these proteins. This accounts for the sclera’s opaque white color and lack of transparency.
Besides the protein fibers, the stroma is made up of a fluid-like ground substance and contains keratocytes, a cell responsible for cleaning up the stromal environment. As is the case with all components of the cornea, there are no blood vessels in the stroma.
Descemet’s MembraneDescemet’s membrane separates the corneal stroma from the endothelium. It is an elastic tissue about 10 microns thick and is analogous to a piece of latex stretched over the back of the stroma. If a piece of latex is punctured, the small puncture whole enlarges from the tension on the material. Descemet’s behaves similarly and cannot regenerate. This tissue is important in keeping aqueous from entering the stroma.
EndotheliumThe single layer of cells known as the endothelium is one of the most important components of the cornea. These hexagonally shaped cells are responsible for about 90 percent of corneal deturgescence.
Endothelial cells
As previously discussed, the anterior epithelium interacted with the tear layer to remove fluid from the cornea in a passive way (without the expenditure of energy). The eye has a certain internal pressure (intra-ocular pressure), which is constantly pushing outward against the inner wall of the globe like the air in a basketball. The endothelial layer is charged with pumping fluids into the anterior chamber and has to work against this pressure. For this to happen, each cell has its share of ATPase pumps that move the fluids.
The endothelium is non-regenerative and the number of cells in the layer naturally diminishes with age. A young child has a full complement of cells in his/her endothelium and the fluids are drained from the eye with maximum efficiency. In the elderly, there are simply too few pumps to drain all the fluid even if they work at their maximum rate. This is why children have such bright, shiny appearances to their eyes and the elderly have a dimmed, hazy look to theirs.
With the reduction of cells, certain areas of the endothelium have holes or gaps. The surrounding cells tend to change their size in an attempt to fill in these voids. The change in size by these surrounding cells is known as polymegatism. Unfilled voids in the endothelial layer are called gutatta. Over time, some cells experience a thickening, or change in shape. This phenomenon is called pleomorphism.
Anterior ChamberThe anterior chamber lies between the lens and the corneal endothelium. It is filled with aqueous humor very similar to blood, but contains fewer proteins and no red blood cells. Aqueous follows a convection current that originates at the ciliary body and circulates around the back of the iris and forward through the pupil until it reaches the cornea. The relatively cool cornea pulls the aqueous toward it. It then spreads out toward the anatomical angle and drains through the trabecular meshwork on its way to the Canal of Schlemm and the limbal vessels surrounding the cornea.
The trabecular meshwork is an extension of the endothelium and functions like a sieve. This system controls the drainage of aqueous from the eye. Occasionally, obstructions can occur in the drainage system around the trabecular meshwork.
Blockage of the trabecular meshwork or obstructions in the anterior chamber angle can cause a back up of aqueous, which will result in a buildup of the intra-ocular pressure. The increased pressure then presses on the nerve fibers in the retina causing permanent damage. This is the mechanism that leads to glaucoma.
IrisThe iris is a vascular tissue located just anterior to the crystalline lens. It is the iris that gives color to the eye. The color of the eye is directly related to the amount of pigment that deposits on the back surface of the iris during the eye’s development. Children who are born with blue eyes might experience a change of eye color through the first six months of life.
The iris is separated into two basic parts consisting of the peripheral iris and the iris frill. The frill is the inner portion that surrounds the pupil. The periphery is separated from the frill by the collaret, a brownish ring visible in a slit lamp.
Two muscles are located in the iris. The dilator muscle is a series of radial bands that run over the back surface from the outer edge of the iris tissue inward toward the pupil. This muscle, when stimulated, causes a pulling of the pupil outward leading to dilation of the eye. The other muscle is a sphincter located right at the pupillary border. This muscle, when stimulated by the oculomotor nerve, causes a pulling of the iris tissue toward the visual axis (constriction). The anterior surface of the iris is an arrangement of tissue and holes (or crypts) that are unique from person to person.
The anterior segment of the human eye is a complex series of structures working together to provide the miracle of vision. The delicate balance of tissues and fluids allows humans to survive in what can sometimes be a hostile environment. Your understanding of the nature of the anterior eye may help you to make appropriate referrals of patients. If you are ever faced with a question of whether to refer or not, be sure to err on the side of safety. If in doubt…refer it out.

Effect lo Light on Vision

2005-10-18T04:11:00.000-07:00

THE PROCESS OF VISION
As an artist relies on his paints, our visual system depends upon the availability of light to create images of the world around us. Ironically, the electromagnetic energy that enables our vision is also capable of depriving us of acuity, sight, and even our health. A study of the human eye reveals our vision is dependent not only on the light that enters the eye—but also the light that does not. Darkness, it would seem, is as important to vision as light.
The negative effects of ultra-violet radiation (UVR) on the human body are well documented. A large percentage of the population is unaware of the negative effects of UVR on visual health. Even less awareness is given to the negative effects of the visible spectrum on vision comfort and performance. UVR is defined as electromagnetic energy with wavelengths between 100 and 380nm. The visible spectrum is comprised of electromagnetic energy with wavelengths falling between approximately 380 and 780nm. Both visible and "invisible" light possess energy in the form of photons. As photons strike the retina, the physical force of the photons is converted into chemical energy producing electrical impulses perceived by the brain as "light."
Following is a discussion of some of the most prominent negative effects of both visible and "invisible" light...
NEGATIVE EFFECTS OF VISIBLE LIGHT
Left: View of a golf course showing discomforting glare. Right: Same view with photochromic lenses.NIGHT VISION During normal lighting (photopic) conditions, the eye generates the electrical impulses which create vision using a chemical process involving potassium ions and sodium ions and sensory cells in the forms of cones and rods. In low lighting (scotopic) conditions, a chemical called "visual purple" allows the rods to be particularly sensitive to faint sources of light (such as stars, or the alarm clock on the nightstand). Rod cells are elongated cylinders which contain approximately 2,000 stacked discs. Within these discs, each rod cell may contain upwards of 100 million components of visual purple (rhodopsin). Rhodopsin is the combination of a protein called opsin and a light sensitive substance called retinene.1 Since the body cannot generate the opsin protein without Vitamin A, individuals with extreme dietary insufficiencies may suffer from nyctalopia (night blindness).
By necessity, rhodopsin is extremely reactive to light. This property gives rod cells a sensitivity to light that is an order of four magnitudes greater than cone cells. Even small amounts of light will cause the "bleaching" of Rhodopsin. During photopic conditions, rhodopsin is consumed much faster than it can be produced. When luminance conditions decrease rapidly from high levels to relative darkness the eye is therefore left without a reserve of rhodopsin. A light adapted eye will have an absolute threshold of dark adaptation as much as 10,000 times higher than the level the eye will have after 40 minutes of exposure to dark conditions. The highest level of dark adaptation is known as the absolute terminal threshold. This level is only possible after complete regeneration of rhodopsin levels has occurred.2,3
Prolonged unprotected exposure to high levels of sunlight increases the time cycle of rhodopsin replenishment and can effect an individual's ability to achieve an absolute terminal threshold in the normal range. Individuals who spend lengthy periods of time in bright sunlight (generally defined as luminance levels of 8,000-10,000 lumens) are particularly susceptible to transient nyctalopia. For example, individuals with occupations involving all day exposure to bright sunlight will have longer dark adaptation times than an office worker who lives in a controlled and moderately lit environment. The ability to reach a normal absolute terminal threshold will depend upon the level of exposure, diet and age of the individual.
The use of sunwear with a transmittance no greater than 12 to 15 percent will lessen or eliminate the effects of bright visible light on the speed and level of the individual's dark adaptation.4 Fixed tint lenses can be used during times of exposure, but will need to be removed in conditions with lower levels of lighting. Photochromic lenses with a darkened state having a transmittance of 15 percent or less (such as Transitions Lenses) will provide adequate protection in bright sunlight and can be worn in all lighting conditions. Individuals using sunwear to protect night vision should be advised to wear larger frames. Lenses with a surface area of 20 square centimeters will provide protection over approximately 95 percent of the ocular area.
Protection of night vision is especially important for older individuals. Rhodopsin regeneration cycles become longer with age- even among individuals with excellent macular health. As the human visual system ages, increases in both the time required for dark adaptation and the absolute terminal threshold occur as the capability for night vision decreases. Adequate sunwear protection is therefore particularly crucial for the older person who spends significant time in sunlight (such as golfers).6
GLARE Just as light is essential for vision, too much light can cause discomfort or even disrupt vision. Glare can be broken down into two primary categories- discomfort glare and disabling glare.
DISCOMFORT GLARE Discomfort glare does not generally degrade vision; however, it is certainly distracting and is capable of causing pain and/or eye fatigue. Discomfort glare commonly results from one of two conditions- excessive amounts of illumination and/or reflections within the visual field. Excessive lighting causes actual pain and discomfort when luminance exceeds 10,000 cd/m2. As a point of reference, fresh snow can have a luminance of 30,000 cd/m2 on a sunny day.7 Surroundings including sand, water, or polished surfaces can also produce discomfort glare.
Although the actual causes for ocular discomfort are poorly understood (the retina has no pain receptors), there is evidence that the discomfort felt in bright light is triggered by the constriction of the pupil. The consequences of discomfort glare are completely understood by anyone who has ever forgotten to wear sunglasses on a sunny day. During and after prolonged exposure to uncomfortably bright conditions, symptoms such as asthenopia, headache, eye and even overall fatigue are common. Individuals with light-colored eyes are generally more susceptible to the symptoms previously mentioned. Lenses which reduce transmittance by 85 percent are capable of reducing the apparent luminance levels of a sunny day to levels commonly experienced in shaded areas (about 1,400 to 2,000 lumens).8
Either fixed tint or photochromic lenses can provide relief from discomfort glare in brightly lit conditions. However, fixed tint lenses have a considerable disadvantage in situations where lighting conditions vary. For example, an individual wearing a fixed tint lens capable of relieving discomfort in full sunshine may experience difficulty when he enters shady areas. In a shady area (such as 2,000 lumens), the apparent luminance will be lowered to only 300 lumens- making activities such as reading a book difficult. Fixed tint lenses which are dark enough to provide relief from discomfort glare are also inappropriate for mesopic (twilight) conditions- especially when operating a vehicle. By adjusting their transmission level, photochromic lenses provide comfort when surroundings have excessive luminance without hindering acuity in low-level lighting.9
DISTRACTING GLARE is a form of discomfort glare perhaps best typified by the distracting reflections from ophthalmic lenses. While such reflections do not commonly decrease visual acuity, they do distract the visual system and may cause transient areas of decreased perception in the visual field. For example, light from the headlights of an oncoming car create glare in the form of "ghost images" of the headlights. Although an AR coating actually increases the transmittance of light from the headlight, the intensity of the ghost image is decreased through a process called "destructive interference." AR coating may also increase visual comfort.
DISABLING GLARE occurs when a light source within the field of view has luminance that is disproportional to the other objects being viewed. The source of disabling glare may either be direct or indirect. A prime example of direct disabling glare occurs when an object in the sky passes close to the sun. The direct luminance of the sun is far brighter than any object in the sky, obscuring any object near it. Another source of direct disabling glare occurs during night driving. Although the ghost images of headlights caused by reflections within the ophthalmic lens is distracting, the actual image of the headlight may have luminance sufficient to cause disabling glare. Although headlights have far less luminance than the sun, they are still dramatically brighter than the surrounding darkness. It is important to note that disabling glare occurs only when the glare source is within 30 degrees of the line of sight. Looking away from the glare source reduces its impact on vision.10 In addition, wearing photochromic lenses, which adjust to varying light conditions, may help reduce disabling and discomforting glare.
BLINDING GLARE is a form of disabling glare, and occurs when glare is severe enough to prevent useful vision of objects within the field of view. A good example is the reflection from a body of water. In this example, light (usually from the sun) is reflected in the form of glare. This glare obscures anything under the water's surface. Since the glare from a flat body of water is polarized, it may be minimized through the use of polarizing lenses. Polarized lenses selectively filter light falling on one particular axis. When the glare is largely contained within this axis, it is eliminated- while leaving the luminance of non-polarized objects relatively unaffected.
Indirect disabling glare also occurs within the eye itself, taking place more outdoors than indoors. The indices of refraction of the various structures and fluids within the human eye are not perfectly equal. As light travels through the cornea, aqueous humor, lens and vitreous humor, light is reflected and scattered. Additionally, as the eye ages intra-ocular disabling glare is increased by the presence of particulate matter and conditions such as cataracts.11 Intra-ocular disabling glare may be reduced through the use of either fixed tint or photochromic lenses, which reduce the amount of scattered light available. Polarized lenses will have little or no effect on intra-ocular glare.
CONTRAST SENSITIVITY One consequence of glare is decreased contrast sensitivity. A 1995 study conducted by an automobile insurance company revealed that driver reaction times were slower in the presence of glare. Contrast sensitivity is crucial in detecting objects within the field of vision.12
Contrast sensitivity decreases with age. A 65 year old is has approximately half the contrast sensitivity of a 20 year old. After age 65, contrast sensitivity decreases rapidly. An 85 year old individual with healthy ocular structures requires six times as much contrast to detect objects. Contrast perception by older individuals in scotopic conditions is further hindered by decreased pupil size. The pupils of an average 20 year old adjust from 4.7 to 8.0mm in size in response to varying lighting conditions. The pupils of an average 70 year old adjust from 2.7 to 3.2mm.13 Of course, the presence of cataracts or ARMD decreases contrast sensitivity even further. Photochromic and fixed tint lenses increase contrast in photopic conditions by decreasing the amount of glare created by scattered light within the eye. Decreases in contrast sensitivity are not limited to older patients, however. For example, decreased sensitivity has also been found in LASIK patients.14 A photochromic lens with a very clear lightened state will provide increased contrast during the day without compromising vision during night driving.
Decreases in contrast sensitivity are not necessarily tied to decreases in visual acuity. An individual with 20/20 vision may still suffer from low contrast sensitivity. The impact of contrast sensitivity on overall visual performance was highlighted by a California Department of Transportation study which indicated that the measurable visual performances with the strongest statistical relationships to accident involvement include low contrast-low luminance visual acuity and disability glare. Accident prone subjects in the test were six times more likely to be involved in an accident, but were largely unaware that there was anything deficient regarding their visual abilities. The study concluded that measurement of low contrast-low luminance visual acuity and disability glare should be included in screenings of older drivers.15 Research has also shown a decrease in contrast sensitivity following LASIK surgery.
NEGATIVE EFFECTS OF ULTRA-VIOLET (UVR) LIGHT
Ultra-violet radiation (UVR) is divided into three categories: UVA, UVB, and UVC. UVR with wavelengths shorter than 320nm (UVB & UVC) is considered to be more photobiologically active than UVA. UVR with wavelengths shorter than 290nm is blocked by ozone in the atmosphere. These boundaries have been used to define the UVB range of 320-290nm. However, there is evidence that photobiological activity extends upwards to the 330-340nm range.16
Top: Blinding glare with regular lenses. Bottom: Same view with polarized lenses.Exposure to UVR does not remain constant throughout the day. On a sunny day, approximately 40 percent of UVR exposure will occur between 11am-1pm. Also, not surprisingly, UVR exposure is greater towards the equator. On a summer day, a sunbather in Hawaii is exposed to twice as much UVR as a sunbather in the northern U.S. Clouds have minimal effect on the amount of UVR exposure- complete cloud cover reduces UVR by only 50 percent, and heavy cloud cover rarely reduces UVR by more than 90 percent. Altitude increases UVR exposure by 6 percent per kilometer of elevation (about 10 percent per mile). Surfaces can also contribute to UVR exposure- water reflects 7 percent of UVR, sand up to 25 percent, and fresh snow has been reported to reflect as much as 80 percent.17,18,19
The effects of UVR exposure are divided into acute (short term) and chronic (long term).
ACUTE Erythema, or sunburn is a common result of overexposure to UVR. Depending upon an individual's skin type and the daily UV index, erythema can occur within as little as 16 minutes. Most sunscreens are not appropriate for application around the eyes, so eyelids are usually left unprotected if proper sunwear is not worn. Photokeratitis, or snowblindness, occurs after over-exposure of the cornea to UVR. It is estimated that as little as 2 hours exposure to snow cover and 6-8 hours at a beach can trigger this painful condition.20
Broad-brimmed hats and large UV blocking lenses afford excellent protection against the acute effects of UVR exposure, but lens color is not a guarantee of UV protection. Clear lenses may block 100 percent of UV light if sufficiently treated. Conversely, darkly tinted lenses may not block 100 percent of UVA and UVB radiation. Transitions Lenses block 100 percent of UV radiation in both darkened and clear states.
CHRONIC Basal cell carcinoma is the most commonly occurring cancer affecting the eyelids- with 75 percent of lesions occurring on the lower lid. Lesions are usually small, firm, and painless with a smooth, pearly or reddish appearance. Studies have shown that children who experience multiple sunburns are more likely to develop skin cancer as an adult.
Pterygium are fleshy growths over an otherwise clear cornea. In 1989, a study revealed that outdoor workers are three times as likely to develop pterygium and six times as likely to develop climatic droplet keratopathy. Cataracts have been associated with UVB exposure in laboratory animals since 1974. In 1988, a study of 838 watermen working in the Chesapeake Bay (latitude 37 degrees north) revealed an increased risk of cortical cataracts (factor of 1.6) when UVB exposure was doubled.21 The correlation was clear and consistent and connected UVB with cortical cataracts only- even though most UVB radiation is filtered by the cornea.
ARMD, or Age-related Macular Degeneration, is the leading cause of blindness in elderly Americans. Although definitive links have yet to be found, the Schepens Eye Institute and the American Macular Degeneration Foundation have reported that exposure to UVR and High Energy (blue) Visible Light may be linked to ARMD.22 It is known that high-energy visible and UV light is associated with free-radical chain reactions in the eye.23
There are many other ocular conditions which can be linked to UVR exposure. Protection from UVR is fundamental to preventing or delaying the progress of all. Once again, broad brimmed hats and lenses providing complete protection from UVR are required. A recent national survey (April 2002), sponsored by Transitions Optical, Inc., revealed that only 6 percent of respondents knew that UVR is damaging to the eyes—even though 79 percent of respondents knew that UVR is associated with skin cancer.24
UVR EXPOSURE & CHILDREN
Left: Discomforting glare with standard lenses. Middle: Same view with sunglasses. Right. Same view with photochromic lenses. Approximately 80 percent of the sun exposure an individual will receive occurs before the age of 18. Individuals with extreme UV exposure between the ages of 5 and 25 are three times as likely to develop skin cancer as individuals with similar exposure later in life.25 Studies have yet to link early exposure to UVR to conditions such as ARMD and cataracts. However, it is notable that the crystalline lenses in children under 10 allow more than 75 percent of UVA and UVB light to reach the retina. It should be obvious that ocular protection against the sun is absolutely crucial to the visual health of children.26 Research suggests that UV damage may be cumulative.27
Unfortunately, while parents have become trained to cover their children's skin with high-SPF sunscreens, most children do not receive the benefit of eye protection. Even worse, many of the toy sunglasses available for children provide inadequate protection against UVR- even though they may appear to have low transmittance of visible light. Children requiring eyewear for visual correction should be fitted with eyewear that block 100 percent of UVR. Since children are likely to misplace or lose a spare pair of fixed tint sunwear, photochromic lenses are ideal for protecting children from UVR. Material selection should be either polycarbonate or Trivex (PPG). Both materials provide excellent impact protection and both are available in photochromic lenses.
CONCLUSIONS
Light is capable of giving sight, and light is capable of taking sight away. Photochromic lenses may enhance vision through refining beneficial light (via refraction) and eliminating destructive light (via selective filtering). Photochromic lenses provide good indoor visual acuity, darken quickly in sunlight to achieve a sunglass-quality tint and block damaging UV rays. By restoring the shadows, fixed tint, polarizing and photochromic lenses reveal the beauty that is light