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Archive for June, 2011

How The Lenses of Eyeglasses Are Made

The function of an eyeglass lens is to bend rays of light so that when they enter the eye they focus perfectly on the eye’s retina. This idea of using lenses to improve faulty vision goes back many centuries. While the technology, equipment and precision used in the process of making lenses have changed exponentially in recent years, the basic steps in creating a prescription lens have remained largely the same for the past 50 years. Here’s how eyeglass lenses are made: 

Start with Lens Blanks
The process of creating a prescription eyeglass lens begins with large buttons of lens material that look like clear hockey pucks. These lens blanks are typically injection molded and mass produced using a variety of plastic resins. The front side of these lens blanks have set curvatures that define the nature of the lens. Single-vision lenses, bifocals, trifocals and the myriad of progressive addition lenses each have a unique front surface. Lens manufacturers often put millions of dollars into engineering the precise front curvatures of their progressive addition lenses (just in case you were wondering why your eyeglasses cost so much).

Step 1 – Surfacing
Once the technician selects the proper lens blank, he mounts it to a carrier block made of wax or lead alloy and inserts it into a lens lathe. Using computer settings dictated by your eye doctor’s prescription, the lathe shaves layers of material off the back side to create very precise curvatures and the desired thickness. It is the combination of the front side curvatures and the back side curvatures that determines how the lens will bend the light rays to correct various degrees of myopiaor near sightedness, hyperopia, or far sightedness, presbyopia, or reading vision, and astigmatism.

Step 2 – Fining
When the lens comes out of the lathe, the back side is somewhat rough. This gives it a frosted appearance. The technician now puts the lens on a fining machine, which uses a precisely tooled polishing block, called a lap, to polish the back side of the lens. The convex side of the lap is covered with a pad that is impregnated with a fine grit and is then mechanically agitated against the concave surface of the lens until the lens is totally clear. To picture this, make a fist with one hand, and then drape your other hand over the top of it and rotate your palm. That’s the fining process.

Step 3 – Edging
Now that the prescription has been ground in and polished, the lens is much thinner than when it started, but it is still the diameter of a hockey puck. The next step is to cut the lens down to size. In most labs today this is done with a computerized lens edger. The newly surfaced lens goes into the edger along with the frame you have selected. The edger uses a digital tracer to capture an exact three-dimensional image of the frame and then, using the fitting measurements provided by your optician, the edger passes the lens over a diamond cutting wheel until it is reduced to the proper size and shape.

Step 4 – Coatings 
Now the lens is ready for finishing touches. For example, lenses can be specially coated to resist scratching, to block ultraviolet light or to reduce reflections. Anti-reflective coatings are a rapidly growing segment. In dust-free coating equipment, the lens typically receives up to 16 ultrathin layers of metal oxide coating. These various layers combine to dampen glare, repel water and sometimes repel oil.

The Future of Eyeglass Lenses is Getting Clearer
While the vast majority of eyeglass lenses made today follow the procedures above, new technology is entering the market. Knowledge developed in the refinement of LASIK eye surgery is rapidly making its way to lens laboratories. The same concepts that doctors use to custom “carve” a patients cornea during LASIK surgery can be applied to plastic lenses. This emerging science, known as wavefront technology, will ultimately make lens blanks a thing of the past. New wavefront lens lathes will custom carve the front and back surfaces of lenses, resulting in greatly reduced visual distortion and improved lens clarity.

To learn more about eyeglasses, eyeglass lenses, proper eye care, eye wear and Shawnee Optical, please visit our website at www.shawneeoptical.com.   

Is Shining a Flashlight into Your Eyes Damaging?

Like the majority of us, you have probably stared into the beams of a shining flashing at point blank range when you were a kid. If you are a parent, you may have caught your son or daughter doing the exact same thing. The apple doesn’t fall far from the tree!

So did your or your child’s encounter with the bright light damage your eyes? With typical outputs of 10 to 20 lumen, flashlights pose little or no threat to the eyes. One reason for this is the built-in defense ability that the eyes possess. Through the coordinated efforts of the pupil, iris, retina and the optic nerve, the eye can adapt to a variety of light conditions, including a flashlight. Here’s how your eyes work to protect you from these acts of curiosity.

Your Pupils
The pupil is the portal through which light and images enter the eye. It varies from 3mm to 9mm, depending on conditions. This opening is covered by the cornea. It only looks black because the internal tissues of the eye, which would otherwise be visible, absorb almost all light.

Your Iris
The iris, the colored part of the eye, is like the retractable sun screen of the eye. In dim light, dilator muscles in the iris contract and open the pupil to allow more light in. In bright conditions, such as that flashlight shining in your face, sphincter muscles in the iris contract to constrict the pupil and block any potentially damaging or uncomfortable light.

Your Retina and Optic Nerve
The retina is the nerve center of the eye and the optic nerve is the information conduit to the brain. Together, they control this protective mechanism, which is called the pupillary light reflex. When the rod and cone receptor cells of the retina become overstimulated with light, the optic nerve triggers the sphincter muscle and the pupil constricts.

The Fall Back Plan
If the offending light is too strong, or the eye needs a few extra seconds to make the needed pupillary adjustment, our brain calls in the big guns–the palpebrae–also known as the eyelids. An impulse shoots to the obicularis oculi muscles and the lids slam shut involuntarily.

Dilated Eye Exam
If you’re curious about what life would be like without the pupillary light reflex, call your eye doctor and schedule a dilated eye exam. During a dilated exam, the doctor uses medicated drops to immobilize this light reflex. This allows him to keep your pupil wide open while he uses his lighted instruments to take a good look at the back chamber of your eyes. When the exam is done, it often takes an hour or two for the drops to wear off, and if you walk from the dim exam room into bright sunlight you’ll probably get a quick lesson in light sensitivity. It should make you appreciate just how good your eyes normally are at controlling light.


To learn more about proper eye care, eye wear and Shawnee Optical, please visit our website at www.shawneeoptical.com.   

The Unavoidable Facts About Bifocals, Part 2

Treating Presbyopia (Eye condition which necessitates bifocals) with Contact Lenses: 

For years contact lens wearers considered the onset of presbyopia to be the time to make the forced switch fromcontacts to eyeglasses. Many are finding that this need not be the case. While contact lens manufacturers scramble to be the first to perfect and market a functional bifocal contact lens, the public has not embraced them in huge numbers. Meanwhile, more and more contact lens wearers are finding that they can treat their presbyopia with Monovision contacts.

Monovision doesn’t denote any special type of contact lens, but rather a special type of contact lens fitting. With monovision, the doctor fits you with a contact lens to correct one eye for distance vision while fitting your other eye with a contact lens for near vision. While this arrangement may sound unorthodox, monovision has been proven to be effective. Erie, Pennsylvania Optometrist James Piersol estimates, “Over eighty percent of my presbyopic patients who try monovision are ultimately successful with it. It normally takes one or two fittings to get the power balanced  properly, but once you do, the patient rarely can tell that each eye is performing a separate function.” Monovision is actually not as far-fetched as it sounds. Dr. Piersol explains, “The brain is always receiving two separate images from your eyes. It normally takes the two similar images and fuses them into one three-dimensional image. With monovision, the brain receives two very different images, one clear only at distance the other clear only at near. Since it can not fuse them into one picture the brain simply chooses between the two images based on the immediate task at hand, much like a director choosing between two camera angles”.

Skeptical? Try this experiment. Take a paper towel tube and with both eyes open look at your television screen. Now alternately close your right eye and then your left. The image from your dominant eye (the right for most people) will hardly move at all while the image from your non-dominant eye will jump noticeably. This is because as you sit there watching TV your brain is “choosing” the image from the dominant eye. If you were being fit for monovision the dominant eye would be fit for distance and the non-dominant eye would be fit for reading.

While monovision contacts do offer a proven and cost effective treatment for presbyopia, they also have a down side. Since your brain would be receiving only one image for distance and only one image for near, there is a resulting reduction in depth perception. This can be a critical problem for some people’s occupational requirements.

Treating Presbyopia with Surgery:

One reason for the growth in monovision contact lens fits is the rising interest in corrective eye surgery. While the public continues to warm to the idea of LASIK and other surgical procedures to correct distance vision, a medical correction is now emerging for near vision. It is called conductive keratoplasty, or CK. CK is a non-invasive procedure where radio waves are used to reshape the cornea in order to improve focus at near. The procedure is quick, relatively painless and a bit pricey. Because CK is done on only one eye the final result is very much like monovision without the contact lenses. That being the case, most people prefer to test their tolerance for monovision with contact lenses before moving on to the permanence and expense of CK.  Since presbyopia advances with age anyone considering CK needs to understand that additional procedures may be required every 3 to 5 years to keep up with their increasing presbyopia.

In the first 200 years after Ben Franklin invented the bifocal there were virtually no changes. Your father’s bifocals were very much like your great-great grandfather’s bifocals. By contrast, in the past 5 years over 176 differentProgressive Addition Lenses (PALs) have hit the market.  Each one is somehow superior to those that came before. The technology is improving exponentially.  Before too many years pass we’ll see doctors inject soft polymer plastic into the eye’s lens sac and thereby restore the lens’s flexibility. At that point it will be bye-bye bifocals.

To learn more about proper eye care, eye wear and Shawnee Optical, please visit our website at  www.shawneeoptical.com.  

The Unavoidable Facts About Bifocals, Part 1

On the list of unavoidable suffering, well behind death but only slightly ahead of  taxes, you’ll find bifocals.  For most of us, bifocals serve as a warning shot that middle age is hot on our heels. It’s a rite of passage that 95% of Americans over the age of 45 have experienced and in the next few years nearly 78 million baby boomers will join the Bifocal Club.

The eye condition is called presbyopia. The word is Greek for “old man’s eyes” but the roots of the condition actually start much earlier than the name implies. As teens, our eyes stop growing, but the lens in the eye continues to generate new cells. As a result, the lens gets progressively denser as time goes by. This increased density results in a stiffening of the lens. By the time we reach our mid forties the lens is so stiff that it can no longer make the transition from distance vision to near. Many of us will try to put off the inevitable by holding the newspaper further away or pushing the computer screen a little further back. Eventually, however, we find ourselves at the eye doctor’s office holding our first bifocal prescription.

Treating Presbyopia with Eyeglasses

Like our grandfathers, grandmothers, fathers and mothers, most of us will fight their emerging presbyopia with eyeglasses. Unlike our parents and grandparents however, many of us want nothing to do with unattractive bifocal lines. This cosmetic concern has made no-line bifocals, more accurately known as progressive addition lenses (PALs), the new clear-cut lenses of choice, but there’s more to PALs than just a pretty face.

“What consumers want,” notes Carol Norbeck, spokeswoman for the Vision Council of America, “is to see as if they never got old. That’s a tall order but progressive lenses are coming closer and closer to that goal”. Our parents’ traditional lined bifocals had two distinct prescription zones. You were either looking through the distance portion or through the reading segment, and when you passed from one zone to the other the image would suddenly change in size.  This is called image “pop”.  Functional? Yes, but it didn’t feel anything like normal vision. Today’s progressive lenses, by contrast, offer a gradual transition from distance to near prescription. The lens provides clear vision at  distance, near and points in-between with no image pop.

    In  PALs introduced in the 1980’s and 1990’s, the trade-off for the lineless design was a pair of  bothersome blur zones on either side of the transition corridor. Today, these older designs are called “hard progressives”. More recent lens designs, called “soft progressives” utilize advanced mathematical formulas and computer graphing to create a much more subtle visual transition from distance prescription to reading prescription. The blur zones have been greatly reduced and pushed to the periphery of the lens where they are safely out of the normal field of vision. The result is vision that feels very natural.

Shawnee Optical Consumer TipIn today’s optical marketplace hard PALs often sell along side the newer soft designs at surprisingly similar prices. Make sure you are being fit with a soft progressive lens. If your optician doesn’t know the difference find another optician.

To learn more about proper eye care, eye wear and Shawnee Optical, please visit our website at www.shawneeoptical.com.

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