## Monday, July 29, 2013

### Digital Lens Technology

The coolest thing happening now in eyeglass optics is digital lens technology, also called “free form” or “high-definition”. As with many new technologies, there is much confusion among consumers about what exactly digital lenses are. That’s because there is much confusion even among opticians about what exactly digital lenses are. The voluminous amount of information online about digital lens technology is either oversimplified and incorrect, or accurate but challengingly technical. I’m going to try to bridge the gap with a clear technically accurate explanation that is accessible to everyone, and will hopefully demystify the technology and illustrate when it is valuable and when it is not.

Curvature

The most primary benefit of digital lenses is that they are custom made exactly to each Rx, and do not suffer the compromises endemic to conventional lens manufacturing. Conventional lenses are partially premade by lens manufacturers, and when Rx lenses are produced a lab selects from among the premade materials whichever is most appropriate for the Rx and then grinds the prescription as closely as possible into the material. However, every unique Rx requires a completely unique curvature of the lens for best correction. When premaking materials, manufacturers cannot economically produce every curvature for every Rx, so they produce a dramatically reduced selection of curvatures that are satisfactory for most Rxs. But satisfactory is far from perfect. Some of the optical distortions and aberrations one experiences with conventional lenses, such as the “fish bowl” effect and blurry vision at the peripheral edges of the lenses, are because the curvatures are just not perfectly matched to the Rx.

For example, your particular Rx may require a 3.25 curvature for best correction. However, the manufacturer may only produce lens materials in 2.00 and 4.00 curvatures, so the lab must select from those. They would round up and use the 4.00, but this rounding constraint guarantees optical error in the lens fabrication even before it has begun. It’s close, but not right.

Digital lenses are produced by a digital lens fabrication process that eliminates the constraints of premade materials. The advanced machines employed in the process cut custom curvatures to exacting specifications as low as 0.01 diopters, and can produce the aforementioned 3.25 curvature, or any other that is called for, with extraordinary accuracy. This enhances visual comfort by eliminating the distortions of merely “close” curvatures, as well as preserves optical clarity in all parts of the lens providing clearer peripheral vision than conventional lenses.

The benefits of digital processing are compounded when astigmatism is involved. To correct astigmatism lenses have two prescriptions ground into them at cross directions. Since every unique Rx requires a unique curve, lenses that correct astigmatism require two unique curves. Conventional lenses that correct astigmatism are still made from the single premade curve selection, so they will err due to rounding in both directions. If the astigmatism is high, it becomes impossible to even come close to proper curvature for both prescriptions with a single premade curve, and wearers end up with lots of distortion and peripheral blur. Digital technology enables perfectly accurate curves for both prescriptions to be fabricated in the lenses, providing high astigmatism patients better vision than possible in conventional lenses.

This custom precision curvature, often called "optimized" as in the curves are optimal for any given Rx, is the simplest level of digital customization. Examples of this technology in single vision and progressive are the Essilor 360 and Varilux DRx line, Shamir Spectrum, and Zeiss Choice Series.

Position of Wear

The Rx of a lens depends on the curves cut into the lens, but also depends on the distance of the lens from the eye as well as the tilt and angle of the lens relative to the eye. The Rx from the doctor assumes a final spectacle lens that sits at the same distance from and angle of tilt relative to the eye as the lens in the exam room. The reality is that there exists a huge variety of frame shapes, angles, and sizes, of which virtually zero replicate the exam room lens position. Therefore, a lens conventionally fabricated to the Rx as written and put into a frame will not exactly reproduce the vision experienced in the exam room. Even the digitally curvature optimized lenses I just talked about only perfectly replicate the doctor's Rx if the lenses are held in exactly the same position in front of the eye as the doctor's exam room lens was.

This is where digital technology gets really cool. Since this fabrication method frees lens makers from the constraints of prefab materials, and the extremely prescise machines can cut just about any unique surface we can imagine, we can customize the lens in ways never before possible. We can measure the distance from, and angles relative to, the eye that a particular frame holds the lens on a particular patient to gather “position of wear” information. Every combination of frame, face, and Rx produces a completely unique set of parameters, and once the data is gathered a uniquely personalized lens is computer modeled to take all those factors into account, and then precision machines individually custom fabricate lenses to those exact specifications. The result is a lens perfectly designed for that patient's Rx, in a particular frame, fit to that patient's particular face, which exactly replicates the doctor's exam room Rx. Anything short of this level of customization will not perfectly recreate the exam room vision. Examples of these fully customized digital lenses are the Essilor Fit Single Vision and Varilux Physio Enhanced Fit, Shamir Autograph, and Zeiss Individual.

Who Should Wear Digital Lenses?

Anyone can wear digital lenses, but some will get more benefit than others, and some will perceive no difference. Patients with mild Rxs, in frames that are fit at a proper distance from the eye and have moderate tilt and wrap, will experience little advantage from the digital technology explained here. As Rxs increase in power, in complexity with astigmatism, and frames deviate from “classic” fitting parameters, conventional lenses will have more error and digital technology becomes more beneficial. Personally, I evaluate every patient individually on their Rx, the frame they select, and how that frame fits them to determine if they should wear digital or conventional lenses. Conventional lenses do fail to exactly replicate the exam room Rx by a calculable margin, but for some Rxs and patients the degree of error is so low it is below the threshhold of perception, i.e. the perceived difference between the conventionally corrected vision and digitally customized vision is zero. If the perceived difference is zero, the benefit of digital lenses is zero. When the frames deviate far from the classic fit parameters, most typically in highly wrapped sport sunglasses, or when Rxs are strong or have moderate or higher astigmatism, that's when digital lenses shine and can provide crispness and clarity of vision never before possible.

As always, find a good optician to design your eyewear. They will counsel you on the particular benefits of digital lens technology for your needs, and ensure you always get the best vision possible.

Want to geek out about this technology and get into gritty detail like I do? Check out these articles by two incredible Master Opticians:

Mark Mattison-Shupnick, ABOM, “The ‘...IZE’ Have It.” 20/20 Magazine, May 2011,
http://www.2020mag.com/ce/TTViewTest.aspx?LessonId=107609

Darryl Meister, ABOM, “Zeiss Individual® Single Vision.” Carl Zeiss Vision, 2010,
http://vision.zeiss.com/content/dam/Vision%20Care/Vision/en_us/PDF/ECP/ZEISS_Individual_SV_White_Paper_15630.pdf

## Thursday, January 3, 2013

### Fun with Antiques

I had the opportunity to make some cool antique spectacles: rimless turn-of-the-century pince-nez style.

They were a lot of fun and work with the custom mounting I did. The customer is a fan of vintage and antiques, and plans to use the pince-nez pairs as "hat and tails" attire.

 Teddy Roosevelt was an iconic pince-nez wearer.
 Morpheus revived the coolness of the style.

 Original 1.5mm thick glass lenses. The clamps on either side sit flat against the lens surface, and screw connects the clamps through a hole in the lens to keep it securely in place.

 My new polycarbonate lenses were thicker (necessary for proper curvature and better vision) so I cut grooves so the clamps could be recessed into the lens. Notice here the clamps are almost flush with the lens surface, not sitting on top of it.

 The full metal round pair was a third for the same customer: a nicely patinaed WWII-era antique. This one was cake compared to the pince-nez.

## Saturday, December 29, 2012

### Chemistrie Magnetic Sunglasses on an Extremely High Power Lens

I make a lot of Chemistrie custom magnetic sunglasses. The claim of the technology is that it fits any eyewear, but I don't think they were envisioning this when they said that. Still, I made it work

A recent customer of mine had an extremely high power Rx, so strong that to mitigate the thickness on the lens some of the power had to be split onto the front surface, which made the lens bi-concave (concave on both front and back surfaces, instead of the usual concave back and convex front of most lenses). That's all fine and good, I've dealt with such powers and lenses before. But this customer also wanted the magnetic sunglasses, and the technology certainly wasn't designed with this lens in mind.

The basic Chemistrie lens is a simple polarized film laminated between their "Hivex" optical material; the front and back sides are identical, differentiated only by curvature. Since the lens is reversible, I just thought why not turn the lens around to have the convex surface match the concave lens front? The sunglasses will be the same concave front curve as the Rx lenses.

Extremely high power eyewear like this is never elegant, but I think they turned out pretty neat. It's  cool unique solution that this customer could only get from a few opticians, and I'm glad I was able to do it.

## Sunday, September 30, 2012

TopTenReviews.com has an article reviewing ten of the most well-known sites for buying glasses online. They give good details for each of the sites, but they understandably lack any professional advice to distinguish among the sites.

Something to look for when shopping glasses online, as with anything, is brands. Brands aren't just overpriced names for the same stuff. They are quality products marked with a reputable name to guarantee your confidence in the product.

The cheapest sites have no recognizable brands, which means there is zero guarantee to you of the quality of the product. Sure they're cheap, and most the sites have 100% refund policies. But when the frames won't fit or hold adjustment, or the lenses seem to strain your eyes, produce weird "fish-bowl" effects, or just seem hazy, what are you going to do? If you paid \$10, even \$20, are you going to contact customer service for a return authorization, package them up, ship them back at your expense, and wait for the refund, all the while without new glasses? Probably not, you'll probably just throw them in a drawer and try again (or just throw them away, as I did when I bought a couple pair online a few years ago just to see how they turned out. They were crap).

The no-brand materials the lowest-price guys use for frames and lenses are so ridiculously cheap, bought in bulk for literally pennies from Asian factories, that much of the time even if you return them for your money back they still turn a profit on the shipping! They have absolutely zero incentive to provide you quality stuff because it costs them more to take the time and use the materials to get it right than it does to churn out poor junk, half of which could be wrong, and just accept the shipping profits on the returns. If you absolutely don't care about the quality of your glasses, go ahead and throw a few bucks around and see if maybe someone accidentally makes you a useable pair. Even a stopped clock is right twice a day.

Not all glasses you buy online are crap. You can get perfectly good quality eyeglass materials online, the same level of quality you get from good opticals, even boutiques. (Note I said materials. The physical product can be the same, however the measuring, product design & selection, and frame fit can all be wrong without a optician's expertise.) The way to be sure you are getting quality is brands.

Several sites in the review sell good brand name frames, which guarantees they were made at least by a slightly decent manufacturer. Framesdirect, GlassesUSA, and Glasses.com all sell the same  kind of brand frames you'll find in mall retail opticals and eye doctors' offices. Of those Framesdirect is by far the biggest and best, with massive selection and a price match guarantee for many of the best designers. If you are going to buy a frame online, buy it from Framesdirect.

However, frames are only of secondary importance for glasses. Is is better to like your frames but not see well, or see well but not like your frames? Neither is good, but the whole point of glasses is seeing better, right? So while frames are more fun to pick out, you really need to think about and put some priority into lenses.

Not all lenses correct vision equally. Poor manufacturing creates lenses with inconsistent prescription and haziness. Vision simply will not be comfortable, clear and crisp even when the prescription is accurate. The only way to know you are not getting \$0.15 per pair Asian special lenses is to buy branded lenses, the same used by reputable opticals in the U.S. Because few consumers know anything about spectacle lens brands, this is where almost every online seller skimps. Some claim to use quality brand lenses, and may do so, but they don't make it clear what you are getting. Framesdirect is great for frames, but they are not clear on their lens brands. They charge enough to suggest they are using quality materials, but there is no guarantee of what you get. Only one site in the review specifically lists it's brands, and they are all good: Eyeglasses.com.

Look at their sample price list above. Essilor, Crizal, Orma, Thin & Lite, these are brands that guarantee you are getting quality produced materials. Note the prices. No \$6.95 glasses here. Because you can't make decent glasses at those prices. However, if you compare to retail stores and doctors' offices, the prices above are excellent and for the very same materials.

These best resource I've found for quality spectacle lenses online isn't in the review, probably because they don't sell frames. Eyeglass Lens Direct only sells lenses, and is and awesome source for most of the high quality spectacle lenses available in the U.S. today. You can select lenses by specific manufacturer, and then by specific lens designs and models. Through this site you can get some of the most technologically advanced and best quality materials there are, products no other online seller offers. The amount if information is daunting and can be impossible for the consumer to decipher, because most of the discrimination among brands and lens designs is the purview of the professional optician. However, because Eyeglass Lens Direct specifically states the products you a re purchasing, you can Google the names and research yourself. Every brand has an informational site to explain their particular benefits.

•  Research and buy the frames you want on Framesdirect.com
• Find a good optician willing to make measurements and guide you to the best lens option for your Rx and frame. Pay him for his professional expertise and time. Don't be a dick. Want my suggestion on a fair price? \$30-\$60 depending upon your needs and his skill.
• Buy the lenses from EyeglassLensDirect.com
You now got the best price on frames, a great price on guaranteed quality brand lenses, and if you found a good optician and compensated him adequately for his skills, you got the same accuracy and guidance you would have if you had purchased eyewear from him.

## Tuesday, June 5, 2012

### Measuring Your Own Pupillary Distance

Pupillary distance (PD) is the fundamental optical measurement for eyeglasses. It's the distance between the pupils of your eyes. This is important because every prescription optical lens has an optical center. I'll let Allaboutvision.com explain it:

The optical center of your eyeglass lenses is the part that gives you the truest vision, and it should be directly in front of your pupils. To determine how to place the lenses in your frames so the optical center is customized for your eyes, the eyeglasses lab needs to know the distance between your pupils, or PD.
It can be tricky to measure your own PD, somewhat akin to trying to cut your own hair. Dispensers need lots of practice to be able to measure PDs correctly, and even experienced opticians have difficulty taking their own in a mirror.
Most online optical providers present you with several ways to go about this important task. Some suggest the simplest method, which is to have your prescribing eye doctor or an optician take the measurement for you. Alternatively, they offer step-by-step explanations of how to take your own PD in the mirror, or how to have a friend take it for you.
There is only one reason you would your want your own PD measurement: your want to buy glasses online, without the expense of an optician's expertise. Ok, fine. I'm a professional optician and I have reasons to hate that people do this. But I just love opticianry. What irritates me more than the erosion of respect for and practice of professional opticianry are the endlessly reused wrong methods of obtaining an accurate PD perpetuated all over the anything-for-a-dollar online glasses websites. So I'm going to tell you why their instructions are wrong, and then I'm going to tell you simply how to do it right.

There are two systems proliferated online to obtain your own PD: manually measure yourself, or via software manipulate an uploaded photo of yourself to produce the data.

To measure yourself the common instructions are (courtesy of random online glasses site justeyewear.com)
Have ready: a straight ruler*, a pencil, and a mirror
1. Facing the mirror, place the ruler on the bridge of your nose, bringing the start of the ruler directly below the center of one eye’s pupil.
2. Looking straight into the mirror, hold the ruler (keeping it steady and parallel to the floor!) and mark the location on the ruler of the other eye’s pupil.
3. Measure the distance in millimeters between the two marks. This is your PD.
4. Repeat this process a couple of times to ensure you have an accurate measurement.
If you have a willing assistant, you can simplify the process even further by having your friend measure your PD using a ruler. (Your job will be to stand still.)
Whether measuring with someone or alone, be sure to hold the ruler steady and parallel to the floor!

 Does this inspire you with confidence or what?

You may get close to your actual PD using this method, but prescription eyewear is neither horseshoes nor hand grenades, so maybe a more precision method is called for.

The second method is to upload a photograph of yourself with some sort of size reference device in the photo. This was initially developed as the iPhone app Pupil Meter using a credit card as the size-reference device. Since most credit cards are fixed size, once you know the size of the card one can calculate the distance between the pupils in the image.

The idea is sound. It's a simple ratio equation. But the app is notoriously flawed and inaccurate. Witness it's awesome 1 star rating and the many uncomplimentary comments.

The system caught on, though, and websites developed their own more sophisticated versions of the same measuring system. I've tried a dozen and the best-designed one I found is this PD Self-Test. This uses you own current glasses as the reference device. You just measure the size of the glasses and highlight your pupils in the image using their slick interface and it does the calculation.

I tried it out. Eight times. With eight different photographs. It measured me from a 61mm to 65mm PD. So call it accurate within ±4mm. How bad can that be?

The whole point of getting a PD measurement is to have the optical center of the lens centered in front of the eye.  If it is decentered, that induces prism: "A lens with prism correction displaces the image, which is used to treat muscular imbalance or other conditions that cause errors in eye orientation." Double vision and focusing difficulties can be caused because your eyes don't work together perfectly, and prism can be prescribed by doctors to correct that and force your eyes to focus together properly. However, prism that is not prescribed and unnecessary will do the opposite: it will create double vision and focusing problems.

So unnecessary prism is bad. But nothing can be measured perfectly, even the most accurate eyewear has some margin of error. How much is too much? Well let's figure out what a pair of glasses I would order myself using the above PD Self Test measurements would be like. If my PD ended up being decentered by the test's 4mm error how much prism would I get?

The Prentice Rule is a simple optician's equation to calculate exactly that. The formula is:
$P = cf$
P is the amount of prism in diopters D
c is decentration in centimeters
f is lens power in diopters D

We are looking for the prism P. I know the decentration is 4mm, which is 0.4cm. My Rx lens power is a moderate 2.5D (-2.50, but signs don't matter for this calculation). P = 0.4*2.5 = 1.0D. To put this in perspective 0.25D is typically the smallest increment prescribed by doctors, generally because it is the smallest amount that will have noticeable impact on vision in most people. 1.0D is four times that. That is literally suffering four times the prescribed dosage. The ANSI standards for prism deviation in a spectacle lens is no more than 0.67D, or at slightly higher power ±2.5mm. The lenses made off that PD calculation at 4mm decentration and 1.0D fail both standards and are worthy only of the garbage bin.

Here's a kicker. My actual PD, measured many times by every method from a ruler held to the face to the Visioffice digital measuring system, is 69mm. The online calc didn't even come close. If I had used it's average result of 63mm, that would not possibly, but exactly cause my resulting glasses to be decentered 6mm, inducing a whopping 0.6*2.5=1.5D unnecessary prism, more than double ANSI standards. Those glasses would certainly cause double vision, strain, and focusing failure.

How can the system be that off? Well, because you have two PDs. When you focus on objects within 20 feet, your eyes converge and the distance between your pupils decreases. How far away is the cameraphone or webcam you're using to take the image for the online systems? 2 feet? 4 feet? Those systems are measuring your near PD, not the 20+ foot distance PD you want in your glasses. They don't even get the near PD right either.

 Look at this handy diagram! It's depicting cameras, but the concept is the same for eyes.

There is no accuracy to online PD systems. It is improbable that glasses made from theses PDs even accidentally end up being correct.

So you've hung out through my lecture and you want to know how to do it right. It's ridiculously simple. I don't know why no online opticals recommend this. I'm sure people simply assume a slick looking digital system like the one above is more accurate than any manual system, and are inspired with confidence in their cut-rate online supplier. But manual is simply the best for doing this on your own. Here it is:
1. Wear your glasses. (Or any glasses if you don't have your own. Even just try on demo glasses at the mall if you are that bold.)
2. Have a felt-tip maker handy.
3. Focus on a single object in the far distance (anything farther than 20 feet works, but farther is better).
4. Raise the marker to your right lens and precisely put a dot on it directly over the distant object.
5. Repeat for your left eye. If done correctly, with both eyes open the two dots should overlap into a single dot over the distant object. If not repeat making the markings until they do form a single dot.
6.  Measure the distance between the two dots on your lenses with a millimeter ruler.