I would like to know your thoughts on Hi-Index lenses. When the patient is sitting in the Drs. Chair and looking through the Phoropter they are looking through GLASS AR coated lenses with almost 100% transmittance.Many of the high Index lenses have reduced transmittance. How Important is Transmittance? What about ABBE? Weight?
Thanks for your input.
Hi Jay,
You're right about the difference in clarity/contrast, etc...between the phoropter and real life. I'm a big fan of high index lenses but with one condition: they MUST have AR on both surfaces. The loss of light due to reflectance (Fresnell's equation) is to the point that a real high index w/o AR can loose up to 18-20% of light. Add night myopia and night driving to the equation, and the client has a real problem.
My opinion for the best design? High index, aspheric with AR for most lenses with any significant power. But as far as high index without AR, the client would get more light transmission through a regular(uncoated)CR-39 or crown glass lens.
Laurie
Jay,
There are other significant differences between the exam room and "real life" that I find interesting. The exam room (which is kept dark) presents black letters/objects on a well-lit, white background. The patient is looking through the optical centers of the trial lenses contained in the phoropter, and those lenses are only about 30-35mm in diameter.
In "real life," the patient deals with varying levels of contrast and has to deal with the periphery of a larger lens (and all the aberrations that necessarily occur). Is it any wonder, then, that patients sometimes exclaim "it looked clearer in the exam room!"
I'll just echo Laurie's comment- dispensing high-index materials without an ARC is just asking for trouble. I won't sell 1.66 without ARC. Fortunately, with a multi-layer ARC, nearly any material can be brought to >99% transmittance.
Pete
PS- It is unfortunate that Crown Glass has so many negative attributes, because glass lenses do have some of the best optical properties (just look at one in a lensometer, and notice how crisp the lines are compared to some of the softer high index materials).
Pete, you couldn't be more right about glass. It is a shame about the bad rap it gets. I still have a pair of Kalichrome lenses made by B&L 20 years ago. Try to duplicate that in plastic.
Ditto on refracting room vision. I have always noticed that the most errors happen when you are dealing with small amounts of cyl.
As to High Index, I'd sell it over poly any day of the week. But and AR is a must! We just did a gentleman in Hypervue 1.66 with Zeiss AR -20.00 - 1.00 x 180 in the OD and he loves them.
Eyeman,
You have to remember Pete is one of those "I like poly" guys :) .. but I tend to agree more with you when it comes to poly.. other then the ViZio poly I like that one pretty well and have not had anyone give the "usual" complaints ..
You use any of those? .. I prefer the 1.66 version over the poly but if I "have to" I don't mind popping in the ViZio version of poly..
I guess you been around as long as I have, and I hate to see "glass" getting the bad rap as well, optically speaking you are not going to find anything better.. BUT I actually had a tech that has NEVER dispensed a pair of glass lens.. and asked me if they made a "photochromic" lens in "glass".. and was it a Transitions lll or X-tra active :)
Also had another account tech call me to ask me how long they should "heat treat" a glass polarized lens :)
Don't get me wrong, I sure don't miss all the "headaches" on the lab side of doing glass (the sparks from the generator, the combustion of the dust over a salt tank, or that nasty slurry, or that unforgettable snap sound of a glass lens "popping" when treating it :) )
Jeff" miss the quality BUT not the headaches" Trail
Well Jay here in the uk we do a lot of glass hi index work but still a lot of it isnt ar coated ? guess thats to do with the cost, and it seems that in 1.7 index where there are 2 abbe value's std 31 and the newer 42 most still ask for the cheaper 31 av.But the biggest problem with them has got to be the weight mostly the 1.9 index these realy do need to go into a small frame thankfully we only supply up to 65 dia but some still ask can you do a 70 dia blank, and dont think of wearing them without an ar coat.
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Every day a grind
Every week a bind
www.iooi.co.uk
Another "Small" problem, as near as I can tell high index plastic lenses are a one year lens. They have a tendancy to: Have internal matrix (small cracks) break down in 12 to 14 mo. Scratch cote and other coats tend to peal away after the same time period. Haven't quite found a good way to let the patient know about this at the point of sale and still make a sale. Of course, the manufacturers and the O.D.'s and some of the greedier M.D.'s say the Rx expires after one year anyway and that is as long as a lens should last.
Let me now if you have a good way to "forewarn" the patient and still make a sale.
Oh Chip. I'm so glad to finally hear someone else say this.Originally posted by chip anderson:
Another "Small" problem, as near as I can tell high index plastic lenses are a one year lens..............
I'm made to feel by lens suppliers that my patients are the only ones to ever have had problems with 1.6 and their coatings.
The only 1.6 lenses that seem to be free from these problems are original German Zeiss lenses with coatings applied at Aarlen
They obviously know something about coatings that others don't and they certainly aren't passing this info onto other Zeiss makers and coaters around the world.
Coming back to your question has made me think.... why not sell 1.6 plastic as a 'one year lens' at about 40% off with the promise that if they fail within 18 months you'll supply another pair, same Rx, same price or even less... you'll still be on the right side but without all the hassle and shouting.
I wear 2 pairs of 1.66 index "plastic" lenses one pair by pentax that are 3 to 4 years old now and i have no problems with them or the coatings, the second pair are home made buy us and i have no problems with theses either.
Maybe it how folks look after them?
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Every day a grind
Every week a bind
www.iooi.co.uk
There is nothing to keep a Hi index of any kind from lasting more than one year.
Chip if you are not getting you Hi index to last longer than this then you might want to find a new Lab.
All of the newest backside coatings have a hi Beyer rating, Labs not applying these newer coatings are not giving their customers the latest in hard coating technology.
Couple this with one one of the newer AR coatings such as Hoya SMP or Transparence and you have a lens that will normally outlast the Rx.
Don't get me wrong All plastic materials will scratch, but so will glass. Just depends on how you treat them.
There is a theoretical equation which calculates the amount of light reflected at the lens surface based on the refractive index. For a 1.50 index material, regardless of glass or plastic, the maximum transmission is around 92%, for 1.6 it is around 89%, and for 1.7 it drops to only 86.5%. The use of AR (anti-reflective) coatings can restore most of the transmission lost by reflection. The conclusion: index, and not material type, determines the amount of light transmitted. Abbe number is a different parameter, it measures the degree of refraction by the blue end of the visible light spectrum compared to the red end, i.e. the chromatic aberration, or the "rainbow" effect. High index materials normally have higher chromatic aberration, but Abbe number at the same index can vary depending on the chemical structures.Originally posted by Jay A. Little:
I would like to know your thoughts on Hi-Index lenses. When the patient is sitting in the Drs. Chair and looking through the Phoropter they are looking through GLASS AR coated lenses with almost 100% transmittance.Many of the high Index lenses have reduced transmittance. How Important is Transmittance? What about ABBE? Weight?
Thanks for your input.
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