high index

[hagi]

Hi people

Is/are there any method(s) by which a high index plastic lens or mid index for that matter be discriminated from a normal cr39 lens. Is the same also true for crown glass lenses. How about discriminating between aspheric lenses and spheric lenses; what are the criteria in this case.

Is the intermediate power of a trifocal lens almost always 60% of the near power. Can you dispense a trifocal intermediate power according to your wants or is it fixed per lens manufacturer.

What advantages do c seg or round seg bifocals have over d segs.

What is the commonest bifocal dispensed nowadays especially in Briton. I have a gut feeling its D segs.

What explanation is the most appropriate to communicate to the patient when describing the qualities of a varifocal lens. From the little that i know, varifocal near power can be checked by the difference between front vertex distance power and front vertex near power; am I correct.

Finally soem plastic frames are made in slab off form others are injection moulded. I have not a clue what these mean. Could you please clarify.

Thanks people.

Knowledge and Understanding comes to everyone; some embrace it others take no heed. Be amongst the wise.

[chip anderson]

Can't answer it all but.


Mid power in trifocals unless specified is usually 50% of total add.

Round segs have base down prism. Straight top segs have base up prism.

If Round segs are used on plus distance powered lenses (which have base up prism when the patient looks down)the patient should not have to look as far down in seg as the prism in seg and distance will at least in part cancell each other. Also power center of a round seg is always in the geometric center of seg. or half the segment diamter.

If straight top segs are used with minus distance powered lenses (which have base down prism when the patient looks down), again the seg and the distance power will tend to cancel each other.

[Robert Martellaro]

I can a tell poly from cr39 by the sound. Try dropping each onto a glass surface (concave side). Tapping on the lens with your fingernail usually works. Poly seems to be higher pitched with a somewhat hollow sound. The mid index lenses have a similar sound. This doesn't seem to work with 1.6 and up, possibly due to their higher specific gravity. Some lenses are engraved like varifocals. Some of these are marked towards the lens perimeter and are lost when cut. The base curve should be lower on higher index lenses if the surface design is the same. If I can't tell what the lens is, I assume that the lens has a fairly high abbe number and fit them with a similar lens.

Robert

[Jeff Trail]

hagi said:
Hi people

Is/are there any method(s) by which a high index plastic lens or mid index for that matter be discriminated from a normal cr39 lens. Is the same also true for crown glass lenses. How about discriminating between aspheric lenses and spheric lenses; what are the criteria in this case.

Some were already mentioned.. you can always go with "smell" as well, take the lens and turn it like placing a safety bevel on it and certain materials will give a distinct odor (yeeww rotten eggs..:)) and even a distinct cut..ie poly "shaves" away..higher indexes have a powder type look and feel more so than CR39 .. you do enough of the different indexes it becomes easier and easier to spot them.. get you a hand full of samples and sit by a hand stone and give it a try..run the stone "dry" makes it easier to determine by watching the lens..
You can always use a set of calipers and measure CT and ET and get a quick idea based on the power if it is anything other than CR.. and using a good lens clock and clocking the front surface and than the back surface putting those numbers together and see if they "match" up. since light travels slower through a more dense higher indexed material you do NOT need as much of an ocular curve to still achieve the same power.. Keeping it simple..say you have a +6 base front curve..clock the back and have a - 4 and check the power in the lensometer it should be around a +2 (give or take a smidgen for TC) ...you check the power and it is a +2.75^ with those two curves than it would be a 1.60 index... the numbers tell you it "should be" based on CR39.. if it does not add up, comparing cures to power than it is a different index... you have good lens clock (prefer digital) and a sliding graph (are a number out there, I think John R made one as well) and know how to convert curves to sag. (makes it slightly easier) you can pretty well refine it down to an exact index with the right tools :)

NOTE: talking rounded curves like "6 base" is not always correct, you should know the TRUE curve.. say like Essilor "6 base is actually 6.20..Sola "6 base" is 5.92 and on and on..so being a "slight" bit off is expected..say a .12th^ or .25^ if you are doing it the quick way with a lens clock and lensometer..Oh and if you are not familiar with all the abbreviations CT= center thickness, ET edge thickness and TC= true curve

The last part about finding if it is "aspherical or "spherical", I have had a number of people tell me "oh you can check it with a lens clock either they have better equipment than I, or they can really spot a very SMALL shift.. in higher powers (especially hyperops) you can tell the difference "visually" as if you were going to hand neutralize the lens.. a spherical lens will show more distortion than an aspherical.. if you really wanted to go down into the numbers game you could run a couple of formula's and figure out the oblique distortion and plot it out..(not really worth all that time and effort though..if worse comes to worse I always tell my accounts to explain the problem to the patient usually replacing both lens is the easier and safer thing to do if the "curve" type is the problem..BTW if the lens is "aspherical" and it was fit "properly" than the OC will not always be right on the dat. line and the PD should be a mono fit..so dotting the OC's up might also give you a clue...

Is the intermediate power of a trifocal lens almost always 60% of the near power. Can you dispense a trifocal intermediate power according to your wants or is it fixed per lens manufacturer.

Chip got this one.. or atleast in the U.S. 50% is the usual ..Can get variations in say 12x35, 14x35 etc. etc.. and occupational (double D's etc.) ..economics and averages, pretty well determined this.. the less molds you have to make, and lens to stock the more money saved..
Same applies to the decentration of the near in a PAL..it is based on the "average".. not that it will fit everyone.. but the majority..

What advantages do c seg or round seg bifocals have over d segs.

Some have mentioned some of the benefits.. also in "high add" situations this design tends to work better.. since it works more like a small round lens on top of the lens while a D is like a lens whacked into.. the higher the add the more the lip of the add protrudes.. can be a real pain for high add people..flashes of light, even inverting the image seen through the edge of the add (seen this complaint often enough with high add people who tend to lean back and angle down their vision while watching tele..) .. a "cheap substitute" for a slab by mixing the seg types and taking advantage of the seg OC's..
Also in aphakic people they tend to be able to cope with a round seg over a D seg..

What explanation is the most appropriate to communicate to the patient when describing the qualities of a varifocal lens. From the little that i know, varifocal near power can be checked by the difference between front vertex distance power and front vertex near power; am I correct.

That is the harder way of verifying ..this is used more so on fairly strong powers..aphakic's and such.. with the "add" marked by laser marks or "water marks" in probably 99.5% of the PAL's out there it is easier just to read the power off of the lens surface :)


Someone else can grab the "frame" question.. and the type of most popular seg in UK ..

Jeff "cut me do I not bleed resin?" Trail

[hagi]

I have posted about 5 or 6 messages on htis site AND i have to admit its the BESSSSSSSTTTTT site around for optometrists and dispensing optometrists.

YOU guys RRR GRRRRREAT. Thansks for the replies. I really DO appreciate it. Keep up the good work. I am reasonably young at the moment. Hopefully when I have learnt more i will be able to provide some worthwhile input to queries on this site.

ONCE AGAIN PEOPLE TA VERY MUCH(also known as thanks very much):) :D ;)

[Darryl Meister]

Hi Hagi,

Are these questions related to exam preparation at all?

Best regards,
Darryl

[hagi]

That is indeed correct Darryll. But that does not imply that I will cease visiting this site once exams are over. In fact I have firm intentions to keep in touch.

At present I am rather panicky. Exams coming up very soon.

But I feel much relieved with the support given by members of this site.

Thanks

[shanbaum]

hagi said:
Finally soem plastic frames are made in slab off form others are injection moulded. I have not a clue what these mean. Could you

This is really reaching back for me, but: "slab off" is probably not the best term to use for the one method of frame manufacturing in which dies are used to stamp individual frames and temples out of plastic sheets. Instead of stamping, pantographing machines may be used; I think this may now be the more common practice. As I recall, the sheets are around 24 inches wide and three to four feet long, so, you can stamp or cut quite a few frames from a sheet. The rough-cut frames are then skived to provide such contour as the design requires depth-wise, the hardware is inserted using ultrasonic equipement to heat the plastic at the point of insertion, and then the hardware is masked off, so as not to damage other frames during the interesting part, which comes next. The frames are put in eight-sided tumblers about six feet wide and two feet in diameter, one-third-filled with small wooden pegs impregnated with a sovent, and tumbled for two weeks. This rounds off all the sharp edges and polishes the surface - it's a lot like polishing rocks. A bit of touching-off by hand on a polish wheel and, voila: zyl frame.

Injection-molded frames, on the other hand, require expensive precision molds, into which plastic material is compressed at very high pressure, similar to the way polycarbonate lenses are made. I believe (but am not certain, having never made frames this way) that the material starts off in the form of pellets. After molding, I think the common treatment is a coating (as opposed to polishing) to provide luster and protection.

I should add a disclaimer that this is the way it worked - back in the 1970's...

[Darryl Meister]

That is indeed correct Darryll. But that does not imply that I will cease visiting this site once exams are over.

I only ask since these exams will generally require very specific answers.

I'll provide some additional insight below, but you may see some redundancy after the previous replies.

Is/are there any method(s) by which a high index plastic lens or mid index for that matter be discriminated from a normal cr39 lens. Is the same also true for crown glass lenses.

There are some ways that are more obvious than others. Jeff and Robert described several of these. Mineral (glass) lenses will be considerably heavier than organic (hard resin) lenses, of course. Hard resin, glass, and polycarbonate lenses will all have a very characteristic sound when they are tapped against a hard surface. The edges of polycarbonate and high-index lenses will have a slightly different appearance than hard resin lenses, including a yellowish or bluish hue with some lenses. High-index lenses are thinner than conventional lenses for a given lens power, which many experienced dispensers can quickly spot. You may not get much of a scent from them unless the lenses are being processed in some way. You may notice more "color fringing" in the periphery of high-index materials because of the low Abbe value.

There are several optical methods as well, which involve testing the critical angle of the material, using an Abbe refractometer, or other esoteric tests.

How about discriminating between aspheric lenses and spheric lenses; what are the criteria in this case.

For a given power, aspheric lenses are generally flatter than conventional ("best form") lenses, which -- again -- many experienced dispensers can quickly spot (a base curve selection chart will provide some indication). For highly aspheric surfaces, you may notice a subtle change in surface power from the center to the edge using a lens measure (or "lens clock"). However, this test isn't too useful for aspheric surfaces with low amounts of asphericity or for lenses with warpage. A series of lap gauges may also work, though this wouldn't be entirely practical.

There are also several esoteric optical tests for asphericity as well, which require specialized equipment.

Is the intermediate power of a trifocal lens almost always 60% of the near power. Can you dispense a trifocal intermediate power according to your wants or is it fixed per lens manufacturer.

It's actually closer to 50% for hard resin (one piece) trifocals. The intermediate power percentage for glass (fused) trifocals will vary slightly, since it depends upon the choice of high-index glasses that are available for the intermediate segment. (The intermediate power is constrained by the distance and near powers, as well as the refractive indices available for the intermediate segment.)

At one point, certain manufacturers provided intermediate segments that came in alternative percentages. However, these are only available through specialty suppliers nowadays.

What advantages do c seg or round seg bifocals have over d segs.

As Chip pointed out, round seg bifocals can neutralize some of the base up prism induced by plus distance prescriptions, which results in less object displacement at near. The line of demarcation between the segment and major portion is less pronounced than with flat-top segments -- particularly one piece plastic flat-tops (however the line or "ledge" of flat-tops is generally opaque to reduce internal reflections and such).

However, flat-tops are generally superior for other reasons, including less image jump and greater fields of view.

What is the commonest bifocal dispensed nowadays especially in Briton. I have a gut feeling its D segs.

"D segs" or "flat-tops" are certainly the most common segments dispensed in the US, and I would imagine that they make up the majority of bifocals in the U.K.

What explanation is the most appropriate to communicate to the patient when describing the qualities of a varifocal lens.

Generally, the beneficial qualities of progressive (varifocal) lenses include:

1. Cosmetics. There are no visible, "tell-tale" segment lines.

2. More "natural" vision. They provide a variable range of focal powers, including intermediate vision, which conventional bifocals do not offer.

From the little that i know, varifocal near power can be checked by the difference between front vertex distance power and front vertex near power; am I correct.

This is generally the approach recommended by most standards, including the ISO and ANSI standards, and this method is also the least sensitive to errors.

Finally soem plastic frames are made in slab off form others are injection moulded. I have not a clue what these mean. Could you please clarify.

Robert gave a pretty detailed answer about this.

Hope these help!

Best regards,
Darryl

[Mary Deas]

Many of the physicals have been covered. Index of refraction 1.81, 1.6 1.39 1.53 start to effect the Rx in the 2.5 to 3.0 D range. Do the math,(I wish I could quote the formula for you) we typically start to convert vertex as a rule at 4.D.