Hello.
please i need to know more about the meaning of Base curve in the attached photo ,in the photo
the base is 450 ,so what is the meaning of this base and which base referenced is1D,2D,3D, OR 4D
PLEASE EXPLAIN .
Thanks
Base curve's Radius
Hello.
please i need to know more about the meaning of Base curve in the attached photo ,in the photo
the base is 450 ,so what is the meaning of this base and which base referenced is1D,2D,3D, OR 4D
PLEASE EXPLAIN .
Thanks
I think it means that the radius of curvature of the "eyewire" portion of the frame is based on 4.5 diopters.
F (diopters) = [n' - n]/r (M)
I would assume n = 1 and n' = 1.50
ergo r = 11.11111 cm
Someone ought to check me on that...
Thank you very much but can you give me more details,give the numerical example please
thanks again
If I'm not mistaken, a +4.50 base curve "frame" is fabricated to hold a +4.50 base curve lens.
But the units of a lens is in diopters, which doesn't help you with mechanical fabrication, so you want a radius of curvature.
So, the question becomes: "what is the radius of curvature of a +4.50 base curve lens"?
In the example above, I was trying to show the radius of curvature of the front surface of a +4.50 base curve lens, if I did the math right (maybe n' should be 1.523 for a glass lens: I used plastic. Some help on that would be good. If so, r = 11.6 cm).
i think you are right about ,a +4.50 base curve "frame" is fabricated to hold a +4.50 base curve lens. a +4.50 base curve "frame" is fabricated to hold a +4.50 base curve lens.
thanks my friend
I'm not sure whether 0.5cm is important or not. I'd assume that the longer radius is the correct answer unless a better poster can confirm otherwise.
Any way its approximately answer but really im understand the meaning of this value from your discus,and may other friends here can be add more info about this.
Thank you very much
Radius = (.53/{base curve})*1000
Radius = (.53/4.50)*1000
Radius = (.11777)*1000
Radius = 117.77 mm
Index of refraction is for standard glass (1.530) and is true for most dioptric measurements.
The "base curve" of a frame being ~ +4.50 is true for "most" standard spectacle style frames, however, the lens that can be edged to fit will "mostly" have a different base curve. The 4.50 "standard" was developed to match an average face fit. Most modern edgers will trace the 3D curvature of the frame bevel and come up with a "best fit" to match the lens curvature and the frame bevel curvature.
(let's not confuse the issue with pantographic tilt please).
Yes , the photo for spectacle style frame and i have noted that most frame have the base 450 ,but why this value was wrote 450 not 4.50 please explain the different.and what about sun lens base 6D PLEASE EXPLAIN IN THE SAME MANNER .
THANKS
Shorthand is my guess. Someone was not being careful to write the decimal, or assumed the reader would understand that a decimal was after the 4.
A 6 base sunglass lens will still fit into a 4 base frame. The edger slightly modifies the curve of the bevel on the lens, and the frame conforms slightly so they line up.
please recommend the best book for more information about this issuesOriginally Posted by MikeAurelius
thanks
I'm not sure there is one. You would have to contact the edger manufacturers to see if there is a common reference book.
thank you my guys
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
Good job, Mike.
What's n= 1.523?
Crown Glass.
In the US and a few other countries, a "shop index" of 1.530 has been used traditionally, to describe lens, lap, and frame curves.
In the late 1800's, the glass lenses of the time had an index of refraction of 1.530. Afterwards, 'crown glass' was developed and it has an n= 1.523. To save costs, the manufacturers of glass lenses collectively decided not to retool their equipment since the new crown glass material caused only about 0.06D error. So the 'shop index' of 1.530 was retained and the Diopter/radius of curvature formula reflects this old 'shop index'.
The reason for the change, according to my Benson Optical Opthalmic Optics course on index of refraction, is that the glass manufacturers discovered that they could not reliably produce glass of 1.530 index and would have to change to another standard. They found that they could achieve an index of 1.523 reliably and this became the new standard for the optical industry.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
The difference between 1.523 and 1.530 is taken into account (in the case of glass lenses) with the difference between a *marked* base curve, and a *true* base curve taken by sag readings.
A typical 6 base glass semi-finished lens will be marked 6.25, but will sag somewhere between 6.19 and 6.21
This allows the shop tools to be cut (adjusting for any padding thicknesses) to exact 1.530 1/8th diopter curves, and the power will come out dead on. Plus power lenses over about +2.50 will need additional compensation for thickness, but that's for the software to calculate.
LOL...not quite. What really happened is that Schott and Corning already had several crown glass types that were at 1.523 index, and all it took was some fine tuning of the recipes to make an ophthalmic quality lens pressing. One of the closest is the (Schott nomenclature) BK-7. This glass type ultimately became S-1 ophthalmic crown, and later S-3, which was optimized for chemical strengthening.The reason for the change, according to my Benson Optical Opthalmic Optics course on index of refraction, is that the glass manufacturers discovered that they could not reliably produce glass of 1.530 index and would have to change to another standard. They found that they could achieve an index of 1.523 reliably and this became the new standard for the optical industry.
Both companies had the resources to make an exact 1.530 index, but chose instead to tweak an existing close index glass type. This became feasible when studies indicated that by adjusting the base curve of the lens (by the lens processors) to a slightly flatter base curve, the index issue essentially disappeared.
I can't believe the collective knowledge of some of you.
What happens when you're dead?
Good question. I don't know. I will be 59 next month, but I don't plan on retiring any time soon...however, I may get hit by a bus LOL
But why was 1.53 abandoned? Richard Cook took his optics seriously, but he did not leave any citations. I'm inclined to think there's some truth to what he said, but that there's probably more to the story than "production problems".
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
It comes down to cost. Schott for one, was never a company to "waste" R&D money on low profit items. If they could tweak an existing product for a "new" market, they will do that.
The ophthalmic lens market, even back in the 1950's and 1960's was never a major profit center, and the overall production quantities were quite small compared to the optical glass market. An "average" melt of S-1 would be on the order of 5 tons (10,000 pounds), while a BK-7 melt would be on the order of 25 tons.
I'm not sure "abandoned" would be correct, it was more along the lines of "not feasible" at going market rates for glass lens blanks. And then, don't forget the segment parts in this. At the same time Schott was working on the S-1 product, they also had to come up with fusible high index "lowers", and the issue there was fusing compatibility. The high index glass for segment parts had to have the exact same Coefficient of Expansion as the major (main lens blank), otherwise, the fusion would result in checking or outright cracking along the fusion line. In some cases, it resulted in outright catastrophic failure of the lens assembly during the annealing cycle.
By modeling the S-1 product on the BK-7 glass type, they were able to easily find compatible high index glass for fusing.
It is a complex process, and one more reason why the glass industry shrank so much after the introduction of the photochromic plastic lens.
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