Is there an assumed add power relationship for the top add power if none specified on lens order? Also if so, please direct or link me to the written rules.
Is there an assumed add power relationship for the top add power if none specified on lens order? Also if so, please direct or link me to the written rules.
Nope. You have to design it yourself.
NVF and near lens designing is the closest thing to prescribing you'll ever do. (In fact, it IS prescribing.)
The only thing you don't get to mess with is distance powers.
First of all, to be a prick, it's not "@" it's "axis" or "X". That bugs me, because "@" means something totally different.
Secondly, labs shouldn't be guessing and designing lenses.
Thirdly, I've had dodo birds from big boxes call me to design computer lenses for them for "my" patients that went to them. Na...ga...da. Wouldn't be prudent.
But, if you understand optics, just a little, here's how you do it:
1. How far away are most computer screens, in M?
2. What is the inverse of that distance (Diopters)?
That is the answer. It's dead simple.
I understand all that. We are the Whole Sale Lab. I have a customer that called to complain that we made the adds the same power. Said 50% was standard. I am making them over for them, but contend there is no standard. He ordered a similar Rx above (example). Guess what? We now have a new policy, if no add power is specified we verify with the customer. (Except for that customer, we will supply.)
I fixed the above as not to bug you. I do thank you for your comments and hope you will offer additional comments.
Last edited by Speed; 05-31-2017 at 02:05 PM.
My personal understanding of Double D's is that it is standard to do 100% for both unless otherwise specified. I was never told this. My reasoning is that, when I order occupational lenses, multiple companies offer Even Adds whereas only one company (that I know of) X-Cel even offers Uneven Adds. They have very select availability.
Have I told you today how much I hate poly?
unless otherwise specified, every lab I've worked with will send me a lens with top and bottom seg the same power. If I want a different power, in the top seg, then I will specify it.
Little late to the game but I always thought because they were cast or molded you could only get a half add power for the top seg in plastic or poly. Any other power required they be made in glass.
This changed?
X-Cel offers a variety of uneven adds as well as even adds in plastic and trivex. Younger offers even adds in plastic only. There are plenty of glass options I am unfamiliar with because I don't process glass.
Have I told you today how much I hate poly?
I have a client who would pay a kings ransom for double Ds in glass, safety thickness, I can't find them for him anywhere
I would look into Precision Optics, an Essilor lab. That is where I get glass safety jobs... though I don't think I've ordered any DDs with them. The Vision Ease lab should also be able to do it. I don't use their lab but they have availability for sure. I would think that Precision could do it because of the X-Cel availability.
Good luck!
Have I told you today how much I hate poly?
Thats good to know, my usual wholesale lab did a search 2 years ago for them and came back saying they couldn't find them. Ended up in Trivex, all he complains about is the ledges everytime I see him. He's a +6.00, large cable temple aviator safety frames - callouses on his nose from all the years of weight.
Glass bi-focals of any sort blow my mind... The fact that the shelf is just IN there baffles me. I want to see a How It's Made on glass FT. I get that is all in the molding, but how?
Have I told you today how much I hate poly?
Higher index glass in seg.
http://www.drdrbill.com/downloads/op...s/Bifocals.pdf
My personal feeling is that the top of a double D occupational bifocal if at the same power as the bottom will probably be too strong and have too short a working distance(WD). For say an electrician or sprinkler fitter that is working overhead while standing on a ladder or scaffold, typically arms would be extended and he would likely be at 25-28 inches.
For a full presbyope with a 2.50 add in the lower seg the WD would be 16". That would be too close if in the overhead position. If a 60% add overhead the WD the add would be +1.50 with a WD of 26.66 inches. Much nicer while working overhead from a ladder or scaffold. For the record in most cases for a full presbyope I prescribe a 2.25 add, and thus a 60% overhead WD would be 29.6 inches which would probably still be quite comfortable.
I would love to find this type of lense in a progressive like used to be made in the Varilux Overview. I believe Janet Benjamin at Laramy has said they can make a lens in some iteration for this purpose.
Have I told you today how much I hate poly?
In a nutshell from Zeiss...
https://www.zeiss.com/vision-care/en...on/bifocal.htm
In a nutshell from Zeiss...
https://www.zeiss.com/vision-care/en...n/bifocal.html.
1. Glass
The Glass Bifocal Lens
To produce glass bifocal lenses, a lens pressing and a so-called button are required.
The circular button consists of two segments with different refractive indices. The top part of the button – known as its crown – has the same refractive index as the base lens, i. e. as the distance portion, while the lower part – the subsequent near portion – has a higher refractive index.
At the surfaces of the dividing edge which is later visible, the two segments are smoothed and then permanently fused in a furnace. The back surface of the button is ground and polished. The function of the button crown is to protect the dividing edge of the near portion when the button is being milled and to complete the distance portion above the near portion.
Manufacture of glass bifocal lenses using the international technique
Production of Bifocal Lenses
International technique
In this process a round hollow is ground into the front surface of the distance portion and then polished. This displays the radius required for the addition.
The button is placed into the hollow of the base lens and both are then passed through a melting furnace for approx. 4 1/2 hours. Depending on the type of lens involved, the temperature ranges from 670°C to 760°C. The button assumes a "pasty" consistency and fuses with the base lens.
In the pressing technique the button is pressed into the red-hot base lens under high pressure
Pressing technique
A base lens with plane surfaces is used for this technique. In this case, the back surface of the button determines the addition. The base lens is heated to beyond its softening point and the cold button is pressed into the red-hot base lens using a punching device. The pressure required for this process corresponds to a weight of approx. 6 tons.
After cooling, the button and the base lens remain permanently fused. The button crown is now totally invisible, as it fuses with the base lens to form a homogeneous glass block. The projecting part of the button is removed by milling. The resultant bifocal blank is then ground and polished on its front and back surfaces, and is then coated.
Thanks Fester! That's definitely the most detailed explanation I've had the pleasure of reading!
Have I told you today how much I hate poly?
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