Cross Cylinders

[packfanrlb]

Hello,Many years ago as an apprentice with B&L,I learned to grind cross cylinders(glass of course).Does anyone know of any labs that can process these in CR-39 or HIX?I do have Gerber surfacing equipment,however,no lap cutter or manual blocker.I guess I'd have to fake out the computer if no one else does this.

[Darryl Meister]

Just out of curiosity, why are you wanting to grind cross cylinders? The only application that comes to mind would be an iseikonic correction for someone with a high degree of astigmatism imbalance, but this is rarely done...?

[packfanrlb]

Primarily cosmetics,the script is +6.50-6.50.A uniform edge would look great in a rimless mounting.

[chip anderson]

Darrell: Or Packy Please explain, I don't think this job can be done with a uniform edge unless I miss-understand cross cylinders very badly.


Chip

[Darryl Meister]

You might get up to a 10% reduction in thickness by splitting the cylinder power between the two surfaces, but -- since there are very, if any, suppliers of plus-cyl lens blanks -- I don't know that it would be worth the additional processing involved. The lens would have to marked up, blocked, surfaced, fined, polished, and de-blocked twice. And the second time around will involve blocking a toric surface, which could easily lead to induced prism as the lens rocks in the block (unless you have some special blocks for that purpose).

I would consider a minus-cylinder lens with a relatively flat base curve, and then recommend a frame that minimizes the dimension containing the cylinder power (e.g., for a -4.00 x 180, the cyl occurs through the 90 meridian, so select a frame with a narrow B measurement). Use of a high-index, aspheric lens design should be a given, as well.

[CDOT]

I have surfaced this kind of lens as per the request of a local university to duplicate a lens from a scope (unknown kind). But for rx, it seems like a lot of work (and expen$e) for a marginal reduction in thickness. I would optimize the index and frame/patient parameters instead.

[QDO1]

Primarily cosmetics,the script is +6.50-6.50.A uniform edge would look great in a rimless mounting.

That is an absoloutly facinating approach, what other optical issues does it throw up?

[rolandclaur]

what do you mean exactly by cross cylinders... having essentially 2 cylinders 90 degrees apart on the front of the lens?

[Darryl Meister]

One cylinder on the front and one on the back.

[QDO1]

what do you mean exactly by cross cylinders... having essentially 2 cylinders 90 degrees apart on the front of the lens?

I would think they need not be at 90 degrees apart either

[Darryl Meister]

I would think they need not be at 90 degrees apart either

They wouldn't. But, in most cases at least, you could replace two obliquely crossed cylinders with a single cylinder if your purpose was anything other than affecting magnification.

[QDO1]

I have surfaced this kind of lens as per the request of a local university to duplicate a lens from a scope (unknown kind). But for rx, it seems like a lot of work (and expen$e) for a marginal reduction in thickness. I would optimize the index and frame/patient parameters instead.

Would have an interesting effect on best form too. Might be a useful way of dealing with higher scripts, eneabling them to be kept (in the worse meridian) within best form parameters

[Darryl Meister]

Might be a useful way of dealing with higher scripts, eneabling them to be kept (in the worse meridian) within best form parameters

True. But, conversely, using a plus-cylinder lens form on a high-cyl Rx may result in a degree of spectacle-induced meridional aniseikonia.

[packfanrlb]

The traditional method was to cross them 90 apart.If you wanted to make a +2.00-8.00,you'd put a +2.00/+6.00 on the front and a -8.00/-4.00 on the back side.As explained to me by an old master at B&L,during WWII when metal for laps was scarce,labs would use two identical laps,one convex and one concave and by changing the opposing axes,you'd pic up a new power every12.5 degrees.Now with no concave laps or plus cyl. semi finished lenses the art of cross cylinders appears to have died.

[QDO1]

The traditional method was to cross them 90 apart.If you wanted to make a +2.00-8.00,you'd put a +2.00/+6.00 on the front and a -8.00/-4.00 on the back side.As explained to me by an old master at B&L,during WWII when metal for laps was scarce,labs would use two identical laps,one convex and one concave and by changing the opposing axes,you'd pic up a new power every12.5 degrees.Now with no concave laps or plus cyl. semi finished lenses the art of cross cylinders appears to have died.

I have worked in a practice where there was an unexplained non-tol to the new spectacles. It turns out the old pair were made like this, and had been used for years. I ended up dispensing a variant on the above, weaning the patient away from the old design to more conventional designs, by moving more and more of the cyl to the rear surface, over a period of 3 years and 4 pairs of lenses. I cant remember the script (was a long time ago), but interestingly the issue was peripheral blur, which, by the time I dispensed the last pair, was not an issue anymore. My friend is my lens clock, which always goes for a spin on most lenses I check

[Darryl Meister]

As explained to me by an old master at B&L,during WWII when metal for laps was scarce,labs would use two identical laps,one convex and one concave and by changing the opposing axes,you'd pic up a new power every12.5 degrees.

That's an interesting approach to the problem. They're basically making a Stokes lens, which uses two crossed cylinders of equal power to produce a variable range of cylinder power by rotating the axes of the cylinders with respect to each other.

[QDO1]

argh - the dreaded stokes construction - what a thought, and before coffee too

[Lady Nicole]

argh - the dreaded stokes construction - what a thought, and before coffee too

Maybe you can help out a newbie, here... What the heck is "stokes construction?"

[Darryl Meister]

A Stokes lens is a lens-like device consisting of a plus cylinder and a minus cylinder of equal power that can be rotated in opposite directions with gears. By rotating these two cylinders, you create an obliquely crossed cylinder that represents the combination of the two individual cylinders. This allows you to create a continuously variable range of cylinder powers, from Plano to twice the power of the individual cylinders, while maintaining a constant average power (spherical equivalent) of Plano. It was used mainly in ophthalmic instruments.

[QDO1]

Maybe you can help out a newbie, here... What the heck is "stokes construction?"

and a Stokes construction is a method of graphicaly repersenting the addition of two cylinders on paper, in such a way as to be able to measure from the paper, the resultant power

On the basis that I always seemed to run out of paper, a good starting place for this method is a whopping great big sheet of graph paper. A stokes lens is as Daryl describes

[Lady Nicole]

Thanks QD, Darryl! :cheers:

Just tell me one more thing: how are Stokes Lenses and Cross Cylinders used for correcting vision? It seems like it could get very complicated very fast, what is the benefit over a "standard" plus or minus cylinder correction?

[packfanrlb]

Let's assume an Rx of +2.00-8.00.If we put half the cyl on the front and half on the back,90 degrees opposed,the edge thickness will be somewhat uniform in all meridians,otherwise we'd see a very thin edge through one meridian and a thick edge through the other.