Power crosses

[Christopher]

so... I'm having a bit of a struggle with power crosses.
I can transpose back and forth in my head, but there seems to be a fundamental point I'm not grasping about working with power crosses.
Anyone got any pointers?

And, just out of curiosity, why does my instructor keep using so many plus cylinder notations? What's the point? We can't even order front side cylinder lenses, they just show up with a negative cyl lens form (my preceptor tried) and if we see a plus cyl Rx every month I'd be surprised.

ps: I now the only real answer is to just keep hitting the books, but I just felt like a bit of idle chat :7

-Chris

[mlm]

What a neat little coincidence! I'm a workshop instructor for the Canadian Association of Optometrists' Optometric Assistant Course. I just delivered a lecture on power crosses this morning. It's a little hard to do this without being able to draw things out for you but l'll do my best. What is it about power crosses that confuses you?

You will see Rx's in plus cyl notation, particularly from ophthalmologists so you need to know what to do with them. I taught my students this morning about transposing; I made them transpose from minus cyl to plus cyl simply to show them that it can go both ways.

[uncut]

Christopher: It's kinda like humanity................some of us are + some of us are -, some are both + and -, and also, AC or DC.

It doesn't matter that you are or aren't.......but you need to know how to recognize it and deal with it!

[Wes]

If you've used a manual lensometer, think about this. You check the power in the two principal meridians, 90° apart. One reads -2.00 and the other reads -4.00. These are the total powers in the principal meridians. An optical cross represents these powers. If you took those readings, you would write the rx as -2.00 -2.00 axis whatever.

[Fezz]

Maybe this will help!

http://www.nfos.org/degree/opt11/module_08b.html

http://www.icee.org/education2/stude...on-Student.pdf


http://www.tedmontgomery.com/the_eye/glasses.html

[gmc]

If you know how to transpose, you're already there. Take the following Rx:

-4.50 +2.00 x 045

Transpose it to

-2.50 -2.00 x 135

-4.50 is the power at 045
-2.50 is the power at 135

Those are the powers in the principal meridians and the numbers that go on the cross. The power at each degree in between changes very slightly between those two extremes, but that's a topic for another time.

[pseudonym]

so... I'm having a bit of a struggle with power crosses.
I can transpose back and forth in my head, but there seems to be a fundamental point I'm not grasping about working with power crosses.
Anyone got any pointers?
-Chris

“When Rx is in plus cylinder form, the cylinder is on the convex surface. When Rx is in minus form, the cylinder is on the concave surface.

"In MINUS cylinder form, the toric surface is on the concave surface.”

Those two phrases confused me so much in my first year of school that I put them on a post-it and stuck it to my wall. As I recall, they stayed up for months. If you have the "Systems for Ophthalmic Dispensing" book by Clifford Brooks, look at page 307 and all will be clear.

I recommend Post Its. I wish I'd taken a picture while I was in school because the wall behind my computer looked like something out of "A Beautiful Mind." School will NOT make you crazy, though. That comes later.

[finefocus]

“When Rx is in plus cylinder form, the cylinder is on the convex surface. When Rx is in minus form, the cylinder is on the concave surface.

Not any more.
99% of the time, the toric surface will be on the back (minus) side, however the Rx is written. Don't confuse plus or minus cylinder Rx notation with plus or minus cylinder grinding.

[pseudonym]

Not any more.
99% of the time, the toric surface will be on the back (minus) side, however the Rx is written. Don't confuse plus or minus cylinder Rx notation with plus or minus cylinder grinding.

I probably meant "lens" and shortened it to "Rx."

You can't get much on a post it.

[Christopher]

Thanks for the input, guys. I've been dreaming about them , and reading a few more articles on them, and seem to be getting a handle on it. I just need to find more time to sit down and do a bunch of drills 'till I can do it without having to think about it.

[mlm]

Okay, now I'm confused. In the CAO course, we teach transposing independently of power crosses. In the lecture, I'll show a couple of crosses so they understand that you just have a different starting point, but most of the exercises are just the math.

We teach that power crosses should be drawn during dispensing to help with frame selection, so you know where the thickness will be in that lens. It also emphasizes to the students that cyl lenses have a range of powers across the lens. It leads nicely into the intro lensometry lecture, so they understand that what they see in the lensometer is essentially a power cross.

[Christopher]

We're using them for more than frame selection. We're given an Rx as well as a base curve and are asked to create three power crosses from that Rx. We're also given three incomplete power crosses, and asked to complete them all and pull the Rx from them. Some are plus cyl, and some are minus cyl.
It seems to be the addition of the BC that is throwing me for a loop.

[gmc]

How do you create three power crosses for a given Rx? For a given correction, whether it's written in plus or minus cylinder form, the power in the principal meridians are the same.

We're using them for more than frame selection. We're given an Rx as well as a base curve and are asked to create three power crosses from that Rx. We're also given three incomplete power crosses, and asked to complete them all and pull the Rx from them. Some are plus cyl, and some are minus cyl.
It seems to be the addition of the BC that is throwing me for a loop.

[harry a saake]

How do you create three power crosses for a given Rx? For a given correction, whether it's written in plus or minus cylinder form, the power in the principal meridians are the same.

GMC, on any cylinder lens, there are always three curves. What your getting confused on is one is on the front, which is the base cuve of the lens. Now, there is also a back base curve on the lens and a cross curve for the cylinder, these two are on the back.

Now lets go into a simple sv cyl rx, and lets say it is -1.00 -1.00 at axis 180
Now let us just for an example choose a 6.00D, front base curve lens
Next leaving our for the moment sag values, thickness or anything else, the back base curve would now be -7.00 and the cross curve -8.00, so the basic job ticket going to the lens generator would read -7.00/-8.00 ct 2.0

[Wes]

GMC, on any cylinder lens, there are always three curves. What your getting confused on is one is on the front, which is the base cuve of the lens. Now, there is also a back base curve on the lens and a cross curve for the cylinder, these two are on the back.

Now lets go into a simple sv cyl rx, and lets say it is -1.00 -1.00 at axis 180
Now let us just for an example choose a 6.00D, front base curve lens
Next leaving our for the moment sag values, thickness or anything else, the back base curve would now be -7.00 and the cross curve -8.00, so the basic job ticket going to the lens generator would read -7.00/-8.00 ct 2.0

I would hazard a guess that as an ABOM, gmc is aware that there are three curves on a cyl lens, but what does any of that have to do with ???three??? power crosses?

[pseudonym]

We did the crosses the same way in school. It's simple algebra, but sometimes the base curve is on the front (when lens in minus cylinder) and sometimes the base curve is on the back (when lens is in plus cylinder.) If the problem states that the lens is being made in plus cyl, you just have to know the base curve value is at all four points of the cross for the backside and then you figure for cylinder and sphere on the front. If the problem states the lens is being made in minus cyl, all four points of the cross on the front side are the base curve value.

I hope this is helpful and not confusing you more, Chris. I had the devil of a time with these myself and I LOVE math.

[gmc]

Thanks Wes.

I understand the combination of curves needed to fabricate a lens. The base curve or front curve, the toric (or back base curve) and the cross curve. That combination is independent of the power cross or optical cross which merely represents the meridional powers required to correct the patient's vision.

You can choose from many combinations to fabricate the lens, but the meridional powers remain the same.

GMC, on any cylinder lens, there are always three curves. What your getting confused on is one is on the front, which is the base cuve of the lens. Now, there is also a back base curve on the lens and a cross curve for the cylinder, these two are on the back.

Now lets go into a simple sv cyl rx, and lets say it is -1.00 -1.00 at axis 180
Now let us just for an example choose a 6.00D, front base curve lens
Next leaving our for the moment sag values, thickness or anything else, the back base curve would now be -7.00 and the cross curve -8.00, so the basic job ticket going to the lens generator would read -7.00/-8.00 ct 2.0

[sharpstick777]

This is what helped me when I was struggling with the same thing. Envision a round lens blank with say, -4.00 sphere, and -2.00 cyl. I will have a thick point, and thinner point on that lens blank. If I started at the thickest point, -6.00 total power and went the other direction from there (+2.00 I would arrive at the plus point). Same lens, but if I started at the thinnest point (-4.00) and kept going, I would add another -2.00, but it would 90 degrees apart.

Transposing is just taking a picture of the side of lens, but describing it a slightly different way.

[harry a saake]

Well, i guess if we want to add another curve, we could always go into cross cylinders

Wes, i didnt bother to look what his credentials are, i just saw a question

[gmc]

That would really open up a can of worms!

No offense taken Harry. I've read tons of your posts and always find them informative.

Well, i guess if we want to add another curve, we could always go into cross cylinders

Wes, i didnt bother to look what his credentials are, i just saw a question

[Christopher]

Sorry guys. I wasn't very clear nor was my question well formulated.
Our instructor wants us to use a three cross format to determine the power of a lens based on using the front curve, and two back curves and coming up with some kind of toric power.
The "first" cross (given to us as: F1) shows the front curve, (which could theoreticaly be different in different medians.), the second cross (labeled F2) showing the back curves as two values 90 deg apart. The third cross is labeled Ft (for toric, i guess). Using the three crosses, in various ways is supposed to allow us to solve any one given any two, or somesuch.
I can visualize this in my head, but I seem to have a tough time translating the algebra.
It feels tantalizingly close, and builds a perfectly logical lens in my though experiments, I just can't get it out :/



* I'll be rooting around in the lens drawer tomorrow eyeballing the plus lenses with lots of cylinder :7 *

[MakeOptics]

Chris,

F1 is the front surface power
F2 is the back or second surface power
Ft is the total power

Anyway the formula is going to be: F1 + F2 = Ft
of course in your case you are using a cross for the front, for the time being both the powers are the same and a cross for the back surface which has two seperate powers (minus cyl). The total powr is calculated by adding the two similar meridians on the front and the back, in the example you have given you would add the front surface along one meridian with the same back surface power meridian and that would be placed on the total power meridian that corresponds. You again do this for the other meridian 90 degrees away on the cross adding the front, the back, and placing it on the total power meridian. If it's a plus cyl then the F1 cross will have differing powers and the back cross will be your base with the same powers in both meridians. Although it will be rare that you deal with different powers in both meridians on both surfaces in ophthalmic optics the rare occasion may present and it's always good to build on a solid foundation of theoretical optics. Just to let you know you will also be rearranging this formula around to figure out which what your surface powers would be given a total power and one surface (that's for future excercises so nail this down). Also this is a simplified version ignoring thickness and if you choose to go furthur with optics you will see the optical cross as a rudimetary matrice, in more advanced optics you will learn to add and subtract obiquely crossed surface powers which will introduce another power component to the mix. Good luck in your studies.

[MakeOptics]

Sorry for the crazy typing, I am using the swipe feature on an android phone most of the time it's quick and simple but when I have to include some interesting nomenclature the swipe feature starts to fall apart on me.

[Wes]

Thanks, I've not seen a power cross used this way.

Edit: come to think of it, I have. It seemed to unnecessarily complicate simple matters, and I paid it little attention.

[pseudonym]

Chris,

F1 is the front surface power
F2 is the back or second surface power
Ft is the total power

Anyway the formula is going to be: F1 + F2 = Ft
of course in your case you are using a cross for the front, for the time being both the powers are the same and a cross for the back surface which has two seperate powers (minus cyl).

I'm awestruck by the greatness of this explanation. We won't even get into the fact that you did it on your cell phone. Jaysus.