|Cylinders and compound lenses. Optical cross. Flat transposition.|
PAGE References to Optical Formulas Tutorial: (first reference is to edition 1 / second reference is to edition 2).
On the lenses that we are dealing with now, when we make these cuts or cross-sections, the curves will be different, depending on how you cut the lens.
The lenses that we make virtually all have spherical front surfaces. When we make toric or sphero-cylindrical lenses, the back surface is of the type where there are a lot of different curvatures, depending on what direction we make the cross-section.
Lets look at a cylinder again. The can that we were dealing with had one flat direction, one circular direction, and every other direction was curved but was not a circle. A true cylindrical lens will have one direction that has no power, a direction 90 degrees away that has power (either plus power or minus power) and every direction in-between will have an approximate power that is in-between the other two powers but which is not based on a true circle and therefore will not give an actual, true power.
If we combine a lens that is a spherical lens, (which means it has the same power in all directions) with a cylindrical lens (which means it has no power in one direction and some power in the other direction) we end up with a sphero-cylindrical lens: a lens with two major powers 90 degrees apart. This lens is a combination of a spherical +2.00 lens with a cylinder that has no power in one direction and another +2.00 power in the other direction. The result is a lens with +2.00 power in one direction, and +4.00 power in the second direction.
Read pages (39-40 / 52-54) in the Optical Formulas Tutorial, if you have not already done so, but do not do any of the exercises in the book yet.
The horizontal meridian is the 0-180 line, when we look at a lens as a circle. So, in the lens diagrammed to the left, we have a power of +2.00 on the horizontal or 180 meridian. This +2.00 power is ACTUALLY the +2.00 from the spherical component plus 0.00 from the cylindrical component.
On the vertical meridian, or the 90th meridian, we had a power of +4.00. This is actually +2.00 from the sphere, plus +2.00 from the cylinder.
We write the strictly spherical prescription as +2.00 D or +2.00 DS or +2.00 sphere. So, if the lens was ONLY the first one in the diagram, where there is +2.00 everywhere on the lens, then the prescription could be written any of those ways.
We write a cylinder as pl +2.00 x180. This is saying that there is pl or plano or 0.00 or no power on the 180 meridian, and there is +2.00 90 degrees away from that meridian. (The x means 'axis'.) So if we had a lens that was ONLY the second one on the diagram, we would have the prescription pl +2.00 x180.
If we want the third diagram shown here, where there is +2.00 D on the 180 and +4.00 D on the 90, we say +2.00 +2.00 x180, which means: +2.00 on the 180 meridian, and another +2.00 ADDED TO THAT on the 90th meridian.
What does a prescription of -3.00 -1.00 x090 mean?
Remember as you put the prescription on the optical cross or take it off the optical cross, all you are doing is showing the two major meridians where there is precise power. If you have been using the focimeter (lensometer) at work, you are showing the powers that you get on the power wheel, plus the axis that you get for one of them.
|PUTTING THE PRESCRIPTION ON THE OPTICAL CROSS
|TAKING THE PRESCRIPTION OFF THE OPTICAL CROSS
Lets do a few more, and I will show you the answers. As before, you do them first, then click on the answers.
Write the prescription in plus cylinder form:
Answers are here.
Now go back to Optical Formulas Tutorial and do the exercises
in the sections that we've covered. DO NOT DO Crossed Cylinder
notation, unless your instructor tells you to do so.