Cross Curve Tool Formula?

[LarryLane]

I know the thick lens formula and how to derive lap tool curves from this equation. What I need to know is how do you come up with the cross curves. It would appear that you just add the cylinder to the tool base, but I notice that sometimes the rxcalc software has slightly different values. Thank You for you help

[lensgrinder]

Let's say you wanted to make an Rx of +6.00 -2.00 X 180. You were going to put this on a refractive curve of +8.75, 6.0 mm thick, an index of 1.586.
You first find the power in each meridian which is +6.00 and +4.00 then you use each power in the back vertex formula (thick lens formula).


Now do the same for the other power


Now put these in lap form




You would use a 2.75/4.50 lap

[LarryLane]

Thanks again for your help.

[Darryl Meister]

Also keep in mind that, once you've calculated the back base or "sphere" curve of the sphere power of the prescription in minus-cylinder form, you only need to add the cylinder power of the prescription to the back base curve to determine the cross curve for the cylinder power.

So you can skip, in this case, calculating the back vertex or thick lens power, instead just adding the cylinder power to your sphere curve (D_2SPH) to determine the cross curve (D_2CYL):



Then you would compensate for the refractive index of the lap tool accordingly.

[lensgrinder]

Also keep in mind that, once you've calculated the back base or "sphere" curve of the sphere power of the prescription in minus-cylinder form, you only need to add the cylinder power of the prescription to the back base curve to determine the cross curve for the cylinder power.

So you can skip, in this case, calculating the back vertex or thick lens power, instead just adding the cylinder power to your sphere curve (D_2SPH) to determine the cross curve (D_2CYL):



Then you would compensate for the refractive index of the lap tool accordingly.

Very good point.
Thanks Darryl