Optics behind plano with prism

**William Walker** · 03-15-2007, 12:03 PM

Hello all! I had a question about an Rx I received about a week ago. This was for over the patient's contact lenses for sunwear and it read plano w/ 2D BO prism O.U. At first, I thought this was an oversight (thinking back to Prentice's Rule), but after talking to a few opticians who've worked in the lab, they explained that it is done by keeping the same curve on the front and back (i.e. - same curve on both sides = plano), but tilting the back curve somewhat so that light would still be deflected. Before this, I have never heard of that, and didn't know if someone could help elaborate.

Thanks!
William

**HarryChiling** · 03-15-2007, 12:20 PM

Their was a great thread about plano's and how the curves were actually different on the back and the front, due to the index of the lens. Check out Donn Whitneys paper in the downloads section.

Attachment 1837

**lensgrinder** · 03-15-2007, 12:48 PM

To create prism in the lab using a manual type generator you will cut the back surface with the lens tilted by a prism ring. This ring is circular so it will fit over the block (what is attached to the lens by alloy or wax). There are different rings used to create different amounts of prism. The rings are thick on one side and thin on the opposite side. The thick side of the prism ring will push one side of the lens closer to the generator wheel which will create the thin side or apex. CNC (Computer Numerically Controlled) type generators will be controlled by a computer that will create a thicker side and thinner side.

Hope this helps.