Part a -- Prism Notation
Part b -- Resultant prism formulas Part c -- Resultant prism graph Part d -- Resultant prism tables Optical Formulas Tutorial, Pages 120-125, 127-131. Supplemental reading: Understanding Lens Surfacing, Pages 5-12, 308-311. |
Part a -- Prism notation.
We have been talking about prism with the base up, down, in or out. When we talked about horizontal imbalance from the glasses optical centers not being correct we talked about prism base in and out even though we know that in many glasses the eyes are above where the optical centers should be. When we talked about vertical imbalance in anisometropia we talked about prism base up or down, even though we know that the eyes converge from the distance pupillary distance when we read. When we are talking about UNWANTED prism we generally use just up, down, in or out because we need only determine if a problem exists as a result of poor optical center placement.
For PRESCRIBED prism the prism base will very likely be in other directions. The base of a prism can be anywhere on the lens, not just on the 90th or 180th meridians.
There are two ways that we can describe where the prism base is to be.
You saw an example of the first way of showing prism base direction when
we talked about splitting prism. If we were given permission to split the
prism for the Rx
pl 3BU
pl 4BI
we would end out with
pl 1.5BU
& 2BI
pl 1.5BD
& 2BI
How do we make this? How do we do final inspection on this?
The second method of showing prism base direction is going to look suspiciously
like cylinder power, but with a twist. It will show the total amount of
prism and give a meridian for the base direction. For the example above,
it may end out looking like
pl 2.5
@ 037
pl 2.5
@ 217
or, just to make life more interesting, it could look like
pl 2.5BU&I
@ 037
pl 2.5BD&I
@ 037
We will start by discussing notation itself. I would like for you to read pages 120-124 in the Optical Formulas Tutorial. Just read. Do not worry about the formula yet, read for understanding of the notation. If you have Brooks Understanding Lens Surfacing, read pages 5-10, left side.
OK, first I want to make sure that you understand the 180 vs 360 notation. Remember way back in the beginning of the first theory class we said that we do not use cylinder axis greater than 180 because what happens in the bottom of the lens is just the mirror image of what happens in the top of the lens? Well, with prism this is just not so. Prism base up at the 90th meridian will be very different from prism base down at the 270th meridian!
So, here are some exercises. You will need the diagram on the top of page 121 in the textbook. (Well you don't HAVE to use it; draw one out yourself is an even better idea.)
Write in 360 notation, and tell me if it is OD or OS:
Use your BACK button to return to the assignment page.