This tutorial is not meant to understand every aspect of prism, it is meant to help you understand prism in your everyday setting as an Optician. In order to help us we need to think of lens as two prisms. A lens with prisms base to base is a plus lens and a lens with prisms apex to apex is a minus lens.

This image shows a minus lens on the right and a plus lens on the left that are vertically oriented. This image is you looking at a patient, the left side is the patients right eye and the right image is the patients left eye. A minus lens has its optical center (OC) located where the apexes of the prism connect and a plus lens has its OC located where the bases of the prism connect. This image will help us determine the direction of prism, Base Up(BU) and Base Down (BD). Notice the pupil in the image, it is located below each OC. Erase the triangle that the pupil is not in or draw a new triangle. Looking at this image

where is the base in relation to the pupil? The triangle on the left the base is up and the triangle on the right the base is down. This is how we determine vertical base direction. Now look at the same image, but with the pupils above the OC. Again, erase the triangle that the pupil is not in and you will get the image below.

Notice where the base is in relation to the pupil and you will see in the right eye (the triangle on the left side) the base is down and for the left eye (the triangle on the right side) the base is up.

Let us take a look at horizontal prism. The image below shows a pair of plus lenses(top) and a pair of minus lenses(bottom). The top image represents prism base out (BO). The bases are located to the outside of the pupil. The image on the bottom is base in(BI).

The base is in in reference to our pupils. The next image is the opposite. The image on top are two plus lenses with BI (i.e. the bases of the prism are located in, in reference to the pupils). The image on the bottom is two minus lenses with BO(i.e. the bases are located out in reference to the pupil).

Remember, you only need to look at the triangle that the pupil is in. When the bases of the prism are in the opposite direction they compound or add to each other. When the bases of the prism are in the same direction they will cancel or subtract from each other. If you have vertical prism, i.e. BU or BD, BU and BU will cancel, BD and BD will cancel. BU and BD will compound If you have horizontal prism, i.e., BO or BI, BI and BO will cancel, BO and BO will compound, and BI and BI will compound.

If you have $2^{\Delta}$BU in the right eye and $2^{\Delta}$BD prism in the left eye, you have a total of $4^{\Delta}$ diopters of prism.

If you have $1^{\Delta}$BI in the right eye and $3^{\Delta}$BO prism in the left eye, you have a total of $2^{\Delta}$ diopters of prism.

You will need to use cancelling and compounding prism when splitting prism for cosmetic reasons or when applying ANSI Standards. In order to apply ANSI Standards we need to find out the amount of prism per lens using Prentice’s Rule.

• $P = c \times f$
• $P$ is the amount of prism
• $c$ is the amount that the OC is from the pupil in centimeters
• $f$ is the power of the lens

Example:

• If the Rx is:
• OD -3.00 DS
• OS -4.00 DS

How much prism will the wearer experience if the patients PD is 63 mm, but they were made at 66 mm?

• We know that the PD is a total of 3 mm off
• We would divide that amount by two and get 1.5 mm for each eye. * This tells us that the PD is 1.5 mm away from the pupil
• Use Prentice’s rule to determine how much prism is present in each eye.

\begin{aligned} P &= 0.15 \times 3\\ &= 0.45^{\Delta}OD\\ P &= 0.15 \times 4\\ &= 0.60^{\Delta}OS \end{aligned}

Now we determine base direction Please note we did use the sign for the power, there is no need too

The red dots represent the patients PD. The black lines represent where the OC is and the N is where the nose is, commonly referred to as the nasal. Notice that the PD is less than the OC, because the OC was made wider than the PD. Now look at the triangle that the pupil is in. The bases are located in on both eyes. Base in and Base in compound each other, so now you add the 0.6 and the 0.45 for a total of 1.05$^{\Delta}$diopters of prism

• If the Rx is:

How much prism will the wearer experience if the patients PD is 60 mm, but they were made at 64 mm?

• We know that the OC is a total of 4 mm off

• We would divide that amount by two and get 2 mm for each eye. This tells us that the OC is 2 mm away from the pupil

• Use Prentice’s rule to determine how much prism is present in each eye. \begin{aligned} P &= 0.2 \times 2\\ &= 0.4^{\Delta}OD\\ P &= 0.2 \times 1.75\\ &= 0.35^{\Delta}OS \end{aligned}

Now we determine base direction

Again, look at the triangle that the pupil is in and notice that the base is out for both eyes. BO in both eyes will compound so we have a total of 0.75$^{\Delta}$. If you have an Rx with cylinder you will need to find the power in the meridian in which you are looking for the prism. For example, if you are looking for horizontal prism you need to find the power on the $180^{th}$ and if you are looking for the vertical prism you need to find the power on the $90^{th}$ meridian. If you are unsure how to find the power on these meridians, read my walk through on the oblique meridian formula