What Is Prism In Eyeglass Lenses & How Is Prism Calculated?

by admin on 24/04/2009

What on earth is prism and how do you calculate prism? This isn’t a word that most people even say everyday, much less calculate. However, if you are an Optician or have plans of becoming an Optician, prism is something that you must not only know about, but know how to calculate. In order to even pass certification or licensing to become an Optician, prism is a topic that won’t ever be overlooked as it plays a vital role in how a person sees out of his or her glasses.wb01254_


First of all, prisms are basically triangle in shape, with an apex(top point or the thin edge on a lens) and a base(flat bottom or the thick edge on a lens).  Looks just like a triangle on paper, however, when looking at a edged lens, things are less obvious.

The purpose of the prism is to modify where the light goes when it enters and passes through it.  For instance, with water filled in a transparent glass, add a drinking straw.  Take note that the straw is straight, however, when it enters the water, the straw appears to be bent.  Yep, this is an example of what a prism does with light.  As light enters, the light then bends according to the amount of prism ground into the eyeglass lenses.  Therefore, the greater in strength the prescription, the shorter the focal length, which is the distance between the point where the light enters the lens to the point where the light converges at the focal point.  This is the case no matter if the lens is plus or minus.

To sum everything so far up, the prism in a prescribed lens is supposed to be used to optimize vision by causing an object that a person is having problems seeing to be brought closer to the eye through the bending of the light via prisms.

Now that we know what a prism is and who it operates in lenses to help people see better, how is then prism calculated by an Optician?  Well, there is a mathematical formula that is used called the Prentice’s Rule.  The Prentice’s Rule is P=hcmD.

P is the power in prism diopters or prismatic effect

hcm is distance(h) in centimeters(cm)

D is dioptric power of the lens

By the way, prism is measured in what is known as Diopters(D).  Seems pretty simple right?  Well, so far, but prism gives some of the most experienced Opticians problems sometimes.  This is exactly why there is another way to calculate prism which is through a manual or automatic lensmeter.  Using this piece of optical equipment allows the Optician or laboratory professional to see just how much wanted or unwanted prism is in the eyewear by counting circles!  Unfortunately, this method is another lesson entirely.  In order to pass the certification and licensing exams for Opticianry, one would need to know both methods.

Let’s say that an Optician needed to find the prismatic effect of a +6.25 lens at the focal point of 5mm away from the OC, or optical center of the lens.

The first thing that needs to be done is the conversion of the mm to cm, thus making it .5cm as the equation only calculates in cm but the optical measurements are taken in mm.  Next would be to plug the numbers into the equation.  D=+6.25, h=.5cm, and P = unknown.

P=(+6.25)(.5cm)  We are now multiplying D and h.

The prismatic effect comes out to be 3.125.  Thus P=3.125, and this is how much prism power is in the lens.

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