Dispensing, Feature

Finding the minimum lens blank size – Part 1

Selecting the right lenses is critical to correctly fitting glasses. LEIGH ROBINSON explains how some simple measurements can ensure the right size lens blanks are chosen every time.

When your lab calls to say “this job won’t cut out,” or “we need to use bigger blanks,” or “we have to grind these lenses,” what do you say? More importantly, what do you do? The many laboratory managers, sales staff and reps I speak with all share concerns about this issue, and are unanimous in their astonishment at how many practice staff respond with “just do the best you can”.

Frankly, it should never have gotten to this point. If the right lenses had been ordered in the first place a lot of time could have been saved.

However, some simple calculations and understandings will help with the issue of cut out every time.

To better manage cut out, we first have to understand what a lens is.

A lens is a transparent object that refracts light, and is a product of its front and back surfaces. It can be made of a various plastics or glass, and is designed to bring light to a focus. A lens is a medical device that transforms visual perception by transforming light. A lens changes visual input, modifies space and affects people’s lives. It is an amazing device that transforms a wearer’s vision.

Although looking at a lens, we see little more than a simple piece of plastic.

It is the primary role of the dispenser to position a spectacle lens in direct relationship to the wearer’s pupil. Doing this incorrectly will cause visual discomfort (asthenopia) at best. At worst, it will cause a prismatic effect and double vision.

We have a profound obligation to our customers to get it right. So, we need some sort of reference point on the lens. We need, a Major Reference Point (MRP). Sometimes called a Prism Reference Point (PRP), it is the point on the lens where any required prism can be found and measured. If there is no prism, the reference point is the optical centre.

This is called centring the lens. If the optical centre of the lens needs to be re-positioned to match the pupil, then the lens is de-centred. The optical centre needs to be aligned with the wearer’s line of sight. This is the primary reason for measuring Pupillary Distances (PD). The optical centre of the lens needs to coincide with the wearer’s pupil, or more correctly the line of sight. (Fig 1)

Figure 1The successful dispensing of lenses depends on knowing the relationship between lenses, frames and how stock lenses might fit into the chosen frame. Decentration is the key.

To decide if the lens needs to be grind or stock, what lens to order, and whether your preferred lens will or won’t cut out, we must figure out the required amount of decentration and the minimum blank size of the available stock lenses.

Frame measurements

All this starts with the frame measurements. The bigger the frame, the bigger our problems will be. Frame manufacturers indicate the boxing measurement used by placing eye size and the Distance Between Lenses (DBL) on the frame, separated by a little box.

The measurement itself comes from a box drawn around the lenses of each eye shape. This enables us to determine the geometric centres of each lens shape of the frame. The geometric centre is the halfway point between the top and bottom (B measurent) of the lenses and the right and left edges (A measurent) of the lenses. (Fig 2)

Figure 2The DBL is the distance between the edges of both lenses and is measured from the right & left edges of the lenses nasally, at the narrowest point. This is not the bridge measurement, but is often incorrectly referred to as the bridge size.Adding together, the eyesize + DBL, gives us the Frame Centre Distance (FCD), which is the distance between the geometric centres of the frame, often called the frame PD.

Effective diameter

Knowing the FCD and geometric centres provides the means to determine the Effective Diameter (ED). The ED is twice the longest radius of the lens, and we need this measurent to determine if the stock lens will cut out.

Available in every practice is the Eye Talk Lens Layout Chart; Eye Talk’s universal chart for lens centration, layout and cut out. This chart is another great initiative from the publishers of The Eye Talk Reference Guide, but is very much under-utilised by those in practice. Use it to improve the service to your patients and the efficiency of dispensing in your practice.