Print

 
 
 

logo_copyright2.jpg

Frame Foundation

 

The changes to the JCAHPO exam content areas has done away with some of the optics subject matter, specifically some of the "opticianry" subject matter, such as lens design and frame specifications. This may be good news for exam takers, but it is unfortunate in that assistants and technicians will be less educated regarding skills that directly affect the visual performance of the patient. This knowledge will make you a better technician and a better caregiver, so eyetec.net is going to cover it.

As a technician who performs refractometry, you are likely to be asked to make recommendations regarding glasses design, and you are likely to be asked to solve glasses complaint problems.  Having a basic knowledge of lens design will help you head off potential spectacle problems, and help you resolve many glasses complaints.

Although it is an asset to be able to consult with a knowledgeable optician, don't assume that every optician that you deal with is more knowledgeable than you are concerning lens/glasses design.  Some of the discount optical shops employ salespersons with little actual opticianry kowledge.  The more you know about opticianry, the better the technician you will be.

We will explore how lens design can have a profound affect upon the comfort and performance of your patient's glasses.

Lens Thickness

One of the main goals of opticianry is (should be) to produce a pair of glasses with lenses that are as thin as they can practically be.  Thick lenses are not only cosmetically unappealing, the thicker the lens is, the more aberrations will affect the patient's vision.  Of course, a main factor that affects the lens thickness is the prescription.  However, there are several other factors that can be controlled in order to minimize thickness.

Lens Size

For a given prescription, as the lens size increases, lens thickness increases proportionally.  For a plus lens, the center thickness will increase as the lens size increases.  For a minus lens, the edge thickness will increase as the lens size increases. 

   
 

 

Lesson learned: smaller lenses are generally better optically than larger lenses.

   
 

 

Lens Decentration

For a given prescription, as the amount of lens decentration increases, lens thickness also increases.  Lens decentration occurs when the optical center of the lens is not the same as the geometric center of the lens cutout for a particular frame.  In the picture below, the patient's PD lines up perfectly with the center of the lens cutouts in the frame.  The lens must be ground from a blank that will accommodate the size of the lens shape.

 

 

In this picture, the frame is wide and the patient's PD is narrow.  The optical centers of the lenses must be decentered inward relative to the geometric center of the lens cutout in the frame.  In order to align the optical center and accommodate the size of the frame cutout, a larger blank must be used.  A larger bank means a thicker lens.

 

   
 

Lesson learned:  It is better to choose frames that line up the patient's pupils close to the geometric center of the lenses.

So how would you know how much decentration would be required for a given frame and given patient?

First of all, we should be familiar with frame measurements:

   
 

   
 

The "A box" is the distance between the temporal edge of the lens and the nasal edge of the lens.  The "DBL" is the "distance between lenses" as mounted in the frame.

The "frame PD" is the A box measurement plus the DBL measurement.  This is equal to the distance from the center of the left lens opening to the center of the right lens opening, on a horizontal line.  This can easily be measured using a millimeter ruler from the outside edge of the right lens opening to the inside edge of the left lens opening.

 

 

The total decentration can be calculated by subtracting the patient's PD from the frame PD.  This measurement assumes that the patient's face is perfectly symmetrical.  Monocular decentrations can be calculated by taking monocular PD measurements and subtracting from half the frame PD.

With this simple information a comparison can be made between the decentration values for two different frames that the patient may be considering, or between the new glasses/frames that the patient is complaining about and the patient's old frames.  Remember that less decentration means thinner lenses.  A decentration value of zero would be optimum.

   
 

Lens Shape

For a given prescription, the more irregular the lens shape is, the larger the lens blanks must be, and consequently the greater the lens thickness will be. 

The key is the effective diameter (E.D.) of the lens shape.  The effective diameter of a lens shape is defined as twice the longest radius of the shape.  The longer the effective diameter is, the thicker the finished lens will be.

 

Let's look at an exaggerated "aviator" sample lens shape:

 

The first thing we do is draw a box around the shape and draw diagonals in the box.  This will give us the center point of our shape.

 

Next we will draw a radius line (red line) from the center point to the lens edge that is farthest from the center point.  This will give us the longest radius of the shape.

 

The lens blank for this shape would have to have this same radius.

 

Compare the above to the size of a lens blank that would accommodate a circular lens that would fit into the same box.

   
 

Lesson learned: Irregular lens shapes increase the lens thickness.  More regular (circular) lens shapes keep the lens thickness to a minimum.

 

Lens Style

Lens style refers to mounting and edge treatments of the lens that are either dictated by the style of the frame or that are added to alter the cosmetic appearance of the lens.  Examples would be a grooved rimless mounting or a metal rim.  Other examples would be rolled and polished edges or faceted edges.

The thickness of a plus lens can be minimized by grinding the edge of the lens to a very thin edge.  The thickness of a minus lens can be minimized by keeping the center thickness of the lens to a minimum.  When we do this, we minimize the saggital value (sag) of the lens.  The saggital value is the millimeter distance of a perpendicular line from the front vertex of a lens to the back plane.

 

 

A grooved rimless mounting of minus lens would work well because there is usually plenty of edge thickness to insert the groove without affecting the sag value.  On the contrary, a grooved rimless mounting of a plus lens is not desireable because thickness would have to be added to the lens edge in order the accommodate the groove.  This would increase the sag value and add to the overall thickness of the lens.  The same can be said for edge facets, edge scallops, and rolled edges.  They work well with minus lenses but add sag value to plus lenses.

   
 

Lens Weight

It is obvious to all of us that excessive lens weight is a major irritant for glasses wearers.  Lens weight is affected by the same factors that affect lens thickness.  The more volume there is to a lens, the heavier it will be.

With modern frame materials, even stocky frames can have a minimal effect on overall glasses weight.  The most important factor affecting weight is lens size.

 

Lens Materials

Plastic lenses are much lighter than glass lenses, with polycarbonate lenses being the lightest material of all.  High index materials provide thinner lenses for a given prescription, but they do not necessarily provide lighter lenses for a given size.  This is because high index materials are denser than lower index materials.

 

 

Back to top