PAGE References to Optical Formulas Tutorial:  (first reference is to edition 1 / second reference is to edition 2).

BICENTRIC GRIND or SLAB-OFF.

Look at the word "bicentric grind". What does it look like it means?

"bi-" means two. "center" would refer to optical centers? "grind" in this field commonly refers to surfacing the back surface of the lens.

So, bicentric grind means that we are surfacing two optical centers on the lens. I suppose that slab-off refers to the traditional way of doing this: attaching a slab of material to the front surface, grinding part of it off with a lot of prism so that some of the original lens is removed, and then popping the extra material back off again. That is a guess. If anyone knows of a better explanation for this term, let me know.

Here is the method used to make a slab-off on a glass lens. The result is a line (not a ledge as you have on executive bifocals) on the front side of the lens. The line looks and feels like a paper fold. It takes some skill to do this and end out with the new line exactly where the top of the segment is.

The slab-off on a plastic lens is done the same way, but since the segment on the front of a plastic lens has a ledge that could be damaged the slab-off is surfaced on the back of the lens. The result is still a line that should be lined up with the top of the segment, and the line still looks and feels like a paper fold.

Look at the material that is being REMOVED from the lens. We are removing a base down prism. Therefore, the result of bicentric grind is the creation of base up prism in the reading area. The process is done on only one of the pair of lenses in a pair of glasses, and it is done on the lens with the most base down or the least base up prism. Therefore, Slab-off is done on the lens with the most minus power or the least plus power.

This process does take skill and time and extra steps to make the lens correctly, so it is time consuming and expensive. There is an alternative for plastic lenses: it is called the reverse slab-off. The reverse slab-off is molded on the front of the plastic lens where the segment is. Since the skill here is in making the mold, and once the mold is made correctly it can be used to make a lot of lenses, the reverse slab-off takes less time because the surfacing lab can order the blank with the slab-off and then surface and edge it normally. There is less likelihood of a redo since the surfacing is normal. It is still expensive, but some labs do not charge as much for it as for the regular bicentric grind.

OK, here we go. You are given an Rx of
      OD +1.00 DS, ADD +2.50       reading level
      OS -2.50 DS, ADD +2.50,       8 mm below oc.
and you are asked to determine what amount of bicentric grind would be used, and on which lens.

1. First, determine what amount of prism is present in each lens at the reading level.
• OD: D = +1.00, d = 8 mm, P = Dd/10 = (1.00)(8)/10 = 0.8 
• The right lens is a plus lens, the wearer is looking below the oc so the oc is UP, the base is where the oc is, so the base is UP. The right lens has 0.8  BU
• OS: D = -2.50, d = 8 mm, P = Dd/10 = (2.50)(8)/10 = 2.0 
• The left lens is a minus lens, the wearer is looking below the oc so the oc is UP, the apex is where the oc is, so the base is DOWN. The left lens has 2.0  BD
2. Second, determine what the total vertical imbalance is. BU and BD are compounding because they are sending the eyes in different directions, so the total imbalance is 0.8 + 2.0 = 2.8 .
3. Last, determine which lens the bicentric grind should be done on. This is regular bicentric grind, which is done on the most minus or least plus lens. The -2.50 is most minus, so the grind will be 2.8 on the OS.

• What about reverse slab-off?


We would order the reverse slab-off for the most plus lens: the OD. Since reverse slab-off is molded, we do not have the choice of doing exactly 2.8, so we would likely get a 2.5. Reverse slab-off is currently available down to 1.

• Will not having exactly 2.8 matter?


No, we do not have to exactly correct the imbalance. We tolerate small amounts of imbalance, and the reading level is approximate. If the wearer's vision is noticeably more comfortable at 7 mm or 9 mm when the old reading level was 8 mm, the wearer is likely to adjust the reading level without even noticing it.

• Why did you ask for slab-off instead of reverse slab-off?


'Cause I get to make up the problems. In real life? If the wearer is using plastic or on discussion agrees to switch from glass to plastic, I will order reverse over regular. But it is not available in all materials or in all bifocal styles; so sometimes the regular bicentric grind is necessary.

Next problem. What reverse slab-off will you order for the Rx
      OD -3.75DS, ADD +1.50       reading level
      OS -2.75 DS, ADD +1.50,       11 mm below oc.

1. First, determine what amount of prism is present in each lens at the reading level.
• OD: D = -3.75, d = 11 mm, P = Dd/10 = (3.75)(11)/10 = 4.1
• The right lens is a minus lens, the wearer is looking below the oc so the oc is UP, the apex is where the oc is, so the base is DOWN. The right lens has 4.1 BD
• OS: D = -2.75, d = 11 mm, P = Dd/10 = (2.75)(11)/10 = 3.0 
• The left lens is a minus lens, the wearer is looking below the oc so the oc is UP, the apex is where the oc is, so the base is DOWN. The left lens has 3.0  BD
2. Second, determine what the total vertical imbalance is. BD and BD are cancelling because they are sending the eyes in the same direction, so the total imbalance is 4.1 - 3.0 = 1.1 .
3. Last, determine which lens the bicentric grind should be done on. This is reverse bicentric grind, which is done on the most plus or least minus lens. The -2.75 is least minus, so the reverse slab-off will be 1.0  on the OS.

• Why 1.0  instead of 1.1 ?


Because the reverse slab-off won't be available in 1.1 . It is not available in every possible amount of prism, so we will choose the closest one.

• Before you said that you usually do this for imbalances over 1.5 . This pair only had 1.1  imbalance.


Some people are more sensitive to imbalance than others. Perhaps this person has a slight hyperphoria that is aggravated by this extra imbalance. I would not automatically do this for a 1 diopter imbalance, but if the wearer already has it and the Rx is not less anisometropic that the old Rx I will not take it away; or if the Rx has changed and the wearer complains of double vision or discomfort while reading only I will go ahead and order the reverse slab-off.

• Why not move the optical centers or do different seg styles?


Depending on the old glasses, the reason that the wearer is experiencing problems, and the wearer's attitude toward the cosmetics, I might try one of those solutions first. This is a judgment call based on experience.

• What if the wearer insists on keeping his photo-gray extra glass lenses?


Then we HAVE to find a different solution, because reverse slab-off is not available in PGX, and bicentric grind is not generally done below a 1.5  prism because it is difficult to get the line crisp and the line placement correct below 1.5 .

We are not going to use the method that we have been using here on this pair, because the adds are not equal. The difference in the adds will also induce some vertical imbalance, and that needs to be taken into account. We do some of these in the advanced Dispensing class and the technique in covered in the textbook if you are comfortable with what we are doing. For this class, we will not do any of these.

• Why would there be different adds? I have never seen one like that before.


Typically this will be the result of the cataract being removed from, in this case, the left eye but not the right eye. There are other instances where this happens, but that is the most common one. In some practices, especially Ophthalmic practices, you will see more of this than in other practices; but you will probably see one of these at some point.

• Have you ever done a slab-off for single vision?


Yes, it happens. For some reason a person with an anisometropic Rx has to move the eyes instead of the head to read. It can be done. It has been done.

Well, this time the adds are the same, so we can proceed.

1. First, determine what amount of prism is present in each lens at the reading level.
• OD: D = +3.75, d = 10 mm, P = Dd/10 = (3.75)(10)/10 = 3.8
• The right lens is a plus lens, the wearer is looking below the oc so the oc is UP, the base is where the oc is, so the base is UP. The right lens has 3.8 BU
• OS: D = +1.75, d = 10 mm, P = Dd/10 = (1.75)(10)/10 = 1.8 
• The left lens is a plus lens, the wearer is looking below the oc so the oc is UP, the base is where the oc is, so the base is UP. The left lens has 1.8  BU
2. Second, determine what the total vertical imbalance is. BU and BU are cancelling because they are sending the eyes in the same direction, so the total imbalance is 3.8 - 1.8 = 2.0 .
3. Last, determine which lens the bicentric grind should be done on. The problem does not specify regular or reverse. If I can order reverse I will, so since the OD is the most plus the reverse slab-off will be 2.0  on the OD. If I must order regular bicentric grind for some reason I will order 2.0  bicentric grind on the OS because it is the least plus.

• My store manager insists that every part of a pair of glasses that we receive from the lab be verified that it is correct before we call the patient to say the glasses are ready. I doubt he would let us dispense something like this without verifying that the amount of prism is correct. How would I verify it?


You verify it by using the lens clock, a.k.a. the Geneva Lens Measure. You might think that you would want to do it in the lensometer; but unless yours has the special prism rings built in this is difficult. And it is made worse by the fact that the bifocal segment itself adds some prism right under the line that is not there above the line. The technique that is described in the link above is accurate and simple to do.
 
 

OK, go back to the textbook. Do the exercises on page (95 / 112, 114) and check your answers in the back of the book. Call or e-mail me if you have questions.