What is slab-off?
It is a special lens grinding technique used to neutralize the unwanted prism effect when looking down through the reading area (usually a bifocal) of widely differing lens powers. Slab off insures that the near image will not be displaced. If it is displaced, the patient will see two different images at near in different places although he is trying to fixate on the same point. Slab off can also be called bi-centric grinding. Though it is usually done on bifocals, it can also be done on single vision.
How did slab off get it’s name?
A slab-off looks as if a “slab” or sections of the lens has been removed, hence the name slab-off.
What lenses can be ground with slab-off?
Bifocals, Single Vision Progressives, Trifocals in virtually any material. On Trifocals, the slab “line” is placed between the reading and intermediate section of the seg, not the top line like a bifocal. On a Progressive it can be placed typically at the optical center of the Progressive or 2-3 below. The optical center of a Progressive is anywhere between 2-6mm below the fitting cross. The slab-off process on a progressive only equalizes the near prism at one point in the progression so every other height will contain some imbalance. A general rule of thumb is to have the prism equalized at 12mm below the pupil location.
When should slab-off be used?
As mentioned, when there is a significant difference in Rx between both eyes. The optical term for this is called Anisometropia. Below are some examples anisometropic Rx’s.
- OD: +4.00 OS: +1.00
- OD: -6.00 OS: -2.00
- OD: +2.00 OS: -2.00
How much of a difference in Rx should be there be before slab-off should be considered?
Though opinions may vary in the industry, as a general rule if there is a difference of 1.5 diopters, then slab-off should be considered. In all 3 examples above slab off should be ground as there is a 3.00 diopter difference in example 1, a 4.00 diopter difference in example 2 and 4.00 diopter difference in example 3.
How do I determine if an Rx with cylinder needs slab-off?
This is a little bit more complicated. First, when looking through a bifocal, we are only concerned with the power at 90 degrees. So, if we look at the following example:
-2.00 -1.00 x 90
-5.00 -1.00 x 90
Since the axis of both eyes is at 90 degrees, we see that all we have to do is calculate the difference between both spheres and we find that the difference is 3.00. Slab-off would be indicated here.
How do I then calculate if slab-off is needed if the axis is NOT 90 degrees?
We have to use the table below and a little math to make that determination.
Let’s look at the following Rx:
OD: -0.25 -1.00 x30
OS: -5.00 -0.75 x 50
Use the following steps and questions.
- Working with each eye separately, how many degrees away from 90 is the axis? In this case the OD is 60 degrees away and the OS is 40 degrees away.
- What is the percentage of cylinder power to be added to the sphere power? Using the table above go to 60 and you’ll see that you have .75 of the cylinder power and 40 is .44 of the cylinder power.
- Do some simple math to determine the resultant sphere power by doing the following: We know in the OD we have .75 of the cylinder power so multiply the cylinder -1.00 x .75 and the result is -.75. Now take the -.75 and add it to the sphere power to get -1.00. The power is -1.00 at 90 degrees. Do the same for the OS. We know in the OS we have .44 of the cylinder power so multiply the cylinder
-0.75 x .44 and the result is -.33. Now take the -.33 and add it to the sphere to get -5.33. the power is -5.33 at 90 degrees.
- What is the difference between both eyes at 90 degrees? In this case, it’s 4.33.
- Doesn’t reading drop have to be taken into consideration? Yes, and here’s how you do the final calculations on this. Looking at the example, depending upon the type of lens, FT or Progressive, calculate the Rx difference as determined in steps 3 and 4, then multiply it by13 (the reading drop for a Progressive) or 5 plus the distance optical center from the seg for a FT. If this Slab-off is a Progressive, multiply 4.10 X 1.3 (decimal moved one place). The resultant slab-off would be 5.33. If this is a FT with the optical center placed 3 above the seg then the drop is 8 (5+3), multiply 4.10 X .8 (decimal moved one place) The resultant slab-off is 3.28.
- Which eye gets the slab-off?
Use the table below:
|Lens Type||Type of Slab:||Apply Slab-off to:||Prism Ground:||Slab placed on:||Price:|
|Glass lenses||Custom||most minus or least plus||Base Up||Front Surface||$100 and up|
|FT-28 Plastic||Reverse||least minus or most plus||Base Down||Front Surface||$45.00|
|All Others||Custom||most minus or least plus||Base Up||Front Surface||$100 and up|
All slab-offs are custom made, take 7-10 days and are not returnable.