In some cases visual problems can arise if a patient with higher anisometropia works in the near. The slab-off technique is a special grinding method used to generate a compensating prism in multifocal and progressive lenses. It results in a barely visible, horizontal dividing line which extends across the entire width of the lens.
Multifocal or progressive lenses produced using the slab-off technique may be necessary for presbyopic patients with anisometropia. The term anisometropia is used if there is a difference of 0.25 D between the far point refraction of the right and left eyes.
How double images occur
However, visual problems which can arise if a patient with anisometropia wears multifocals should only be expected if this difference is greater than approx. 1.5 D. Slab-off lenses can then be prescribed to remedy these problems.
When the wearer moves his eyes across the lens, he no longer looks through the major reference point (with non-prismatic powers = optical centre), but through the peripheral areas of the lens. This results in prismatic effects, i. e. the principal ray is deviated prismatically. When the wearer lowers his eyes, the intermediate image fixated by the wearer lies either above (minus lens) or below (plus lens) the real object.
In anisometropia, the intermediate images generated in front of the two eyes in peripheral vision lie at different heights due to different prismatic effects on the right and left. If this prismatic height difference exceeds a value that can be compensated for by the wearer, he can no longer use the intermediate images of the right and left eyes to form an unimpaired, binocular image, or he can only do so to a limited extent.
Problems for the patient
In extreme cases, the wearer sees double images. More frequently, however, letters appear unsharp when reading, and longer periods of reading become very strenuous. This can result in asthenopic symptoms such as headache or smarting of the eyes.
Slab-off is the answer
With the slab-off technique, the intermediate images obtained when reading with multifocals are at the same height for the right and left eyes. Unimpaired binocular vision without eye strain is then possible again.
With single vision lenses
In single vision lenses the anisometropic wearer can move his head in such away that he always approximately looks through the major reference point of the lens. In this way, he avoids prismatic effects and hence also a prismatic difference between the right and left lenses.
With multifocal lenses
In multifocals, however, the lens design compels the wearer to lower his eyes for reading. Head movement is then no longer possible to compensate for prismatic effects. When an anisometropic, presbyopic patient receives his first pair of multifocals, he may therefore experience visual problems when reading which he did not experience with the single vision lenses he has been wearing until now.
The slab-off technique involves the incorporation of a compensating prism with base 90° for near vision in the mathematically more negative multifocal or progressive lens (weaker plus or stronger minus lens).
Slab-off technique, a compensating prism
When the wearer looks through the near portions or near zones of the lenses, the compensating prism deviates the principal ray behind the mathematically more negative lens to the same extent as the principal ray is deviated behind the mathematically more positive lens. This means that the prismatic effects in the major reference point for near vision are the same again on the right and left.
Binocular near vision without eye strain
As a result, the intermediate images are at the same height for both lenses and can be fixated and fused into an unimpaired binocular image when the wearer looks through the near portions or near zones. Binocular near vision without eye strain is then possible again.
In bifocal lenses the slab-off technique has been in use for a long time. As progressive lenses have now become increasingly popular and have almost replaced bifocals in some countries, there was a need to also provide problem-free binocular vision for anisometropic patients who wished to wear progressive lenses.
However, slab-off production was not technically possible for progressive lenses for a long time. ZEISS is currently the only manufacturer to have succeeded in implementing this extremely intricate process. Unlike slab-off production for bifocal lenses, compensation must be made for prismatic differences not only in the major reference points for near vision, but also in the those for distance vision.
As a bifocal lens acts like a single vision lens in the distance portion, there are no prismatic effects when the wearer looks through the major reference points for distance vision and hence also no prismatic differences between the right and left lenses. In progressive lenses without a slab-off prism, the prismatic difference only totals zero in the prism measuring points. In the major reference points for both near and distance vision there is a prismatic difference between left and right as soon as the dioptric power differs on the two sides.
In slab-off progressives, the prismatic difference in the major reference points for distance is removed by grinding a prism over the entire Rx surface. As in bifocals, the prismatic difference in the major reference points for near vision can be compensated for with the aid of the slab-off technique. The compensating prism with base 90° is produced on the mathematically more negative lens (weaker plus or stronger minus lens).
Slab-off progressives allow the anisometropic wearer to benefit from the many advantages of progressive lenses. Although the lens cannot be used at the height of the prism measuring point due to the slab-off edge, the anisometropic wearer still has more visual zones at his disposal than with a pair of bifocal spectacles.
Example of prismatic powers in a progressive lens:
R +3.0, L +5.0, Add.: 2.0
b. d. = base down b. u. = base up
|Distance measuring circle
||R: 3.3 b. d. L: 4.5 b. d.
||R: 4.5 b. d. L: 4.5 b. d.
|Pr. measuring point||R: 1.2 b. d. L: 1.2 b. d.||R: 2.4 b. d. L: 1.2 b. d.|
|Near measuring circle||R: 4.7 b. u. L: 7.2 b. u.||R: 7.2 b. u. L: 7.2 b. u.|
The slab-off edge which is located on the front surface of bifocals and coincides with the segment top can only be produced for prisms over 1.5 cm/m. This approximately corresponds to anisometropia of 1.5 D.
A prismatic "slab" of material is removed from the distance portion on the front surface of the semi-finished lens, producing a base-down prismatic power.
This is followed by the grinding of the Rx surface. Here, the modified semi-finished lens is blocked at a slant in order to remove the prism in the distance portion again and to obtain a base-up prism in the near portion.
The slab-off edge which is located on the back surface in the case of progressive lenses and lies approx. 0.5 mm below the prism measuring point can only be produced for prisms over 2.0 cm/m. This approximately corresponds to anisometropia of 1.75 D.
Compensation for the prismatic difference in the distance centration crosses is already taken into account when the Rx surface is being ground. The actual slab-off process does not commence until the grinding process is complete. After an auxiliary lens is cemented onto the Rx surface, the bond is left to harden for approx. 24 hours. The progressive lens is then blocked at a slant on its front surface so that the required prismatic power for near vision can be ground onto the lower area of the back surface.
Tipps for the optician about limitations for progressive lenses with slab-off technique: