Monday, May 22, 2006


Multifocal Design

Bifocals are made in two different ways. (Figure 40) One piece and fused lenses.

The one-piece type is made from one piece of glass or plastic. The lens surface is ground with two different curvatures. The shorter radius of curvature in the bifocal area creates the additional power. Fusing two different types of glass together makes fused bifocals. Each type of glass has a different index of refraction, which is not possible with plastic lenses. The segment button has a higher refractive index (flint n = 1.7) than the basic lens (crown n = 1.523).

Most flat top fused segments are designed with the optical center 4 mm below the segment top and produce only a very small amount of image jump (see below). Larger, flat top segments are similar to an executive bifocal, in that they will have little to no image jump.

Click on image to enlarge.

a. Image Jump is produced by the sudden introduction of the prismatic power at the top of a bifocal segment. The object the individual sees in the inferior field suddenly jumps upward when the eye turns down to look at it. If the optical center of the segment is at the top of the segment, there is no image jump. Image jump is worse in glasses with a round top bifocal, because the optical center of the bifocal is farther from the distance lens optical center. A flat top bifocal is better because the optical center of the bifocal is close to the distance optical center.

b. Image Displacement is the prismatic effect induced by the combination of the bifocal type and the power of the distance lens prescription in the reading position. Image displacement is more bothersome than image jump for most people. Most bifocal corrected presbyopes read through a point about 10 mm below the optical center of their distance lenses. If that position is also at the bifocal segment’s optical center, as in most fused flat top bifocals, the bifocal segment produces no prismatic effect at all. The prismatic effect that is there is induced by the distance lens correction, not the segment. However, if the optical center of the bifocal segment is located below or above the reading position, the bifocal will contribute to image displacement at the reading position. The total prismatic displacement will be the sum of that produced by the bifocal and that induced by the distance lens.

A flat top lens is essentially a base up prism, whereas a round top lens is a base down prism at the normal reading spot, 10 mm below the optical center of the lens. A myopic distance lens has base down prismatic power in the reading position; thus, image displacement is worsened with a round top lenses. The prism effects are additive. Similarly, a hyperopic spectacle lens is a base up prism in the reading position; thus a flat top lens makes image displacement an issue.

Most individuals will physiologically adapt, or learn to fuse small vertical deviations. If they cannot, there are several ways to compensate for the problem including, using contact lenses instead of spectacles, prescribing dissimilar segments, or providing "slab-off" prism.

In the past, slab-off prism (base up) was added to the spectacle lens that had the most minus or least plus correction for distance. Now, with modern plastic lenses, slab-off prism is taken off the mold, effectively adding base down prism to the most plus or less minus lens. This is called "reverse slab". Slab off is fabricated by way of bicentric grinding which creates two optical centers in the lens. One optical center is for the distance correction and the other is for the reading correction of the lens.

Question: If a patient comes in wearing glasses: OD +2.00, OS -2.00, and complains of vertical diplopia when reading. Both eyes are reading 5 mm down from the optical center. How much slab-off do you prescribe?

Answer: 2.00 prism diopters base up OS. A slab-off prism is always put in front of the more minus eye because slab off provides base up prism. In this example, the right eye will have induced 1.00 prism diopter base up and the left eye will have induced 1.00 prism diopter base down.

Question: Should a hyperope use a round top or a flat top bifocal?

Answer: A plus lens will have significant image displacement with a flat top lens. Image displacement is lessened with a round top lens. Although image jump will be present, it is less disturbing than image displacement.

Question: Should a myope use a flat top or a round top bifocal?

Answer: A round top lens has significant image displacement with a minus lens. A flat top lens minimizes image displacement and image jump for a myope.

Question: What is the induced prism for an individual wearing +5.00D OU, when reading at the usual reading position of 2mm in and 8mm down from the optical center of his lenses?

Answer: PD = hF, therefore, vertically +5.00D x 0.8 = 4PD BU per eye horizontally +5.00D x 0.2 = 1PD BO per eye

Question: What vertically compensating prism is needed for an individual wearing +5.00D OD and +2.00D OS when they are viewed in the normal reading position of 8mm down from the optical center of the lens?

Answer: +5.00D x 0.8 = 4PD BU and +2.00 x 0.8 = 1.6PD BU, 4-1.6 = 2.4PD BU needed for the left eye.


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