|Fig. 1: Three Optical Centers of a Bifocal|
|When reading the powers of a bifocal, you’ll see two lensometer targets. As you focus on one, the other will become blurred. The upper one is the distance optical center, the one inside the segment is the resultant optical center.|
Each issue, Tech Tips will explore some interesting aspect of optical technology. This month we look at the resultant optical center.
This is when the prismatic effects of each lens interact with each other.
The optics of ophthalmic lenses is, for the most part, invisible. Single vision lenses have no identifying markings, neither do progressive addition lenses. On the other hand, bifocals and trifocals have lines that outline the segment zones. Even so, what creates the power in these lenses is invisible to the wearer and to observers of these lenses. That means that in order to locate lens parameters such as the distance optical center, cylinder axis and prism power, you have to use instrumentation.
A common task eyecare professionals perform is locating the optical center (OC) of a lens. That’s pretty easy to do; just align the center of the lensometer’s target with the center of the reticle circles and there it is! Bifocal lenses are a little more complicated. This lens style has two zones; a distance and a reading zone. Because of the two zones, you might expect this lens to have two optical centers, one for the distance zone and one for the reading zone…but that’s not so. A bifocal lens has three optical centers.
The simplest way to understand this is by using an example of a glass lens that has its segment glued onto it. Yes, many years ago (early 1900s), opticians cemented segments onto single vision glass lenses to create bifocals. The single vision lens has an optical center of its own so that’s optical center #1. The segment is a small round lens…let’s say 22mm in diameter, so it has an optical center of its own, making optical center #2. So where’s the third OC?
The answer lies in the interaction of the two lenses when they are combined. In other words, the prismatic effects of each lens interact with each other and it’s this interaction that creates another optical center known as the resultant optical center.
Fig. 1 illustrates the three optical centers. The distance OC is where you’d expect it to be for a single vision lens…on the Datum (180) line and a bit decentered. Since the cemented segment is a little 22mm round spherical lens, it has an optical center right in the middle of the lens. Assuming that the distance portion of this lens has plus power and the add has plus power, the upper portion of the segment delivers base down prism while the lower portion of the distance lens recreates base up prism. At some point, the interaction of the prismatic effect of these two lenses will equal zero. That’s the location of the resultant optical center. When this occurs inside the segment area, it’s known as a real resultant OC. When it falls outside the segment area, it’s a virtual point that can be calculated.
When using a lensometer to read the powers of a bifocal, you’ll be able to view the distance optical center in the distance area of the lens and the resultant optical center within the segment zone but you will not be able to see the segment optical center. Why? Remember the resultant optical center is created by the interaction of the prismatic effect of the distance and near lenses. The segment optical center was produced when the segment was made. Before it was cemented onto the distance lens, you could easily view it but once it’s combined with the distance lens, it’s no longer locatable because its optics interact with the distance lens (see fig. 2).
The principles detailed above apply to all segmented multifocal lenses, even though they are not cemented together. Now what about a progressive lens…does it have a resultant optical center? In a sense, yes…a progressive one. As you move down the corridor, the position of the resultant center changes progressively.
Now what about a trifocal? How many optical centers does it have? There are five — a distance OC, a bifocal OC, a trifocal OC, and two resultant OCs (one for the bifocal segment and one for the trifocal segment).
Ed De Gennaro is Director, Professional Content of First Vision Media Group.