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An Ultex bifocal lens is a specific style of multifocal eyewear that was very popular in the early 20th century. It is characterized by a very large, round reading segment at the bottom of the lens. It is technically known as a "one-piece" bifocal because it is made from a single piece of lens material, rather than two pieces fused together.
read more about ultex bifocal lens ...Most glass bifocals from the past were "fused," meaning a button of high-power glass was melted into the main lens. The Ultex is different. It is ground from one solid block of glass or plastic. The change in power for the reading segment is created by grinding a different curve onto the back surface of the lens. This makes the lens smooth on the front.
The reading area on an Ultex lens is a very large round segment. Because of the way it is ground, this segment can be much wider than other round-segment bifocals (like the Kryptok). This provides a very wide field of view for reading, which was a major advantage of this design when it was first introduced.
While the Ultex offers a wide reading area, it has a significant optical flaw called "image jump." Because the top of the reading segment is round, the optical center is far away from the line where your vision crosses over. This causes objects to seem to jump suddenly when you look down. This annoyance led to the invention of the Flat-Top bifocal, which largely replaced the Ultex.
One of the major optical benefits of the Ultex design was its visual purity. Older "fused" bifocals created the reading power by melting a button of high-index glass into the main lens. The boundary between these two different materials often acted like a prism, causing "color fringing" or rainbow halos around the reading line. Because the Ultex is made from a single, continuous piece of material, it eliminated this chromatic aberration entirely.
Unlike a Flat-Top or Executive bifocal where you can feel the ridge of the segment on the front of the lens, the Ultex is physically distinct. The change in power is created by a change in curvature on the back surface of the lens. This leaves the front of the glasses perfectly smooth to the touch. This design made the lenses easier to clean and gave them a streamlined profile, even though the outline of the reading zone remained visible to others.
They are very rare today. Most optical labs no longer stock or surface them because modern Flat-Top and progressive lenses offer better optical performance and cosmetics. You might find them through specialty vintage eyewear restorers.
Yes. Although the front of the lens is smooth, you can clearly see the large, curved line of the reading segment. It is more visible than the line on a modern Flat-Top bifocal.
Both have round segments. The Kryptok is a "fused" lens with the segment on the front. The Ultex is a "one-piece" lens with the segment ground onto the back. The Ultex segment is usually much larger.
You typically would not choose an Ultex lens for a new pair of glasses today unless you are trying to replicate a specific historical period or vintage look. If you are having trouble reading with your current glasses, your optician will likely recommend a Flat-Top 28 or a progressive lens, as these provide a more comfortable and seamless visual experience.