Bifocal Lenses correct TWO Viewing areas, most commonly reading and distance. The change in power for the reading segment (the bifocal) is visible on the lens. The SEG (Bifocal window) is for near vision (reading), the remaining lens is for distance viewing. They are available in 28mm and 35 mm Seg widths. Also called “Lined Bifocals,” “Straight Tops,” or “Flattops;” Bifocal Lenses are readily recognizable by their half moon shaped reading seg.
Trifocal Lenses are similar to Bifocal lenses, except that they correct THREE Viewing areas: reading, intermediate (arms length), and distance. The change in power for the reading and intermediate segment (the trifocal) is visible on the lens. The top portion of the SEG (trifocal window) is for intermediate viewing, and the larger, bottom portion of the SEG is for near vision (reading). The remaining lens is for distance viewing. They are available in 28mm and 35 mm Seg widths. While the Trifocal Lens is a true multi-focal lens, allowing the wearer to see at all distances, the SEG is visible to people looking at the wearer.