The progressive addition lens (PAL) was a wonderful invention and the concept of it was simple; make a lens that provided accommodative-like assistance to the presbyopic eye with no visible segments or segment demarcation lines. These lenses deliver that accommodative-like assistance through the use of an intermediate corridor and reading zone. In order to provide these zones (and a distance zone), these lenses deliver some portions of the lenses that are less clear for foveal vision, usually found in the periphery and lower periphery of the lenses. These less clear areas create clear zones. Since increasing power creates unwanted astigmatism, the intermediate powers section has the narrowest zone width so you’ll sometimes see manufacturers claiming corridor widths of a certain size.
CENTER OF THE CORRIDOR To ensure a PAL wearer receives maximum efficiency from the lens’ intermediate corridor width, the eye must track down the corridor’s length as it gazes from the distance zone into the intermediate zone or the reading zone. If it does not track down the direct center of the corridor, the wearer may come close to the boundaries of less clear vision, which may cause visual confusion, blur, or discomfort.
Taking a PD measurement is the start of having a patient track down the center of a lens’ corridor. The problem with this measurement, even one that was taken highly accurately, is these measurements are a simple geometry measurement. They don’t take the lens optics into consideration.
PRISMATIC EFFECT Lenses with power induce a prismatic effect that pulls the eye in or out depending on distance prescription. This is fine in a single vision of a segmented multifocal lens but in progressives, the corridor width is relatively narrow (3mm to 4mm on average). In addition, the pupil of the eye is about 3mm in diameter in normal daylight and 5mm or more in dim light.
For a PAL wearer to avoid experiencing blur using the intermediate zone of the lenses, the pupil of the eye must track down the center of the corridor without overlapping the corridor boundaries on each side. If that boundary is crossed, the wearer will experience visual confusion, blur, or discomfort. As you can see from Fig. 1A, the power of the distance portion of the lenses will influence the path of the eye as it tracks down the corridor. In this example, these eyes will converge more due to the base out prism in the distance lenses.
Fig. 1B illustrates how the eye will be influenced by prismatic effect in downward gaze. In this case, the base up prism will influence the eye to look further down in the lens than you would expect from simple geometric measurements.
MAKING ADJUSTMENTS Many PALs have addressed these problems by varying the inset and length of the corridor. The lens in Fig. 2 has been compensated this way. Notice that both the corridor length and inset are adjusted. If done properly, this compensation positions the corridor so the eye can fully use the corridor and add zones and avoid visual confusion, blur, or discomfort.
Variable inset and corridor length compensation is a useful feature. Without it, PAL wearers run the risk of having trouble adapting to and using their lenses.
Ed De Gennaro is Director, Professional Content of First Vision Media Group.