Intermediate Progressives can be a confusing lens to understand. In this article, we’ll explain what information you have to give the lab, what the final Rx be at the top and what the design of these lenses are like. Let’s look at the Rx below:
Digression used based on the add power is -2.25.
The design of an Intermediate Progressive emphasizes a wide reading and Intermediate area and depending upon the design, a small area for distance viewing. Since the reading and intermediate is wide from edge to edge, we call this “windshield optics.” The distance area is just wide enough to view objects out to approximately 8 feet however, cannot be used for intensive distance such as TV, driving or movies. Intermediate Progressives are task-specific lenses. Our most popular Intermediate Progressives are the Essilor Computer, Shamir Office and Sola Access.
What’s different about an Intermediate Progressive? Every Intermediate Progressive has a digression. What’s a digression? Just like you have a Progressive where the power increases or progressives, the digression power of an Intermediate Progressives decreases as you move up the lens or digresses. Most Intermediate Progressives have several digression options. For instance the Shamir Office is available in 4 options: 0.75, 1.25, 1.75, and 2.25. These are actually minus powers and to reduce the confusion, manufacturers really should express these options -0.75, -1.25, etc. The digression option is chosen based on the add power. The digression and add power are not the same however they do interact with each other.
To determine what the Rx will be at the top of the lens, let’s use the Rx shown and apply the following rules:
Algebraically add the distance Rx to the add to get the total add power at the bottom (in this Rx this would be +3.50).
Algebraically subtract the digression power selected from the total add power (in this example, this would be +3.50 -2.25 or +1.25). +1.25 is the final or stabilized power at the top of the lens.
It’s really that easy if you apply these simple rules and it doesn’t have to be confusing.
When you order an Intermediate Progressive, give us the Rx as if it was a Progressive (distance Rx and add power). Do not give us an Intermediate Rx. Also, give us the height and p.d.’s. The reference point for the height and p.d.’s, distance or near, may vary depending upon the design.
Last, it’s also important to mention that when you verify the Rx of an Intermediate, it’s always checked at the reading circle. It cannot be verified at the top of the lens or in the intermediate area. Each Intermediate Progressive has the digression power engraved on the lens. If the total add power is correct at the reading circle and the correct digression is engraved on the lens, then the Rx is correct as ordered!