PRISM RX FLAT TOPS
PRISM RX FLAT TOPS
When a flat top bifocal lens Rx is ordered with prescribed prism... (flat top bifocal or a progressive for that matter), the amount of prescribed prism is placed in front of the pupil, in line with the visual axis. The optical center (OC) is moved to create the correct amount of prism for the way in which the patient should wear their lenses. The fitting height of the segment is ordered as measured without changes.
That means, at the patient’s PD, any prism measured should be equal to the prescribed prism. Or, the opposite is also true.
Using a lensmeter, locate the center of the target at the point of prescribed prism on the lensmeter reticle, then, dot the lens. Using a ruler, measure the distance between the two dots. The distance between the two dots should be equal to the patient’s PD. If not, apply ANSI tolerances to know whether to accept the lenses as OK.
Definitions to know:
PRP, Prism Reference Point.
In a flat top bifocal, or in a progressive, it is the point on the lens where the value of prescribed prism can be verified. In progressives, that is at the dot 2, 4, 6 below the Fitting Cross. (or in SEIKO lenses, 0mm) When there is no prescribed prism, the PRP is the same as the optical center (OC).
The vertical distance from the bottom most lens point to the topmost segment edge.
How do you measure PRP height?
In flat top bifocals or trifocals (round segs too!), the PRP is typically located at the lens’ vertical midline (half the frame’s “B” measurement). In most orders for flat tops, the OC is located at the vertical midline. It’s the same when there is prescribed prism. Some opticians may alter the height of the OC and/or PRP in a frame where the patient’s eyes are high up vertically in the frame. However, that is the exception.
How do you measure distance between Optical Centers (OC) and the PRP?
When there is no prescribed prism, they are the same point. With prescribed prism, in flat tops, the PRP is located at the PD at the frame’s vertical midline, the OC is the number of mm’s away that the prism equals (Prentice Rule). However, if the Rx is a cylinder and/or the prescribed prism is both horizontal and vertical, then the OC will drift along an oblique line opposite the prescribed prism.
Do you measure lens center thickness at the PRP or OC?
Thickness is measured at the lens’ prism reference point (PRP). In minus lenses, the lens at the OC is always its thinnest point but as the OC moves from the PD point, thickness changes. See the following gallery of examples of thickness; the right eye has no prescribed prism, the left is the same power but with prism.
These pages are from software called SOLARx that allows the virtual surfacing of lenses so the final forms can be seen.
To read the page, See the Rx, there will be examples of Plano, minus, plus and astigmatic Rx’s. The right lens has No Rx Prism (0.00D). The left lens in this example is 2 prism diopters (Δ) Base Out. The center thickness arrow shows the thickness at the PRP for the right and left lens. The rest of the details are listed in the screen. These lenses are representative of polycarbonate flat tops.
How do you measure if there is unwanted vertical prism?
When verifying a prescription, just back from the lab or your own in-office lab...
1. Verify the lens with the most power in the vertical (90th) meridian first.
a. Approximate the power of each lens at 90 degrees
2. Center the OC of the strongest lens first, frame flat on the stage, axis correctly aligned.
3. Then, verify the other lens. Do not change the height of the stage. With the frame against the stage, where is the center of the target vertically? If it is at the center of the reticle, there is no unwanted vertical prism. If there is, estimate how much. ANSI Z80.1 allows 0.33Δ. To see the ANSI tolerance table,