The American National Standards Institute (ANSI) X80.5 Standard (Requirements for Ophthalmic Frames) provides the following definition for pantoscopic angle:
The angle formed by the front and temple with the temples in the open position, as viewed from the side of the frame. The numerical value in degrees is determined by measuring the angle between the centerline of the temple (straight line from the center of the ear bend) and a line drawn perpen-dicular to the vertical axis of the corresponding eye of the front.
This is an interesting measurement to take but it is not the one you need for fitting premium free-form lenses that ask for POW measurements. What you should be measuring is the tilt of the lenses as they sit before the eyes. This is known as pantoscopic tilt.
The image at left illustrates three people all wearing the same frame. In all three cases, the pantoscopic angle of the frame is the same but notice how different the lenses are tilted in relationship to the wearer’s face plane (an imaginary line touching the forehead, lips, and chin). The person at left has a normal ear position and the eyeglass lenses have a mild pantoscopic tilt. The middle person has a low ear position, which causes almost zero pantoscopic tilt. And the person at right has a high ear placement, which results in a dramatic pantoscopic tilt.
|Notice how a frame with the same pantoscopic angle has different pantoscopic tilt angles due to ear position of the wearer. This means pantoscopic tilt must be managed by adjustment.|
Even though all of these frames have the exact pantoscopic angle, they have a different pantoscopic tilt for their respective wearers. This means you should not measure the angle created between the temple and the frame and send that value in for the pantoscopic tilt of the frame. Instead, you’ll need a way to measure the tilt of the lenses before the patient’s eyes.
When discussing the face plane concept, understand that this is an imaginary line and is theoretical since everyone’s forehead, lips, and chin do not align so neatly. Even so, the concept is an essential one. Lens tilt like pantoscopic tilt is referenced to this imaginary line. Lenses have zero tilt when they are fitted parallel to the face plane (known as orthoscopic tilt), they have pantoscopic tilt when the lower portion of the lens is closer to the face than the upper portion, and they have retroscopic tilt when the lower portion is further away from the face than the upper portion.
Pantoscopic tilt does not take ear position into consideration; it is simply the angular relationship between the face plane and the lenses. Sure, ear position is going to affect pantoscopic tilt, but the optician will be managing this during the fitting of the frame before POW measurements are taken. That’s the reason accurately fitting the frame before measuring is so important.
Now that you understand that pantoscopic angle and pantoscopic tilt are two different things, you won’t mix them up.
Ed De Gennaro is Director, Professional Content of First Vision Media Group.