Progressive lenses must be accurately measured or fit because the progressive power placement is crucial for comfort and function. This type of lens provides a progression of power instead of only two or three lens power -- the way bifocal and trifocal lenses work. Progressive lenses are also called PALs, invisible or no-line bifocals. Measuring a PAL properly ensures the wearer will experience the best possible vision.
- Skill level:
Things you need
- Pupillary distance ruler
- Clear tape
- Washable marker
Sit directly across from the progressive wearer. Adjust your posture so that you are at the same eye level as the other person. Ask him to adjust his posture so he is sitting comfortably.
Check the fit of the frame. Be sure the frame sits comfortably on the nose and ears and is in a place that feels right for the wearer. Make any necessary adjustments and replace the frame.
Place a dot on the right lens over the person's pupil. If there is no lens in the frame, place a piece of clear tape horizontally across the front of the lens, and put the dot on the tape. Repeat with the left lens.
Use the pupillary distance, PD, ruler to measure the progressive height in each lens. Measure from the centre of the dot down to the bottom of the inside edge of the frame, also called the eyewire. This measurement is called the seg height.
Measure the person's pupillary distance with the PD ruler. Place the zero edge of the PD ruler on the centre dot of the right lens, and measure horizontally across to the middle part of the frame. Repeat with the left lens, placing the zero part of the PD ruler on the centre of the frame and measure across to the dot on the left lens. Take this measurement at least twice for accuracy.
Tips and warnings
- Keep in mind that the seg height and PD measurement for the right and left eye may not be the same.
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