How do I Check PD on a Lensometer?

In order to see correctly, the optical center of a spectacle lens should be positioned in front of pupil. PD, or pupillary distance, is the measurement of the distance between the pupils and is used for determining where you look through your lenses and positioning the optical center. A lensometer is an optical instrument that is used to measure power in a lens and mark the location of the PD.

Things You'll Need

• Lensometer
• Eyeglasses
• Ruler with millimeters

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Instructions

• 1

  Place the right lens of the glasses onto the spectacle table and lower the lens holder down to keep the glasses firmly in place. Eye care professionals always work with the right lens first to avoid confusion. Make sure the glasses are placed so the front surface of the lens is facing up. The temples, or earpieces, of the glasses should be pointing away from you. If you are measuring a lens that is not in the frame, place it on the spectacle table facing the same way.

• 2

  Look through the eye piece and adjust the power wheel on the right side of the lensometer until a set of three parallel lines come in to focus. If the prescription does not contain astigmatic correction, there will be a clear, fourth line, forming a cross. If the prescription does contain astigmatism, the fourth line will not be in focus.

• 3

  Line up the center of the cross, whether all the lines are in focus or not, into the center ring of the reticle. The reticle looks like a target with numbered rings and allows you to place the lens's optical center in the middle. Once the center of the cross of lines is in the center circle of the reticle, lower the lens holder handle located on the right of the lensometer near the eyepiece. This will hold the lens in place.

• 4

  Mark the lens with the marking device, also located to the right of the eyepiece. Move the handle away from you and down, which will place three dots of ink on the lens. The center dot is the optical center of the lens. This is used to calculate the PD. Repeat with the left lens.

• 5

  Remove the lens from the lensometer to measure the PD. If the lenses are already in a frame, lay the frame down, the lenses up and, using a ruler with millimeters, measure the center dot from one lens to the center dot of the other. An average PD will measure between 58 and 73 millimeters but can vary, according to the University of Cambridge.