Using a pupillary distance (PD) ruler involves measuring the distance between the centers of your pupils. This measurement is crucial for ordering prescription glasses online or ensuring frames are fitted correctly.
How to Use a Pupillary Distance Ruler
Measuring your PD with a standard ruler or specific PD ruler is a straightforward process, often done with the help of another person. As noted in guides on this process, it's important to ensure the patient is looking at a specific point during the measurement. For instance, as highlighted in the provided video snippet, you get the patient to look at your right eye throughout this procedure (or a distant point).
Here's a step-by-step guide:
- Gather Your Tools: You'll need a small ruler marked in millimeters (mm) and a mirror or another person to help you. A dedicated PD ruler often has a small notch to rest on the bridge of the nose.
- Position Yourself and the Patient:
- If measuring yourself, stand about 8-12 inches (20-30 cm) away from a mirror.
- If another person is measuring you, sit down while they stand in front of you, at the same eye level, also about 8-12 inches away.
- Fixate Your Gaze: This is a critical step for accuracy.
- If using a mirror, look straight ahead at your own eyes in the mirror.
- If someone is measuring you, look straight ahead at their eyes, specifically focusing on one eye (e.g., as suggested in techniques like the one mentioned in the video reference, look at the examiner's right eye while they measure). Do not look at the ruler.
- Position the Ruler:
- Place the ruler horizontally across the bridge of your nose.
- Ensure the ruler is held steady and level. A PD ruler with a notch can help rest it comfortably.
- Align the Starting Point:
- Close your left eye. Align the zero mark (0 mm) of the ruler with the center of the pupil of your right eye.
- Alternatively, some prefer to align the zero mark with the edge of one pupil (e.g., the inner edge of the right pupil) and measure to the corresponding edge of the other pupil (e.g., the inner edge of the left pupil). Consistency is key.
- Measure the Distance:
- While keeping the zero mark aligned and without moving the ruler, open your left eye and close your right eye.
- Look straight ahead (or at the designated distant point/examiner's eye).
- Read the measurement on the ruler that aligns with the center of your left pupil.
- Record the Measurement: This reading is your binocular PD (the distance between both pupils). Repeat the process a couple of times to ensure accuracy and consistency.
- Monocular PD (Optional): Sometimes, you might need a monocular PD, which is the distance from the center of the bridge of your nose to the center of each pupil separately (Right PD and Left PD).
- Close one eye and align the ruler's zero mark at the center of the bridge of your nose.
- Read the measurement at the center of the open eye's pupil.
- Repeat for the other eye. The sum of your monocular PDs should roughly equal your binocular PD.
Key Tips for Accuracy:
- Use a well-lit area.
- Keep the ruler as close to your eyes as possible without touching your lashes.
- Maintain a steady gaze on the distant fixation point throughout the measurement.
- Have another person measure you if possible, as it's often easier and more accurate than using a mirror.
Pupillary distance is typically between 55 mm and 75 mm for adults and 40 mm and 55 mm for children. Having an accurate PD ensures the optical center of your lenses aligns correctly with your pupils, providing clear and comfortable vision.
