PD Ruler Material: PVC, plastic PD instrument / PD ruler. Measuring the distance. Keep your eyes straight ahead and focus on distant still objects. Place the gap on the nose as close to the eyes as possible, then find the middle and read the line closest to the middle of the pupils. Measuring range: 0-100mm. Size: about 21 x 5 x 0.7 cm / 8.26 x 1.96 x 0.27 inches (length * width * height). This is a very good promotional product, which is ideal for hospitals, medicines or medical supplies.