CR-39 Flat Top Bifocal Lens 1) Refractive index (ND): 1.499 2) Abbe value: 58 3) Gravity: 1.32g/m3 4) Diameter: 28 / 70mm CR-39 Round Segment Bifocal Lens 1) Refractive index (ND): 1.499 2) Abbe value: 58 3) Gravity: 1.32g/m3 4) Diameter: 28 / 70mm