BiFocal Gauge Reading Scuba Dive Snorkeling Purge Mask Prescription Lenses Promate Pro Scanner Scuba Dive Purge Mask (Rx-Able)/MK650: Bifocal Optical correction lenses, also called Gauge Reader available in +1.0, +1.5, +1.75, +2.0, +2.25, +2.5, +2.75, +3.0, +3.5, +4.0.