The digital PD ruler is used for testing the distance the pupil of eyes during optometry and fitting for glass. It uses direct sampling-point and extremely accurate aligning.Highly sensitive line-sensor and intellight system allow objective and precise measuring data. Range of focusing is adjustable by knob. Automatic shutdown function will effectively extend battery time. Power supply is available in DC 3.7V or 1 piece of battery ONLY. Pass CE ROHS certification