reasons, it cannot be used during the 4-hour charging period. What is a dioptre? A dioptre (dpt) is the unit of measurement of the optical power of a lens. In the case of spectacle lenses, negative dioptre values [...] of the field of view. With only two standard batteries, the necessary voltage is stepped up to the 3.6 volts required. The step-up converter used here guarantees bright, even illumination over a long period