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 correct short [...] short sightedness, positive values correct long-sightedness. The dioptre is equal to the reciprocal value of the focal length of a lens measured in metres: a power of +5 dpt therefore has a reciprocal length [...] a visual acuity of 0.8 is required for reading travel timetables. Other minimum values include: 0.4 for watching television, 0.5 for reading the newspaper and 0.7 for reading a telephone directory. Is