This part of ISO 9342 specifies requirements for reference lenses for the calibration and verification of focimeters that are used for the measurement of spectacle form lenses. It also gives a method for the determination of the back vertex power of the reference lenses. NOTE It is accepted that other reference lenses can also be used with powers within the given range, manufactured to the same standard of accuracy and form, but different back vertex powers. However, only lenses with integer nominal powers, as described in paragraph 4.1, can be used for the calibration of digitally-rounding focimeters.