It is advisable to use reference meters with detector diameters of ≥ 3 mm, because they can
easily be irradiated with an open beam, and they are less susceptible to contamination (dirt and
dust). The reference meter's surface reflections should be as small as possible. If the source
emits a divergent beam, then a reference meter with an integrating sphere may be advisable. It
is also acceptable to use meters with "flat" detectors and mathematical correction, based on
multiplying the emitted far field distribution with the measured angle-dependence of the
detector of the reference meter, and integrating over the range of far-field angles.