1. Bureau International des Poids et Mesures (BIPM),Sèvres Cedex,France,92312
2. National Metrology Institute of Japan (NMIJ) – National Institute of Advanced Industrial Science and Technology (AIST),Ibaraki,Japan,305-8563
3. Physikalisch-Technische Bundesanstalt (PTB),Braunschweig,Germany,38116