On secondary representations of the second

The emergence of microwave and optical frequency standards with accuracies close to those of caesium primary frequency standards has led to the concept of secondary representations of the second. The recently-formed Joint Working Group of the CCL / CCTF has now recommended as secondary representations to the CCTF / CIPM one microwave radiation in ⁸⁷Rb and three optical radiations in ¹⁹⁹Hg⁺, ⁸⁸Sr⁺ and ¹⁷¹Yb⁺. This paper describes the rationale, evolution and development of criteria for acceptance of a radiation as a secondary representation, together with a review of the JWG assessment and recommendation of the four radiations already proposed.