ISBN 978-0-12-374697-9. ↑ With ISO 31-5, NIST and the BIPM have adopted the notation c0 for the speed of light in free space. ↑ "Current practice is to use c0 to denote the speed of light in vacuum according to ISO 31. In the original Recommendation of 1983, the symbol c was used for this purpose." Quote from NIST Special Publication 330, Appendix 2, p.