Scotopic/photopic ratio (S/P ratio, or Rsp) is an absolute measure of a light source's performance under the mesopic vision state. However, there is more potential for a light source to reach higher mesopic performance at higher correlated color temperatures (CCTs), and these cannot be evaluated. We proposed an Rsp ratio metric to examine relative performance of the S/P ratio at a corresponding CCT. Based on examination of a large set of light sources, we established the Rsp-CCT and Rsp ratio-CCT relations of different light source types. On investigating previously reported high S/P ratio light sources with the proposed metric, it shows high effectiveness in evaluating relative mesopic performance when considering CCT issues.