The theory of mesopic vision provides an important theoretical foundation for the choice of road lighting sources. Based upon a number of recent mesopic photometry studies, an equation (E mes = B •E P ) to deduce from the E p (photopic illumination) to Emes (mesopic equivalent illumination) is proposed, where B is instant for modified coefficient. The coefficient B can be used easily to calculate the mesopic equivalent illumination by using the measuring results of photopic illumination for different correlated color temperature (CCT) light sources under mesopic light levels. Using the equation, we analyze the variation of coefficient B with background lighting level of the different correlated color temperature lighting sources under mesopic vision levels. By calculating the mesopic equivalent illumination of the different sources, our results showing that the higher correlated color temperature LED sources have better visual effects. Moreover, the results provide a basis for further studies on the illuminometer, which might be suitable for mesopic vision.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.