CIE Mesopic Model
The outcome of the work of CIE TC1-58 is published in a CIE technical report 191: 2010.
The CIE system for visual performance based mesopic photometry describes spectral luminous efficiency, Vmes(), in the mesopic region as a linear combination of the photopic spectral luminous efficiency function, V(), and the scotopic spectral luminous efficiency function, V’(), and establishes a gradual transition between these two functions throughout the mesopic region.
The system is of the form:
The values of m and Lmes for this system as a function of photopic luminance and light source S/P-ratio (ratio of scotopic-to-photopic luminous output) are given here .
You can calculate the mesopic luminance with the CIE mesopic model
using the Excel-file here.