The normalization of the measured spectrum is carried out by forming the ratio of the measured earthshine spectrum and the spectrum of direct sunlight, the solar irradiance.

The natural logarithm is taken to linearize the exponential from Lambert-Beer's Law. Taking the logarithm of this ratio converts the result to an optical density.

In this way normalisation cancels out the Fraunhofer lines in the earthshine and also any multiplicative instrumental effects that might also affect the measurements.