Further, during data acquisition it is also important to allow for the measurement of a region unobstructed by the sample. This allows for recording of the incident flux, I0, that could be used to obtain relevant optical density images following equation 1. In some cases simultaneous recording of an unobstructed region alongside with the region of interest is not possible (for example in planar thin films), the best practice would be to record the incident flux before and after each data set acquisition with the same spectral definition. Spectral points or energy points need to be carefully chosen in order to account for the sharp near edge fine structure features.