This equation is often used as a rough estimate of the appropriate thickness of the sample before the sample is prepared for STXM measurements. For example, a log(I0/I) value (also called OD or optical density) should always be targeted to be <~1.5 to prevent excessive absorption and misinterpretation of data. However, when the sample is unstable, or is degrading/decomposing, such as it is often the case for perovskite materials, it is imperative that close attention be paid during the acquisition of the data as well as on its interpretation. Figure Another important but trivial assumption inherent to conventional STXM measurements is that in a mixture of materials, transmitted X-rays do not retain the order of "material encounters".