The Coulomb repulsion, impeding electrons motion, has an important impact on the charge dynamics. It mainly causes a reduction of the effective metallic Drude weight (proportional to the so-called optical kinetic energy), encountered in the optical conductivity, with respect to the expectation within the nearly-free electron limit (defining the so-called band kinetic energy), as evinced from band-structure theory. In principle, the ratio between the optical and band kinetic energy allows defining the degree of electronic correlations. Through spectral weight arguments based on the excitation spectrum, we provide an experimental tool, free from any theoretical or band-structure based assumptions, in order to estimate the degree of electronic correlations in several systems. We first address the novel iron-pnictide superconductors, which serve to set the stage for our approach. We then revisit a large variety of materials, ranging from superconductors, to Kondo-like systems as well as materials close to the Mott-insulating state. As comparison we also tackle materials, where the electron-phonon coupling dominates. We establish a direct relationship between the strength of interaction and the resulting reduction of the optical kinetic energy of the itinerant charge carriers.
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