Emissivity of an optically thin plasma as a function of temperature observed by the ACIS-I detector on Chandra in two energy bands: 0.5–3.5 (filled) and 3.5–7.5 (dashed) keV. Abundance of heavy elements is assumed 0.5 relative to solar. Shaded region shows a range of gas temperatures in the core of the Perseus Cluster (3–6.5 keV). Notice, that for the 0.5–3.5 keV band, the emissivity is almost independent of the temperature within this region. Therefore, the X-ray surface brightness in this shaded band I_X is just a function of gas electron number density n_e, namely, |$I_{\rm X}\sim \int n_{\rm e}^2 \mathrm{d}l$|⁠, where l is the length of the line of sight.