The selective catalytic reduction (SCR) based on urea water solution (UWS) is an effective way to reduce nitrogen oxides (NOx) emitted by engines. The high potential offered by this solution makes it a promising way to meet the future stringent exhaust gas standards (Euro6 and Tier2 Bin5).UWS is injected into the exhaust upstream of an SCR catalyst. The catalyst works efficiently and durably if the spray is completely vaporized and thoroughly mixed with the exhaust gases before entering. Ensuring complete vaporization and optimum mixture distribution in the exhaust line is challenging, especially for compact exhaust lines.Numerous parameters affect the degree of mixing: urea injection pressure and spray angle, internal flow field (fluid dynamics), injector location …. In order to quantify the mixture quality (vaporization, homogeneity) upstream of the SCR catalyst, it is proposed to employ non intrusive optical diagnostics techniques such as laser induced fluorescence (LIF). Such techniques can first of all enable a detailed characterization of the UWS spatial distribution upstream of the SCR catalyst. The capability to quantify urea mixture distribution in real exhaust geometries is a promising way to speed up the development and improve the performance of SCR systems.In this article is detailed the study of an SCR system as a whole, from the nozzle of the injector to the inlet of the SCR catalyst. Visualization applied to an SCR system is a good way to understand, analyze and optimize an SCR system. Indeed, our experimental tools allow to quickly qualify a technical definition of an SCR exhaust line. Experimental results obtained on an engine test bench are presented and discussed.