Quantitative measurements of fluid flow properties can be achieved by background-oriented schlieren (BOS). In this paper, it is shown that this depends on several factors. Image-quality index is used to investigate the influence of the image sensor and the quality of its output. Image evaluation is applied to synthetic images, which are treated with a step function, so that they simulate the sharp density jump. The gradual change of the evaluated vector shift revealed the major dependence on the interrogation window, and revealed less of a dependence on background features. BOS applied to shock-wave reflection from a wedge in a shock tube gave qualitative results, due to large uncertainties. But, the application to cooling by natural convection gave satisfactory results, comparable to thermocouple data and theory.