A modification of JET is presently being prepared to bring operational experience with ITER-like first wall (Be) and divertor (W) materials, geometry and plasma parameters. Reflectivity measurements of JET sample tiles have been performed and the data are used within a simplified model of the JET and ITER vessels to predict additional contributions to quantitative spectroscopic measurements. The most general method to characterize reflectivity is the BRDF. For extended sources however, like bremsstrahlung and edge emission of fuel and intrinsic impurities, the results obtained in the modeling are almost as accurate if the total reflectivity with ideal Lambertian angular dependence is used. This is in contrast to the experience in other communities, such as optical design, lighting design or rendering who deal mostly with point-like light sources. This result is so far based on a very limited set of measurements and will be reassessed when more detailed BRDF measurements of JET tiles have been made. If it is true it offers the possibility of in-situ monitoring of the reflectivity of selected parts of the wall during exposure to plasma operation, while re-measurement of the BRDF is performed during interventions. For a closed vessel structure like ITER, it is important to consider multiple reflections. This makes it more important to represent the whole of the vessel reasonably accurately in the model, which on the other hand is easier to achieve than for the more complex internal structure of JET. In both cases the dominant contribution is from the first reflection, and a detailed model of the areas intersected by lines of sight of diagnostic interest is required.