Such devices act consequently as an efficient in-plane wavelength demultiplexer for SPPs [45] as it was indeed observed experimentally (see Fig. 12D and E). While the results discussed here were achieved within the visible spectral range, plasmonic crystal devices are expected to perform even better (e.g., in terms of spectral selectivity) in the infrared (telecom) spectral range due to significantly lower ohmic losses [46]. In general, the use of multiplexers, splitters and tritters [45] in photonic applications might be specifically appealing due to their small footprint in the range of 10 × 10 µ m $^{2}$. Furthermore, the use as building blocks for classical [38] or quantum [47] optical computing can be envisaged. In this article we reviewed the field of leakage radiation microscopy (LRM) theoretically and experimentally. Theoretically we analyzed how SPP can generate leaky wave in the glass substrate by tunnelling from the air side through a thin metal film supporting SPP waves. We showed in particular that for thick film ( D ≥ 70 nm) leakage radiation (LR) does not affect the dispersion relation on the air/metal interface. Importantly the angular distribution of LR is located on a cone matching the SPP dispersion relation. We also reviewed the first experimental results reporting the observation of LR on rough surface and using near field optics methods. We analyzed more recent application of LRM to SPP nano- devices fabricated by electron beam lithography. From all these results we can conclude that LRM is a convenient and versatile far field optical method for analyzing quantitatively SPP propagation on flat film and their interaction with various nano-devices of direct practical interest. Such versatility positions LRM as an appealing alternative to near field optics for studying SPP propagation on a scale of, or larger than the wavelength. For financial support the European Union, under project FP6 2002-IST-1-507879 is ac- knowledged. [1] H. Raether, Surface Plasmons (Springer, Berlin, 1988). [2] W. L. Barnes , A. Dereux, T. W. Ebbesen, Nature 424 , (2003) 824. [3] C. Genet, T. W. Ebbesen, Nature 445 , (2007) 39. 21 IV. CONCLUSION