corresponding Fourier space image. The different observed arcs of LR rings correspond to the reflected (R), and transmitted plus incident (T) beams. The L beam is associated with a SPP launched in the direction to the left, i. e, away from the mirror. C is the directly transmitted laser beam distribution. By acting in the Fourier plane image F ′ of the LRM microscope we now block the information associated with the central beam and with the R or T beams [39]. Thereby the according SPP beam images are erased from the image plane and consequently weak intensity beams otherwise observed by interference can be directly analyzed. For further analysis we extracted radial cross-cuts along the white lines as shown in Fig. 10 B [39]. Results are shown in Fig. 11 A for the cross cut along L . The data points agree very well with a Lorentz fit given by Eqs. 20, 21. The FWHM of the Lorentzian distribution gives us a SPP propagation length of L$_{SPP}$ = 20 µ m. This value is in perfect agreement with the cross-cut made along the beam L in the direct space image 10 A (see Fig. 11 B). Both data agree also with values given by the dispersion relations discussed in sect. 1 (see Figs. 2B and 3B). LRM was subsequently applied by us to many SPP in-plane devices such as beam splitters [10], dielectric lenses, prisms [18] and wave-guides [42]. In particular, LRM experiments were compared to near field optical experiments ( photon scanning tunnelling microscopy) and showed good agreement in the cases considered [18, 42]. LRM appears thus in this context as a complementary far-field optical method to NFO such as NSOM. LRM was applied as well for analyzing SPP Bragg mirrors (with high reflectivity R ≃ 90 − 95%), interferometers [40, 43] and SPP elliptical cavities [35] or 2D SPP microscopes [44]. In this context we observed [40] stationary SPP waves with very high visibility V = ( I$_{max}$ − I$_{min}$ ) / ( I$_{max}$ + I$_{min}$ ) by using LRM. This proves directly that SPP wave coherence is conserved in LRM and can exploited for quantitative analysis down to the spatial resolution limit λ$_{SPP}$/ 2. Actually SPP interferometers such as the ones described in [34, 35, 43] reveal clear interference pattern and oscillation characteristics of these devices. It is thus possible to develop 2D interferometry for SPP waves having all the advantages of current macroscopic interferom- etry techniques. We also mention the realization of plasmonic crystals (i. e., photonic crystals for SPPs) which were studied using LRM (see Fig. 12). In such devices [45] rectangular 2D latices made of gold nano-protrusions (200 nm diameter, 50 nm height) deposited on a 50 nm thick gold film (see Fig. 12B) are used to create photonic band gaps at specific SPP 19