## 3 Results
### Object Composition Identification with Local Field Measurement  
Phase  
Figure 4b shows the dependence of the phase of the rf signal as a function of the angle between the object's surface normal and the sensor axis recorded for copper plates with different lift-off values. In contrast to the amplitude data set in Figure 4a, the angular dependence of the signal's phase show consistency regardless of the lift-off value. For small angles there is a small correlation between measured phase and lift-off, but phase converges to the same value at larger angles, e.g., \(\pm 50^{\circ}\). A similar trend is confirmed by the results recorded for different object shapes (not shown), and dimensions, Figure 5b. All data sets indicate that the phase of the inductive signal contains information unique to the object's composition that is immune to a change in lift-off and the object's dimensions. This statement could be made more general, i.e., the secondary rf field phase values and their distribution over the object do not vary with lift-off value, to include cases of the magnetically permeable objects. This is particularly well illustrated by the inductive images of the ferrite plate, which also indicates some limitations of the phase measurement.  
Figure 6 shows a series of amplitude (a) and phase (b) images of the ferrite plate recorded at \(0^{\circ}\) deg with different lift-off values. As discussed in ref. [23], the surface of the ferrite plate (negligible electric conductivity and high magnetic permeability) does not create an observable signal and only generates a non-zero signal around the plate edges. The amplitude of this signal decreases with increasing lift-off. The phase image has the form of a vortex, which reflects the change of the secondary field orientation inside and outside the plate [10; 12; 23]. The phase images recorded for different lift-offs not only share the same generic pattern (vortex) but also the value of the phase recorded over the same part of the plates are equal for all lift-offs. As shown in ref. [23], the phase image of a magnetically permeable object does not vary with relative orientation between the object and the sensor. To some extent the same property, phase saturation limited angle r ge, e.g., from \(+30^{\circ}\) to \(+15^{\circ}\) in Figure 3, was also demonstrated for electrically conductive objects. While these two observations prove that the phase is immune to variation of these two important factors in the inductive measurement (relative orientation and lift-off), the accuracy of phase measurement deteriorates with the decrease in signal's amplitude associated with increasing lift-off. To quantify this statement we use data from Figure 6. We define signal amplitude as an averaged signal along the plate edge. Figure 7a shows a decrease of this amplitude with increasing lift-off value. To demonstrate the phase dependence on the lift-off we average values along the radius of the vortex. Figure 7b shows these phase values averaged along different directions as a function of the lift-off. While the data set confirms that the phase does not change with lift-off, the accuracy of the phase measurement represented by the standard deviation deteriorates with decreasing signal amplitude. The relatively small lift-offs studied in this paper results from using a small primary rf field coil (outer diameter of 3 mm) in our experiment. A bigger coil or a coil array might be used to increase the achievable lift-off [24; 25]. Similar behaviour, i.e., increasing phase uncertainty has been observed in the same measurement with a copper plate, i.e., electrically conductive object.