## 4 Conclusions  
In the process of developing a concept from proof of principle to a complete instrument, it is important to address real-life measurement complexities, which have been avoided in the initial stages when evaluating the feasibility of certain aspects of the technology. Here, we presented possible options to address the issue of variable orientation and lift-off in object composition identification based on inductive measurement with an rf atomic magnetometer.  
Our approach relies on the ability to identify the surface vs. volumetric character of the secondary rf field generation through the signal's angular dependence. Essentially, it is a consequence of our measurement strategy where we concentrate on the secondary field components orthogonal to the primary rf field. The ability to determine the character of the secondary field combined with the signal's frequency dependence allows selective operation of the frequency range where the object response produced by eddy currents has purely surface character regardless of object composition. Identification of composition independent variables enables calibration of the detection system, estimation of the lift-off value for each measurement and subsequently material identification. Measurements of the angular and frequency dependencies of the rf signal might be complemented by the implementation of a set of the primary rf field coils that enables independent evaluation of the lift-off. Subsequent object composition identification was performed for objects with the same geometry, future work will study the signal response for different geometries.  
While concentrating on the development of a measurement toolkit we recognise that actual implementation of the discussed techniques, e.g., operation with lift-off at 50-100 cm range, requires the development of dedicated hardware, e.g., rf field sources and portable detection systems. The works towards this goal are underway.  
Conceptualization, J.D.Z., S.S., P.B. and W.C.; methodology, P.B. and W.C.; investigation, J.D.Z., S.S., P.B. and W.C.; resources, P.B. and W.C.; data curation, P.B.; writing--original draft preparation, P.B. and W.C.; writing--review and editing, J.D.Z., S.S., P.B. and W.C.; supervision, W.C.; project administration, P.B. and W.C.; funding acquisition, P.B. and W.C. All authors have read and agreed to the published version of the manuscript.  
This project was partially funded by the Knowledge Asset Grant Fund provided by the Government Office for Technology Transfer (GOTT) and the National Quantum Program funded by the Department for Business, Energy & Industrial Strategy.  
Not applicable.  
Figure 11: Signal amplitude measured across a 1D scan over the middle of a \(40\times 40\) mm\({}^{2}\) plate which is tilted at 30\({}^{\circ}\), where the differences in the two coils lift-offs are (**a**), \(+5\) mm, (**b**) 0 mm, and (**c**) \(-5\) mm. The coil on same/opposite side as sensor is shown with red triangles/ blue diamonds and the two coil configuration is given by the green circles.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data that support the findings of this study are available from the corresponding author upon reasonable request.  
**Acknowledgments:** We would like to thank [PERSON] and [PERSON] for critically reading this manuscript.  
**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.