  We propose a theoretical principle to measure the mass, spin and direction of
spin axis of Kerr black holes (BHs) through observing 2 quantities of the
spinning strong gravitational lens effect of BHs. Those observable quantities
are generated by 2 light rays emitted at the same time by a source near the BH:
the primary and secondary rays that reach a distant observer, respectively, the
earliest and secondary temporally. The time delay between detection times and
the ratio of observed specific fluxes of those rays are the observable
quantities. Rigorously, our proposal is applicable to a single burst-like
(short duration) isotropic emission by the source. An extension of our
principle to cases of complicated emissions may be constructed by summing up
appropriately the result of this paper, which will be treated in future works.
