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We propose a theoretical principle to measure the mass, spin and direction ofspin axis of Kerr black holes (BHs) through observing 2 quantities of thespinning strong gravitational lens effect of BHs. Those observable quantitiesare 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, theearliest and secondary temporally. The time delay between detection times andthe ratio of observed specific fluxes of those rays are the observablequantities. Rigorously, our proposal is applicable to a single burst-like(short duration) isotropic emission by the source. An extension of ourprinciple to cases of complicated emissions may be constructed by summing upappropriately the result of this paper, which will be treated in future works.

How to Measure Black Hole's Mass, Spin and Direction of Spin Axis in Kerr Lens Effect 1: test case with simple source emission near BH