the characterising process comprising the steps of:

(a) controlling a transmitter to:

generate first input coherent electromagnetic radiation using a plurality of electromagnetic radiation sources;

apply a sequence of spatial modulation patterns to said first input coherent electromagnetic radiation, each spatial modulation pattern setting each one of plurality of electromagnetic radiation sources to an on state or to an off state; and

direct the first input coherent electromagnetic radiation toward the target via the disordered medium;

(b) controlling a receiver to:

receive a signal representing an intensity of output electromagnetic radiation at the target, and generate a corresponding total output signal;

store output data representing each total output signal whereby, once the sequence of spatial modulation patterns has concluded, said output data comprises a plurality of particular spatial modulation output signals, each particular spatial modulation output signal corresponding to one of the spatial modulation patterns;

wherein a complex-valued transmission matrix comprising a plurality of complex-valued transmission constants approximates a relationship between the first input coherent electromagnetic radiation and the output electromagnetic radiation, the characterising process determining a real-valued transmission matrix comprising a plurality of real-valued transmission constants which approximate a relationship between the intensities of the first input coherent electromagnetic radiation and the output electromagnetic radiation, each real-valued transmission constant associated with one of said plurality of electromagnetic radiation sources, the method further comprising the steps of:

(c) for each electromagnetic radiation source of the plurality of electromagnetic radiation sources determining the associated real-valued transmission constant as a function of said output data and the on and off states of that electromagnetic radiation source over the sequence of spatial modulation patterns;

(d) storing said plurality of real-valued transmissions constants, each real-valued transmission constant comprising a real number;

(e) determining a focusing spatial modulation pattern of said plurality of electromagnetic radiation sources by:

determining a group of real-valued transmission constants from the plurality of real-valued transmission constants by examining the real number of each real-valued transmission constant and including the real-valued transmission constant in the group so that the group comprises real-valued transmission constants which have real numbers greater than the real numbers of real-valued transmission constants not in said group;

and

setting a state indicator for each electromagnetic radiation source for causing the electromagnetic radiation source to be in said on state if the associated real-valued transmission constant is in said group and in said off state if not, the state indicators for said plurality of electromagnetic sources forming said focusing spatial modulation pattern;

the focusing process comprising the steps of:

(f) generating second input coherent electromagnetic radiation and applying a spatial modulation to said second input coherent electromagnetic radiation using said focusing spatial modulation pattern; and

(g) directing said second input coherent electromagnetic radiation toward the target via the disordered medium.