  We introduce a simple but practical method to measure the optical
transmission matrix (TM) of complex media. The optical TM of a complex medium
is obtained by modulating the wavefront of a beam impinging on the complex
medium and imaging the transmitted full-field speckle intensity patterns. Using
the retrieved TM, we demonstrate the generation and linear combination of
multiple foci on demand through the complex medium. This method will be used as
a versatile tool for coherence control of waves through turbid media.
