Functional phasor diagram of the double-Bracewell nuller with complex amplitudes of inputs 1 (green), 2 (yellow), 3 (red), and 4 (blue). DC1, DC2, and DC3 are three directional couplers with a 50/50 splitting ratio in intensity. DC1 and DC2 produce constructive outputs at #1 and #4, while the two nulled outputs are feeding DC3. The nulled outputs #2 and #3 feature two transmission maps, which are centro-symmetric to one another with regard to the central star.