A transform spectrometer measurement apparatus and method for a planar waveguide circuit (PLC). The spectrometer typically includes an input optical signal waveguide carrying an input optical signal; a plurality of couplers, each connected to the input optical signal waveguide, and each including a coupler output for carrying a coupled optical signal related to the input optical signal; and an array of interleaved, waveguide Mach-Zehnder interferometers (MZI), each having at least one input MZI waveguide, each MZI input waveguide receiving a coupled optical signal from a respective coupler output. A phase shifting circuit is applied to at least one arm of the MZIs to induce an active phase shift on the arm to thereby measure phase error in the MZIs. Light output from the MZIs is measured under intrinsic phase error conditions and after an active phase shift by the phase shifting circuit.