An apparatus for measuring wavelength composition and power of a dispersed spectrum of light comprises a diffractive light modulator and a detector. The diffractive light modulator comprises an array of light modulating pixels operable in a first mode and a second mode. In operation, the dispersed spectrum of light illuminates the diffractive light modulator along the array of light modulating pixels, which selectively directs a subset of the dispersed spectrum of light into the first mode while directing a remainder of the dispersed spectrum of light into the second mode. The detector is optically coupled to the diffractive light modulator. In operation, the detector detects the subset of the dispersed spectrum of light while not detecting the remainder of the dispersed spectrum of light. Each of the light modulating pixels is controlled by signals sent from a controller. The signals comprise a time division multiplex modulation, a code division multiplex modulation, or a combination thereof.