A photonic approach to the instantaneous measurement of multiple frequency components using a spectrally sliced incoherent source (SSIS) is proposed. In the proposed system, a broadband incoherent source is spectrally sliced using an etalon to generate an SSIS. Each channel of the SSIS is externally modulated by a microwave signal containing multiple frequency components. The modulated SSIS is then sent to a second etalon. Thanks to the difference between the free spectral ranges of the two etalons, multiple frequency components are simultaneously estimated from the power distribution at the output channels of the second etalon. Compared with the use of a laser source array, the use of an SSIS provides a simpler way to perform multiple-frequency-component measurement.