A spectral detector includes a grating panel including a first grating pattern having a first period, a second grating pattern having a second period that is different from the first period, and a light exit surface through light exits the grating panel, and an optical measurement panel arranged to face the light exit surface of the grating panel, and configured to measure a change in intensity of first light passing through the first grating pattern according to a propagation distance of the first light, and to measure a change in intensity of second light passing through the second grating pattern according to a propagation distance of the second light.