The transmittance of a filter periodically varies with respect to the incident light frequency. Provided that fk is the sum of Fk and νk, or the difference obtained by subtracting Fk from νk, depending on the kth light source among multiple light sources, an incidence guide causes light from the plurality of light sources to be incident on the filter such that the propagation angle of light when light from the kth light source propagates through the interior of the filter equals θk obtained by computation using fk. The incidence guide causes light to be incident on the filter, taking fk to be the sum of νk and Fk for at least one of the light sources, and taking fk to be the difference between νk and Fk for at least one other of the light sources. A detector detects the intensity of transmitted light that transmits through the filter.