PROBLEM TO BE SOLVED: To provide a compact and simply constructed light measuring device with which it is possible to reliably detect a small amount of dental plaque, and which can be easily assembled into a toothbrush.SOLUTION: The light measuring device comprises: a first and a second light source 2, 3 for emitting light of a first wavelength and light of a second wavelength of greater length； a detection unit for detecting first and second fluorescence intensity generated when a sample (tooth) is irradiated with lights of first and second wavelengths； an emission control unit for causing the first and the second light sources 2, 3 to alternately emit light in accordance with a reference signal in a first period and only a third light source to emit light in accordance with the reference signal in a second period, out of the first and second periods that are alternately repeated, each longer than the cycle of a reference signal (timing signal 101)； a phase detector 104 for performing phase detection in accordance with the reference signal, and generating a first output signal and a second output signal； and a control circuit 30 for calculating the amount of a fluorescent substance to be measured, by computation using the first and the second output signals.SELECTED DRAWING: Figure 9COPYRIGHT: (C)2020,JPO&INPIT【課題】少量の歯垢を確実に検出でき、しかも歯ブラシに容易に組み込むことができる小型でシンプルな構成の光測定装置を提供する。【解決手段】光測定装置は、第１の波長の光及びより長波長の第２の波長の光を出射する第１及び第２の光源２、３と、試料（歯）に第１及び第２の波長の光を照射したときに発生する第１及び第２の蛍光強度を検出する検出部と、交互に繰り返されそれぞれが基準信号（タイミング信号１０１）の周期よりも長い第１及び第２の期間のうち、第１の期間では基準信号に従い第１及び第２の光源２、３を交互に発光させ、第２の期間では基準信号に従い光源３のみを発光させる発光制御部と、基準信号に従い位相検波を行って、第１の出力信号、並びに第２の出力信号を生成する位相検波器１０４と、第１及び第２の出力信号を用いた演算により測定対象の蛍光物質量を算出する制御回路３０とを有する。【選択図】図９
