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A new method is presented of on-chip optical spectroscopy, functioning without the need for a bandpass filter or grating. The principle of optical spectroscopy is based on the difference of optical absorption coefficients with wavelength. The optical intensity is calculated from the different penetration depths in Si. The key to the new spectrometer is the use of a photogate active pixel providing the selective control of photo-generated charge using the gate voltage. To demonstrate this spectrometer, a novel filterless fluorescence detection sensor has been fabricated in our laboratory, using standard 5-μm CMOS silicon integrated circuit technology. The SYBR-Green label fluoresces at 520 nm when exited by 470-nm radiation. In a simulation experiment using two LEDs, the fluorescent intensity detected was 1/300 of the excitation light intensity (intensity of fluorescence was 1 μW/cm2, while the excitation illumination was 300 μW/cm2). In an experiment using actual DNA solution containing SYBR-Green, it was confirmed that the fluorescence detection sensor successfully detected the fluorescent label without the need for a filter.