We developed a new illumination method called the simultaneous illumination method. This method does not require synchronization between light sources and sensor signals, which drastically simplifies the instrumentation. As a proof-of-concept, we applied this method to an oceanographic fluorometer. In principle, using this method, one can easily increase the number of characterized emission wavelengths by mounting optical sensors for as many emission wavelengths as needed. Our fluorometer has two emission-wavelength channels and twelve excitation wavelengths. The aim of this prototype is to demonstrate a viable in situ N-channel emission fluorometer with multiple wavelengths of excitation, which has not been previously realized. (C) 2011 Optical Society of America