A method and apparatus to detect fluorescence from a sample is provided. The optical system allows the user to continuously tune the wavelengths and bandwidths of the excitation source and the emission detection system. A peaking function automatically peaks the detected fluorescent signal by fine tuning the excitation and emission detection systems. A look-up table allows the fluorescent signal for a specific wavelength to be corrected for the wavelength dependence of the optical train. In one embodiment of the system the sample is simultaneously irradiated in more than one wavelength band, each wavelength band being independently tunable.