A low cost portable fiber optic fluorometer is packaged as a personal computer peripheral and is based on interchangeable modules. One embodiment of the fluorometer comprises an excitation source, a detector, a signal processor, frequency source, wavelength selector, and an optical interface. The optical interface is exclusively fiber optic-based, thereby simplifying optical alignment and reducing the cost of the fluorometer. In another embodiment, the excitation source is an inexpensive monochromatic excitation source. In this case, the monochromatic excitation source and the first wavelength selector are preferably removable and replaceable, so that the fluorometer is advantageously able to generate different excitation wavelengths and detect different emission wavelengths. A fluorescence measurement method comprises the steps of generating an excitation signal; transmitting the excitation signal to a system under study which, in response, generates an emission signal; and detecting both the excitation signal and the emission signal with the fluorometer, and then using the detected excitation signal as a reference signal to calibrate the fluorometer and to nullify distortion errors in the emission signal. The use of the excitation signal as a reference signal provides an advantageously simple way to calibrate the fluorometer and to nullify distortion errors in the emission signal. An alternative method allows the precise shapes of the emission signal and the excitation signal to be profiled. The fluorometry method and apparatus can both advantageously be used for performing both direct fluorescence lifetime measurements and for performing phase fluorometry.