Fluorescence is a form of luminescence. Fluorescence occurs after a molecule absorbs energy in the form of light.  The fluorescent molecule absorbs a photon, promoting the molecule to an excited electronic state.  It then emits a photon (of lower energy), decaying to the ground (unexcited) state.   The emitted photon is always of lower energy than the absorbed light because the molecule releases heat in addition to the photon.  Since the wavelength of radiation is inversely proportional to the energy, the wavelength of the emitted radiation is always longer than that of the excitation light.

Principle of a fluorescence detector

The layout of a fluorescence detector cell is seen above.  Detection is at 90 degrees to the incident light to minimize interference from scattered light.  The parabolic reflector maximizes the collection of emitted photons.  Fluorescence detectors typically excite fluorophores with a specific wavelength (selected with either a filter or a monochromator), then monitor emission at a different (longer) wavelength selected with another filter or monochromator.  The excitation light is removed by the second filter or monochromator, allowing only the emitted light to strike the transducer. 

A limited number of compounds exhibit their own (native) fluorescence. Generally speaking, the more double bonds (especially conjugated double bonds) present in a molecule, the larger the fluorescence intensity.  Because only a limited subset of molecules are fluorescent, fluorescence detectors are considered selective detectors.  The selectivity of a fluorescent detector can be increased by proper choice of excitation and emission wavelengths.  Ideally, interfering components are not detected because they do not absorb at the chosen excitation wavelength and/or do not emit at the chosen emission wavelength.  In addition to its selectivity, fluorescence is very well suited for trace analyses due to its signal to noise properties.  Fluorescence detectors are often used in series with a variable wavelength UV detector, so both signals can be monitored for optimal sensitivity and selectivity.