A spectrofluorometer is an instrument which takes advantage of fluorescent properties of some compounds in order to provide information regarding their concentration and chemical environment in a sample.
Generally, spectrofluorometers use high intensity light sources to bombard a sample with as many photons as possible.
The emission is also either passed through a filter or a monochromator before being detected by a photomultiplier tube, photodiode, or charge-coupled device detector.
Systems vary greatly and a number of considerations affect the choice.
The standard would be fluorescein in NaOH, typical values for a high end instrument are in the femtomolar range.