Design of phase-modulation fluorometers

Before describing the instruments, it is worth making two preliminary remarks  

The values measured in these two ways should of course be identical and independent of the modulation frequency. This provides two criteria to check whether an instrument is correctly tuned by using a lifetime standard whose fluorescence decay is known to be a single exponential. 

Note that the measurement of a decay time is fast (a fraction of a second) for a single exponential decay because a single frequency is sufficient. Note also that a significant difference between the values obtained by means of Eqs (6.35) and (6.36) is compelling evidence of non-exponentiality of the fluorescence decay. 

Historically, the first instrument for the determination of lifetime was a phase fluorometer (designed by Gaviola in 1926) operating at a single frequency. Progress in instrumentation enabled variable modulation frequency by employing a cw laser (or a lamp) and an electro-optic modulator (0.1-250 MHz), or by using the harmonic content of a pulsed laser source (up to 2 GHz). These two techniques will now be described. 

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