Electroluminescent lamp-based phase fluorometer and oxygen sensor

We have tested 454-nm violet-emitting solid state electroluminescent lamps (ELLs) as inexpensive intensity-modulated excitation light sources for phase fluorometric oxygen sensors. Compared with blue-emitting silicon carbide LEDs, planar surface ELLs can be produced in various shapes and in large sizes. Accordingly, the overall optical output power emitted by ELLs is much higher than that of blue LEDs. By arranging a large-size ELL close to a large-size fluorescent chemical sensor, we obtained a large number of fluorescence photons allowing for the use of a pin photodiode instead of a photomultiplier tube as the detector. For a sinusoidal driving voltage at a frequency f, the ELL output light is modulated at 2 f and at harmonics of 2 f. Because of this nonlinear modulation characteristic, we used a square wave driving signal, resulting in a pulsed light output at a repetition rate twice the square wave frequency. The shortest light pulses obtained had a FWHM close to about 1 μs. This means that the violet ELLs used in our tests provide modulation frequencies at twice the square wave driving frequency and at all harmonics thereof up to about 1 MHz. This would allow the use of fluorescent chemical sensors with decay times as short as 30 ns, assuming that a phase shift of 10° is adequate for the application. Due to the high ELL driving voltage, effective shielding is required to avoid electromagnetic interference between the modulated light source and the photodetector. Depending on the driving frequency and voltage applied, the ELLs showed a decrease in the optical output power to 50 or even 10% during the first 100 h of operation.