Light emitting diode excitation of Cr3+:Al2O3 as thermographic phosphor: experiments and measurement strategy

Light emitting diode (LED) excitation of thermographic phosphors for temperature measurements was investigated in the present work. A near-UV and a green LED excited the phosphorescence of sol-gel deposited chromium-doped alumina (Cr3+:Al2O3, ruby) on silicon in different experiments. The influence of the pulse length on signal intensity was measured and is discussed theoretically. From this, measurement strategies are recommended in order to obtain high signal levels. The temperature-dependent phosphorescence between room temperature and 800 K is compared to previously obtained laser excited phosphorescence. The phosphorescence decay time of the ruby films is found to be slightly dependent on the excitation pulse duration but neither on the excitation wavelength nor on the oxygen partial pressure in the atmosphere. In many situations the less expensive LED excitation appears to be a good alternative to the laser excitation methods. In addition, a strategy is proposed for measuring two-dimensional surface temperatures using two gated cameras; the validity was proven for pointwise measurements.