The development of a light-collecting probe with high spatial resolution applicable to randomly fluctuating combustion fields

A high-luminosity-light-collection system for highly spatial detection of c hemiluminescence of radical species in flames has been developed. The syste m, multi-colour integrated Cassegrain receiving optics (MICRO) is based upo n a Cassegrain-type configuration, which implies that it employs only refle ctive components (in combination with an optical fibre for light collection ). It provides therefore spherical- and chromatic-aberration-free detection , which is of importance for high-spatial-resolution measurements and for t he simultaneous monitoring of signals in different wavelength regions from a given spatial volume. The effective light-collection volume has been esti mated to be only 1.6 mm x 0.2 mm x 0.2 mm by ray-tracing techniques, which is more than three orders of magnitude smaller than that provided by a corr esponding simple single-lens system and comparable to that of laser-based t echniques, e.g. Doppler anemometry. The system is also easily aligned since the active probe volume can be visualized by sending in visible light thro ugh the system in the reverse direction. In order to demonstrate the perfor mance of the system, OH-radical chemiluminescence in a Bunsen flame was mon itored using MICRO and compared with the ion-current signal from a Langmuir probe with a minute sensor tip. A good correlation between the fluctuation s in the two signals could be obtained, proving the high spatial and tempor al resolution of the MICRO system.