Sapphire spectral transmission

The optical properties of the wall also contribute to the emitted spectra. The usual arc tube wall material, polycrystalline alumina (PCA), has rather low "in-line" transmission and scatters most of the incident radiation. That portion which is scattered back into the discharge medium and that is also in the spectral region inside or near the peaks of the self-reversed resonance lines, is likely to be absorbed. Thus we have found greater Na reversal for PCA arc tube lamps than for sapphire lamps with the same diameters, Na-Hg alloy composition and reservoir temperatures. Indeed, various authors have reported 3-8% higher efficacies for sapphire vs PCA lamps depending upon the quality of the PCA and its surface treatment 3). 

Alternatively, it is possible to install fibre optic probes directly in the main stream in-line while the IR spectrophotometer remains remotely in a low vibration laboratory environment. In-line analysers, which do not remove any sample from the line, have the minimum possible lag time and do not change the sample physically or chemically from its nature in the process. Recently, bundles of 500 /xm optic fibres have been developed for the 5000-900 cm (2000-11,000 nm region), which permit transmission of IR energy over distances of several metres. Lowry et al. [76] have evaluated fibre-optic cables that might prove useful in FTIR remote sampling applications. The various optical fibres (chalcogenide, silver halide, heavy metal fluoride or sapphire) differ in their spectral window [77]. Due to the thermal stability and the spectral window, sapphire fibres are considered suitable for in-line characterisation of polymer melts in a production line (e.g. in an extruder head) as an alternative to discontinu-ously operating conventional off-line transmission IR spectroscopy of polymer films [78]. 

---