Examination of various research efforts has revealed that optical techniques best satisfy the requirements of on-line, non-contact inspection; other methods are usually deficient with regard to speed, flexibility and ability to perform non-contact measurement. Of the optical methods considered, techniques based on statistical analysis of scattered light from the sample are best suited for the purpose of surface inspection. A general scheme for an on-line, non-contact, optically based inspection machine has been developed. The system has the important attributes of fast measurement, simplicity and flexibility. The system's roughness measurement capability was successfully tested on ceramic components of varying surface finish.
Raman scattering in optical fibres provides a means of measuring continuous temperature distributions over extended distances. However, in many applications it has been found that the background transmission properties of the optical fibre either change after deployment of the fibre or vary over time due to ageing effects. Changes in the fibre transmission can be distinguished from thermal effects by measuring the Rayleigh backscatter, which is relatively insensitive to temperature. The combined use of Raman and Rayleigh data is discussed here in the context of a particularly simple and cost-effective sensor design, where a single, fixed optical filter and a single light source are used for both measurement modes. The Rayleigh backscatter measurement allows the accurate correction of background transmission changes in a probe arrangement that includes several splices between different fibres with different attenuations. In this way, the temperature in a 290 °C test region is accurately recovered from the anti-Stokes Raman signal. However, preliminary measurements of the fibre attenuation as a function of temperature and exposure to an accelerated ageing environment show that these two effects will be difficult to separate in practice. Therefore it remains challenging to perform accurate, unambiguous background corrections in situations where high-temperature ageing occurs.
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