A double-wedge reflector for emissivity enhanced pyrometry

A double-wedge integrating cavity is described for possible applications to on-line bright metal pyrometry. A Monte Carlo simulation, assuming internal cavity surfaces of given specular reflectance, a diffusely reflecting sheet, and radiation leakage losses through the central fibre bundle aperture and through the variable gap between the cavity and the fluctuating sheet, provides an insight into the effect of different cavity parameters such as the wedge angle and the cavity size compared to stand-off. The double-wedge cavity configuration is shown to increase consistently the equivalent emissivity of a metal sheet even in the presence of substantial sheet fluctuations, and thus to reduce temperature readout errors due to unpredictable variations of the real surface emissivity. Such a design is particularly insensitive to spurious ambient radiation. A preliminary prototype of a cavity-assisted double wavelength fibre optic pyrometer was assembled for on-line tests on a continuous aluminium annealing line. Details of the sensor calibration and performance are given.