A laser emissivity free thermometer (LEFT) for thermodynamic temperature measurement of hot objects is being developed at the KRISS. The LEFT has two spectral bands, 850 nm and 1450 nm to excite temperature rise on a target and measure induced photo-thermal signals from the different band. The thermal excitation is supplied by only laser diodes of which cw power is more than 1 Watt. A spherical mirror collimates the lasers on the target and thermal radiations from the target. A fiber bundle delivers laser lights from the laser diodes to the mirror and the thermal radiation from the mirror to the detector modules. The detector signals from the laser modulation are detected by lock-in amplifiers. The thermal signal without the laser excitation is calibrated against the blackbody to obtain the effective wavelength of each spectral band. A tungsten strip lamp was used to test the thermometer from 900°C to 1500°C. The excitation power dependency of the photo-thermal signal is measured at 1075°C to convince linearity. The modulation frequency dependency on the photo-thermal signal is also measured from 2 Hz to 35 Hz. The LEFT is calibrated by determining the parameter at only a point, 1500°C. The measured temperatures of the lamp are compared to the thermodynamic temperatures that have been corrected from the 650 nm radiance temperatures considering emissivity of the tungsten and transmittance of the lamp window. The minimum temperature the LEFT can measure is expected to be 600°C.
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