Disappearing filament pyrometry

The filament calibration curve can be obtained by comparison against previously calibrated pyrometers or from the output of high-temperature thermocouples in thermal contact with 

Planck s law becomes Wein s law if the —1 term is considered insignificant combining constants yields  

By using absorbing filters, the radiation from blackbody sources at higher temperatures can be down-rated to temperatures within the calibration range of the pyrometer. As a result, the range of the pyrometer can be extended well above the melting temperature of gold. 

One advantage of a spectral radiation pyrometer is that the emissivity or emittance at only a specific wavelength (e.g. 0.653 pm) is of importance. A non-blackbody source will be less luminescent than a blackbody source at the same temperature. Thus, a falsely low temperature will be determined by sighting a calibrated disappearing filament pyrometer on the non-blackbody. This temperature has been referred to as the brightness temperature . 

Again assuming Wein s law can be substituted for Planck s law ( —1 term is negligible) and taking logarithms  

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