Why can a thermal camera misread hot metal by hundreds of degrees, and can a phosphor thermometer do better?

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Accurate temperature measurements are critical in manufacturing because they affect both product quality and energy consumption. At elevated temperatures, non-contact thermometers are often the only option. However, these instruments require prior knowledge of surface emissivity, which is often unknown or difficult to determine, leading to large errors.

The researchers presented a new imaging luminescence thermometer using magnesium fluorogermanate phosphor and the intensity ratio technique, with the potential to overcome this limitation. In an experimental comparison, they measured several engineering alloys undergoing heat treatment at temperatures up to 750 °C and compared the measurements with those from a traditional contact thermocouple and a thermal imager system.

The luminescence temperatures differed from the embedded thermocouple temperatures by less than 45 °C at all temperatures tested. By contrast, thermal imager measurements on bare metal samples, with the instrument emissivity set to 1.0, showed differences of up to 500 °C at 750 °C—described as a factor of 10 larger.

To improve the thermal imager's accuracy, the researchers adjusted its instrument emissivity until its temperature agreed with the thermocouple temperature. For bare metal, the effective emissivity depended strongly on the sample, with mean values ranging from 0.205 to 0.784.

Because the phosphor-derived temperatures showed substantially smaller errors than the thermal imager measurements, the researchers suggest using this method to complement thermal imaging. It could provide a robust way to adjust the thermal imager's instrument emissivity until its readings agree with those of the phosphor thermometer.