Surface Emission for Infrared Thermometer
All infrared‑measuring instruments obtain temperature information of electrical equipment by measuring the infrared radiant power from the equipment surface. When an infrared diagnostic instrument receives the same infrared radiant power from a target, different detection results will be obtained due to variations in the target's surface emissivity. That is to say, for identical radiant power, a lower emissivity yields a higher displayed temperature.
The surface emissivity of an object is mainly determined by material properties and surface conditions, such as surface oxidation, coating materials, surface roughness, and contamination status. Therefore, to accurately measure the temperature of electrical equipment with an infrared thermometer, the emissivity value of the inspected target must be known. This value shall be entered into the computer as a critical parameter for temperature calculation, or the ε correction value of the infrared measuring instrument shall be adjusted to implement emissivity correction for output temperature readings.
Two additional countermeasures are available to eliminate the influence of emissivity on detection results. When measuring with an infrared thermal imager, perform emissivity correction by looking up the surface emissivity of the measured equipment component. Reliable temperature‑measurement results can thus be obtained, and detection reliability is improved. For equipment components prone to frequent faults in infrared inspection, appropriate paint may be applied to increase and stabilize emissivity. This delivers good comparability of test results and helps acquire the true surface temperature of the measured equipment.
Slope is defined as the ratio of emissivity under the monochromatic broadband temperature‑measurement mode to emissivity under the monochromatic narrow‑band temperature‑measurement mode, and it is used for temperature calculation in the two‑color temperature‑measurement mode. Since the emissivity for narrow‑band mode cannot be adjusted, it is calculated by dividing the monochromatic broadband emissivity by the slope value.
Pay attention to narrow‑band temperature; adjust the slope and broadband emissivity so that the calculated narrow‑band emissivity does not exceed 1.00 or fall below 0.10.
Emissivity is a measure of an object's capability to radiate infrared rays. Its value ranges from 0 (mirror‑like surface) to 1.0 (blackbody). If the set emissivity is higher than the actual emissivity, readings from the sensor head will be lower than the true temperature. For example, if the actual object emissivity is 0.9 while the setting is 0.95, the measured temperature will be underestimated.
