Guide to Emissivity (Part One)


Written by:
L. Terry Clausing, P.E., Fluke Corporation

Figure IR1

Figure 1 shows a typical three-phase fused power disconnect. The corresponding infrared image, Figure IR1, was taken with the emissivity setting at 1 on our thermal imager. The temperature span and color scale for the infrared image is set to 95.5-deg F for black, with warmer temperatures indicated progressively by blue (105-deg F), green (115-deg F), red (125-deg F) and white (133-deg F and hotter). We also measured the load in phase A, B and C (from left to right), at approximately 34 amps each.

A simple analysis of the thermal image indicates that phase A is significantly hotter than phases B and C. The fuse clip at the top of phase A indicates 133.4-deg F, while the end of the fuse, specifically the metal cap of the top of the fuse, appears much cooler with a temperature of 103.6-deg F and the fuse body just below the cap appears to be 121.9-deg F.

Can this be true? Is the metal cap only 103-deg F? No. You are seeing an example of the apparent temperature and the effect of emissivity. The fuse end cap is a highly reflective metal-in this case copper. Notice that the body of the fuse also appears hotter than the metal cap. The temperature of the cap is actually as hot as the fuse body it contacts.

To explain why the apparent temperature seen through a thermal imager can be significantly different than the actual temperature, we will review our knowledge of physics

Thermal Radiation and Properties of Materials

All objects emit infrared (thermal) radiation. The intensity of the radiation depends on the temperature and nature of the material's surface. At lower temperatures, the majority of this thermal radiation is at longer wavelengths.

As the object becomes hotter, the radiation intensity rapidly increases and the peak of the radiation shifts toward shorter wavelengths. The relationship between total radiation intensity (all wavelengths) and temperature is defined by the Stefan-Boltzmann law:

            Q=eσT4

Where:

 

            Q = Radiation intensity

            e  = Emissivity of material

            σ  = Stefan-Boltzmann constant

            T  = Absolute temperature

At a given temperature, the maximum radiation is achieved when the object has an emissivity of 1. This is referred to as blackbody radiation, because with an emissivity of 1, the object is a perfect radiator. However, in our real world, there are no true blackbodies or perfect radiators. Since real materials are less than perfect radiators, the relevant issue is "How much less than perfect are they?" Emissivity is defined as the measure of how much less than perfectly a material radiates when compared to a blackbody. Emissivity is only one of three factors that cause an object to be a less than perfect radiator.

The Thermal Nature of Materials

Materials (objects in everyday life, whether they be solids, liquids or gases) are constantly affected by their surroundings. Thermally, all objects attempt to exchange energy with other objects in their natural drive toward thermal equilibrium with their surroundings. In this search for thermal equilibrium, heat is exchanged between objects via three mechanisms: conduction, convection and radiation.

        Conduction is defined as heat transfer between two solid bodies that are in physical contact with each other.

        Convection is heat transfer, usually between a solid material and a liquid or gas. Conduction and convection are dependent on physical contact between materials.

        Radiation is a process of heat transfer, characteristic of all matter (at temperatures above absolute zero). Radiation passes through a vacuum, and can also pass through gasses, liquids and even solids.

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