The infrared heat energy of an object doesn’t appear visible to human eye. However, this infrared energy can be transferred in basically three forms of energy as shown in the figure. They are:
Amongst these three types of energy, only emitted energy is considered suitable for determining the correct surface temperature of an object. Reflected energy is rendered inappropriate for surface temperature measurement because there is no specific relation between the temperature of an object and reflected energy. Besides, transmitted energy is also irrelevant for temperature measurement since it is associated to both the temperature of the target object as well as “transmitted to” object. An infrared thermometer always needs some sort of adjustment prior to measurement since it is capable enough to measure all three forms of energy. Consequently, the thermometer should be arranged in such a way that it indicates only emitted energy.
Emissivity is a variable which is considered very crucial for infrared temperature calculations of a target object. It is basically defined as “the ratio of the energy radiated by an object at a given temperature to the energy emitted by a perfect radiator, or blackbody, at the same temperature”.1 In other words it is “the ratio of energy emitted by a material in comparison to how much infrared energy it emits”.
The value of emissivity for all objects ranges between 0.0 and 1.0. The emissivity of a blackbody is found to be 1.0 as it happens to be very near to the emissivity range of infrared. Objects having higher emissivity values can be easily handled for infrared temperature measurements while the objects having lower emissivity values are difficult to deal. Therefore blackbodies offer trouble free temperature measurements while extremely reflective and shiny metals like chrome, silver and aluminum having lower emissivity values poses problems in correct temperature measurements. However, almost all IR radiation thermometers provide compensation for varying emissivity values for diverse materials.
Emissivity v/s Emittance
The term emissivity determines the characteristics of a material while emittance refers to the characteristics of a target object. Hence, emissivity is only one variable which helps in finding out emittance of a specific object. There are some other factors also which determine emittance. They are:
- Shape and form of the object
- Material oxidation
- Surface conditions i.e. surface finish etc.
In spite of having a slight technical division between emittance and emissivity, these two terms are frequently used interchangeably.
The temperature detection via infrared radiation thermometer considerably depends upon emissivity. The basic equation establishing relationship between emissivity and output of a radiation thermometer is given below:
V(T) = e K TN
In the above equation: e denotes emissivity, V(T) denotes thermometer output with temperature, K is a constant, T is the temperature of the target object and N is the N factor. This N factor is equivalent to 14388/ (lT), where l is the equivalent wavelength.
Higher values of N factor i.e. shorter equivalent wavelengths are considered better for temperature measurements. Even an improper optical system will not have any undesirable effect on temperature measurements if instrument having high value of N is employed. There is a reference literature available for emissivity values of nearly all materials. According to this published literature, emissivity of opaque and non-metallic materials is found to be high and it remains stable in the range of 0.85 to 0.90. On the other hand, the emissivity of unoxidized, metallic materials keeps on varying between 0.2 to 0.5. However, metals like Gold, silver and aluminum are some of the exceptional materials which offer extremely low emissivity in the range of 0.02 to 0.04. Owing to their very low emissivity values, they are considered inapt for temperature measurements via an IR radiation thermometer.
Determination of Emissivity
Five techniques available to determine the emissivity of a material are mentioned below. Via these techniques one can make sure whether temperature measurements resulted in accurate results or not.
- Take a sample of the material and heat it to a known temperature by means of an accurate sensing device and then dtermine the temperature via an infrared thermometer. Now, adjust the emissivity value so that the indicator is forced to show the right temperature.
- In case of quite low temperatures i.e. up to 500°F, one can experiment by putting a piece of black masking tape on the surface of the target object. Then determine the temperature of the tape surface by making use of an IR radiation thermometer having an emissivity value of 0.95. Now remove the tape and again measure the surface temperature of the target. Hence by calculating the difference in two readings, one can easily adjust the emissivity to a desired value.
- To measure high temperatures, a hole (having depth around 6 times the diameter) can be bored into the target object. This hole will behave somewhat like a blackbody having emissivity value of 1.0. Then measure the temperature in the hole, and accordingly adjust the emissivity value so that the indicator is forced to display the correct temperature of the material.
- One can coat a portion of the target object with a dull black paint having emissivity value around 1.0. Then determine the temperature of the paint, and accordingly adjust the emissivity value so that the indicator is forced to display the correct temperature.
- Known and published emissivity values for all materials are available in the form of reference literature. One can simply enter these values into the instrument and determine the emissivity value of a material.