Emissivity and its role in Infrared Temperature measurement

Introduction

Infrared (IR) temperature measurement has become a widely used application in numerous industries, from manufacturing to scientific research. Through the use of infrared, temperature can be measured remotely with precision, and without the need for physical probes or invasive instruments. However, there is one determining factor that significantly influences the accuracy of these measurements: emissivity.

This parameter directly affects how infrared sensors interpret thermal radiation, and any incorrect setting can lead to inaccurate results. In this article, we discuss its concept, its role in temperature measurement, and the techniques available to adjust it correctly.

What is emissivity?

Definition and basic principles

It is defined as the efficiency with which a surface emits thermal radiation or energy in the infrared spectrum compared to a perfect emitter, known as a blackbody, which has an emissivity of 1.0. In essence, it is the measurement of an object’s ability to emit infrared energy, and ranges from 0 (no emission) to 1 (perfect emission). The higher it is, the more infrared radiation it emits at a given temperature.

The concept is based on thermodynamics, particularly Planck’s law and Stefan-Boltzmann’s law, which describe how all objects emit thermal radiation as a function of their temperature. Materials with high emissivity emit more heat, while those with low emissivity emit less, even at the same temperature.

Emissivity in different materials

Not all materials emit infrared radiation equally. Metals, for example, generally have a low emissivity, ranging from 0.02 to 0.3, which means that they reflect more infrared radiation than they emit. On the other hand, materials such as ceramics, organic materials, and paints tend to have high values, often between 0.7 and 0.95.

Knowing this property in materials is of great importance in the use of infrared thermography, its ignorance can drastically affect the temperature readings. On the other hand, the values vary with the wavelength within the infrared spectrum used by the equipment.

The emissivity of a material depends not only on the type of material, but also on factors such as surface texture, oxidation, and coating. For example, polished metals have a lower emissivity compared to oxidized or rough surfaces of the same metal.

Importance of temperature measurement

It is a fundamental property in infrared temperature measurement, as infrared cameras and pyrometers rely on the detection of emitted infrared radiation to determine the temperature of an object. A material with high emissivity will emit more radiation, leading to more accurate readings, while low emissivity materials, which reflect more than they emit, may give erroneous results unless corrected.

Inaccurate values can cause infrared sensors to overestimate or underestimate the actual temperature of an object. For example, a polished aluminum surface, with its low emissivity, might appear cooler than it actually is, while a high-emissivity surface might give a reading closer to reality. Compensating for this is a necessary step to obtain reliable temperature measurements. In the following table, values for various materials are shown to serve as a guide¹.

Table 1. Emissivity values for various metals.

Influence on different materials

Dado que los distintos materiales presentan valores variables, la precisión de las mediciones de temperatura puede variar en función de la superficie que se vaya a medir:

• Metals: According to this statement, metallic materials generally have low values, which makes them prone to errors in infrared measurements if they are not properly compensated. For example, measuring the temperature of a polished steel surface without adjusting for its low emissivity can result in a lower-than-actual reading.
• Non-metals: Materials such as wood, plastic, rubber, and ceramics have higher values, which results in more accurate infrared temperature readings. For these materials, adjustments may still be necessary, but the margin of error is smaller than for metals.
• Painted or coated surfaces: Such a characteristic of a material can change drastically if its surface is painted or coated with another substance. Many paints have a high emissivity, which allows for more accurate temperature readings.

Common errors due to incorrect emissivity

Inaccurate temperature measurements are often due to not properly accounting for emissivity. Common errors include:

1. Assuming the emissivity of a material: Generally, property calculations tend to assume values without knowing the actual value, which leads to significant inaccuracies. A reflective metallic surface, for example, can lead to underestimated readings if a generic configuration is used.
2. Ignoring surface condition: Surface finish, oxidation, and surface roughness can alter the emissivity of a material. A metal with a polished finish will have a lower emissivity than metal with a rough or oxidized surface.
3. Viewing angle: Infrared radiation emitted by an object is best detected when the camera is perpendicular to the surface of the object (90° viewing angle). As the angle deviates from the ideal position, the amount of radiation reaching the camera detector decreases, since at oblique angles, the amount of radiation reflected rather than emitted directly from the object increases, which can result in lower or inaccurate temperature readings.
4. Environmental factors: Ambient temperature, humidity and other environmental factors can affect the perceived emissivity of a material, leading to incorrect readings. Of special consideration are heat sources in the environment of the measurement object².

Emissivity adjustment techniques

To account for these variations and to avoid inaccurate temperature measurements in evaluations with infrared thermography, several methods are used to adjust this property:

Tape application

There are certain high emissivity tapes that can be applied to low-emissivity surfaces to standardize their value. These tapes are designed with a known value, often around 0.95, which allows more accurate temperature readings to be obtained when placed on a material with a lower emissivity.

Basically, the procedure consists of placing the tape on the surface of the material of which the property is unknown, with a certain temperature, then the value of the tape is set in the thermograph and the temperature on the tape is measured. Finally, the thermograph is pointed at the material next to the tape and the emissivity is adjusted until it equals the value of the temperature on the tape previously obtained, thus determining the emissivity of the material.

Use of contact thermometers

This technique is particularly used when evaluating tubes in furnace radiant zones, where the temperature values of the contact thermocouples attached to the surface of the tubes are used to adjust the emissivity of the thermography.

Manual adjustments and emissivity tables

Most infrared cameras allow you to manually enter the emissivity value of the material you are measuring. The tables provide values for this property of common materials, which serve as a reference. However, these tables provide approximate values, so they should be used with caution, especially for materials whose surface condition significantly impacts the emissivity.

Factors affecting correctness

Material type

The type of material is the first factor to consider. Metals, ceramics, and organic materials behave differently in terms of their ability to emit infrared radiation. The material must be accurately identified in order to make the correct adjustments.

Surface condition

Polished metals reflect much more infrared radiation than rough or oxidized metals, leading to a lower value. Surface coatings, such as paint or corrosion, can increase the emissivity of a material, and these factors should always be taken into account during temperature measurement.

Viewing angle

It should always be evaluated with the camera pointing perpendicular to the surface of the material. If measurement from an angle is unavoidable, camera parameters should be adjusted, or corrections made to compensate for reflected radiation.

Environmental conditions

Environmental conditions such as ambient temperature, humidity, and surrounding radiation sources can also influence the accuracy of infrared temperature measurements. In some cases, it may be necessary to adjust the emissivity according to the specific environmental context, especially in environments with high humidity or significant background radiation.

Conclusion

Understanding and adjusting for emissivity is crucial to accurate infrared temperature measurement. Whether using an infrared camera in an industrial setting or a pyrometer in a research laboratory, failure to account for it properly can lead to significant errors. By using gauges, applying tapes and making manual adjustments using charts, professionals can ensure more accurate and reliable temperature readings.

References

1. FLUKE. What is Emissivity?; Accessed September 09, 2024. https://www.flukeprocessinstruments.com/es/service-and-support/knowledge-center/infrared-technology/what-emissivity
2. MESUREX. ¿Qué es la emisividad y por qué es tan importante en Termografía?; Accessed September 09, 2024. https://mesurex.com/emisividad-en-la-medicion-de-temperatura-mediante-termografia/