Keysight Technologies How to Measure the Unknown Thermal Emissivity of Objects/Materials Using the U5855A TrueIR Thermal Imager

Application Note

Introduction

In an infrared (IR) thermography application where an IR camera is used, understanding the object’s surface emissivity is crucial because it significantly impacts the temperature measurement result. This is especially true for a quantitative thermography application where temperature accuracy is the primary concern. This application note explains practical ways to acquire or estimate the emissivity ([) of the target objects objects when using an infrared imaging radiometer.

Emissivity Basics

Emissivity relates to the radiation of thermal energy from the surface of a target object. The emissivity, [, is defined as the ratio (between 0 and 1) of the thermal radiation from a target’s surface to the radiation from an ideal black body, ([ = 1), at the same temperature. Objects which have high emissivity emit more thermal energy through radiation compared to objects with low emissivity. As emissivity gets higher, reflected thermal radiation lowers. The relationship between emitted and reflected IR energy from a typical opaque object is shown in Figure 1. For reference, Table 1 contains the emissivity values of some commonly-found objects

Figure 1. Emissivity and reflectivity relationship on thermography application.

Target object External thermal energy source

Emitted thermal IR energy from object (E)

Reflected thermal IR energy (R)

Infrared (IR) thermal imager E + R = 1

t (E)

(R)

Infrared (IR) thermal imag

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Table 1. Emittance value on some common materials

Item Emittance value

Iron (polished) 0.10

Aluminum (oxidized) 0.15

Stainless steel (polished) 0.20

Nickel (oxidized) 0.37

Copper (oxidized) 0.40

Brass (oxidized) 0.64

Iron (oxidized) 0.75

Stainless steel (oxidized) 0.80

Snow 0.85

Cloth 0.90

Paper 0.90

Glass 0.90

Rubber 0.90

Clay 0.92

Concrete 0.92

Brick 0.93

Paint (typical) 0.94

Graphite (carbon) 0.95

Water 0.96

Skin 0.98

There are a few factors that affect the emissivity of an object:

Object temperature Emittance is a dynamic variable that will change with object temperature. Depending upon the material, emittance can range from quite low to quite high depending upon an object’s temperature. For clean surfaced metals, emittance increases as temperature increases. For nonmetals, emittance decreases as temperature increases.

Viewing angle Viewing angle can also significantly affect an object’s emittance. Because object emittance is highest from a perpendicular aspect, it is preferable to image perpendicular to an object’s surface.

Object geometry or shapeObject shape can have a profound influence on emittance. Compared to a flat surface, changes in object shape will cause changes in emittance thereby affecting thermal imagery and radiometric temperature values.

Cavities on an object will have higher emittance values compared to flat surfaces on the same object. Depending upon the size and depth of the cavity, emittance values can reach nearly 1.0. Compared to a flat surface, convex areas on an object will cause will cause emittance values to decrease.

Surface condition Emittance can be greatly influenced by surface condition. Polished metals that have smooth surfaces are quite reflective and usually have very low emittance values. Because of this, shiny metals offer the greatest challenges for thermal imaging and non-contact temperature measurement.

In addition to surface roughness, dirt, dust, oxidation, and non-metallic coatings on an object’s surface will generally result in higher emittance values. Depending upon circumstances, [ values may approach 1.0

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An IR thermal imager works better when scanning a high-emissivity object surface be-cause the object is able to emit more IR energy to the camera, which then converts that IR radiation to a temperature measurement. As emissivity lowers, the IR emissivity also lowers and the reflected IR energy from other heat sources near the target may raise the temperature measurement conversion, thus providing an inaccurate temperature reading of the target object.

Users have to enter the emissivity, [, into the IR thermal imager to correctly compensate for surface temperature. Figure 2 shows a USB cup warner with an aluminum plate that is spray-painted with different colors, each with a different emissivity value. The black paint has higher emissivity compared to the red paint, thus on the IR thermal imager, the black paint area appears to be hotter versus the red paint area. In order to get the correct temperature on the red paint area, the emissivity value on the camera setting will have to be reduced accordingly. The question is, what emissivity value should the user enter into the IR thermal imager?

REDBLACK RED

BLACK

Figure 2. Emissivity effect on thermography

05 | Keysight | How to Measure the Unknown Thermal Emissivity of Objects/Materials Using the U5855A TrueIR Thermal Imager - Application Note

Determining Emittance Values

There are a few methods to estimate or measure the emissivity value of an object’s surface.

Common material emissivity tableUsing the common material emissivity table shown in Table 1 is the quickest way to estimate the emissivity of a target object. This is particularly useful in qualitative thermography applications where the main test objective is to detect the hot spots by comparing hot and cold (or normal) areas of the target object. For example, in electrical maintenance thermography inspection work, the inspectors normally enter an estimated emissivity value and then start scanning for hot spots around electrical systems, including items such as electrical switches, cables, connectors, fuses, circuit breakers, and other components. With an IR thermal imager, such as the Keysight Technologies, Inc. U5855A TrueIR thermal imager, set to a common emissivity value of 0.90, Figure 3 shows how the IR image capturers a hot spot on the left cable.

Figure 3. IR thermography hot spot on electrical system

06 | Keysight | How to Measure the Unknown Thermal Emissivity of Objects/Materials Using the U5855A TrueIR Thermal Imager - Application Note

Temperature comparisonAnother method for measuring the target object’s emissivity is by comparing the temperature measurement result on the IR thermal imager with the measurement from a contact thermocouple sensor. As illustrated in Figure 4, the target object surface temperature is first measured by a thermocouple or a similar type of temperature sensor measuring system. The emissivity value on the IR thermal imager is adjusted accordingly until it obtains the same temperature measurement result. The last entry of the emissivi-ty value represents the surface emissivity of that target object.

This kind of emissivity measurement can be easily done when there is some kind of contact-type temperature sensing acquisition working in parallel with the IR thermal imager. In certain use cases, the IR thermal imager serves as a quick temperature scanning tool to identify hot spots on target objects. The temperature sensing probes are located at those hot spots for temperature measurement acquisition.

Figure 4. Acquiring the emissivity value by comparing the temperature measurement from thermocouple sensor

Target object

DAQ+ meter

Emitted thermal IR energy from object

Thermocouple sensor

Adjusting the emissivity, ε, on the camera to get the similar temperature measured by the thermocouple sensor

Thermocouple sensor temperature measurement

1

2

ct

missivity, ε, on the camera temp t

07 | Keysight | How to Measure the Unknown Thermal Emissivity of Objects/Materials Using the U5855A TrueIR Thermal Imager - Application Note

Emissivity coatingAnother method for measuring the unknown surface emissivity is by applying an additional high emissivity coating, such as blackbody paint or a high emissivity tape such vinyl electrical tape, onto the object itself (see Figure 5). Coating materials like the tape typically have a thermal emissivity value of > 0.90. The procedure for this method is as follow:

1. Clean the object surface from any dirt or stains if possible. Apply a layer of blackbody paint or vinyl electrical tape to area A.

2. Do allow time for the entire surface of blackbody paint or tape to reach an equilibri-um temperature with the target object. If the targeted object is not radiating heat, it is better to heat up the object until the temperature of the entire surface reaches an equilibrium state.

3. Perform the reflected temperature calibration function (if available) and set the known emissivity value of the blackbody paint or tape on the thermal Imager. Measure the temperature of area A where the blackbody paint or tape was applied. Ensure that the thermal imager is focused and perpendicular to the target object. Record the center spot input reading on the thermal imager.

4. Move and refocus the imager outside the uncoated area, B, and while measuring the temperature adjust the emissivity value on the imager to obtain a similar temperature reading as the coated area. This will provide a very good estimation of the unknown thermal emissivity value.

This method is practical and easily implemented if the thermography is having difficulty estimating the thermal emissivity of the target object. However, there are limitations to this method. For example, the coating or tape can only be applied to accessible target objects. Additionally, the environment and setting must be safe for the operator to apply the coating.

Figure 5. Using high emissivity coating/tape to measure unknown emissivity

1

Apply the known emissivity value of the coating/tape, measure the temperature

Refocus the IR thermal imager to the uncoated area, adjust the emissivity on the imager to get the similar temperature reading

A B

Unknown emissivity objects/materials

Apply blackbody paint or blackbody tape (with known emissivity value)

2 known emi

e

vity va

08 | Keysight | How to Measure the Unknown Thermal Emissivity of Objects/Materials Using the U5855A TrueIR Thermal Imager - Application Note

Conclusion

Finding the unknown thermal emissivity value of a target object has always been a test challenge when performing IR thermography analysis. In field work, uncertainty of the target object’s emissivity value can cause thermographers to question if the correct temperature measurements and profile have been captured on their IR thermal imager. The various methods illustrated in this application note serve as a practical reference for accurately measuring a target with an unknown emissivity, from both a quantitative and qualitative thermography analysis perspective.

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