2 Methods for Calculating Emissivity

Today we'll be looking at two methods of calculating the emissivity of an objects surface, however before we dive into it, let's re-hash on what emissivity actually is.

Emissivity is a measure of the efficiency in which a surface emits thermal energy. It is defined as the fraction of energy being emitted relative to that emitted by a blackbody, at the same temperature (and at the same wavelength). A blackbody is a material that is a perfect emitter of heat energy and has an emissivity value of 1. A material with an emissivity value of 0 would be considered a perfect thermal mirror.

For example, if an object had the potential to emit 100 units of energy but only emits 90 units in the real world, then that object would have an emissivity value of 0.90. In the real world, there are no perfect "blackbodies" and very few perfect infrared mirrors so most objects have an emissivity between 0 and 1.

Most infrared analysis software has the ability to compensate for emissivity so that accurate temperature measurements can be made of materials with emissivity below 1.00. The accuracy of the measurement, however, is determined by the precision to which the emissivity value and ambient temperature are known. Additionally, the temperature of objects in the environment should ideally be uniform. Radiance from objects that are hotter or colder than the surroundings can reflect off of the target and affect the accuracy of emissivity compensation.

Small changes in an object’s emissivity can result in noticeable affects on measured temperature. A 0.02 reduction in emissivity, for example, can decrease the measured temperature of an object at 100°C by approximately 2°C. Likewise, variations in the ambient temperature can affect measured temperature. An increase in ambient temperature of 5°C, for example, can increase the measured temperature of an object at 40°C with emissivity of 0.80 by approximately 1°C.

In order to compensate for the emissivity of an object, its emissivity must first be determined. There are two basic approaches to determining surface emissivity; surface treatment or material heating. Surface treatment involves applying a treatment that is of a known high emissivity (usually tape or paint) to the surface of the object and then heating the surface. Material heating involves uniformly heating the object to a known steady-state temperature that is above ambient temperature. During both procedures, best results are achieved when the object is heated to a temperature close to the temperature at which measurements are to be taken during testing. If performed properly, correct emissivity values can be obtained using either approach. The chosen method will depend on the characteristics of the surface and size or shape of the object.

Surface Treatment Method

This method should be employed when the object’s size and shape facilitates applying a small section of masking tape. Masking tape is the preferred surface treatment for object temperatures below 100°C due to its uniform emissivity (0.95) and thickness. Alternatively, a thin dab of paint or white-out can be used on objects with small or uneven surfaces where tape cannot be applied. The disadvantages of using paint or white-out are the possibility of deviations in coating emissivity and thermal diffusion due to variations in application thickness. If care is taken during the application of the coating, however, uniform results can be obtained.

To determine an object’s emissivity using the surface treatment method, follow these steps.

1. First, determine the Reflected Apparent Temperature (TRefl/Tbak/Reflected Background Temp). This is done by setting the "distance" to 0m and "emissivity" to 1. Then measure the average temperature of the reflected angle to which you will be viewing the target from. This measurement will be your reflected apparent temperature and you will need to input the value into your camera settings. You need to pay extra attention to reflected apparent temperature in situations where the radiation emitted by your target differs from the radiation emitted by its surroundings, such as a warm object in a cold environment – or vice versa. For higher emissivity objects, reflected temperature has less influence. For lower emissivity objects, however, it’s a critical factor that must be understood. As emissivity decreases, what you are measuring (and seeing thermally) is coming more from the surfaces of surrounding objects (including the camera and operator), not the target you are inspecting.
2. Apply a small section of masking tape to the area of interest making sure to leave a section of the original surface exposed. There are many types of masking tapes with different emissivities, mainly from 0.92 to 0.98. Therefore it is necessary to obtain a calibrated tape with well-known emissivity, for instance this one from Testo: https://www.testo.com/en-AU/emission-tape/p/0554-0051
3. Heat the surface to a temperature that is below 100°C. Heating can be accomplished by different methods including powering the device or heating the surface using a heating plate or hot air gun. The aim is to heat the surface to at least 10°C higher than the ambient temperature. and ensure the surface is being heated consistently to attain a steady state temperature.
4. Capture a thermal image of the heated surface. Note: Make sure that the heating source is not reflecting off of the exposed surface when the image is captured.
5. Draw a small region enclosing the tape and a second small region enclosing the exposed surface.
6. Set the emissivity of the region enclosing the calibrated tape to 0.95 (or whatever the predetermined emissivity of the calibration tape is).
7. Adjust the emissivity of the region enclosing the exposed surface until the temperatures within both regions are equal. Record the emissivity of the object.

Material Heating Method

This method should be employed when tape or paint cannot be applied to the surface due to an object’s small size or surface characteristics. Material heating can also be used to determine the emissivity of different materials comprising a complex object with many different surfaces.

To determine an object’s emissivity using the material heating method, follow these steps.

1. Determine the Reflected Apparent Temperature as per Step 1 of the surface treatment method above.
2. Heat the object to a known uniform steady-state temperature and at least 10°C higher than the ambient temperature. One of the most common methods of heating small and thin objects, such as semiconductors chips, is using a heating plate. A thermal chamber can also be used provided there is an opening or infrared window on the chamber through which to image the object.
3. Measure the steady-state temperature of the object by measuring the temperature of a high emissivity area in the thermal image or by using a contact temperature probe.
4. Draw a small region enclosing each different surface to be measured.
5. Adjust the emissivity of each region until the temperatures within the regions are equal to the temperature of the object measured in step 2. Record the emissivity of each different surface.

A Note about Contact Temperature Probes

If used in appropriate situations and applied correctly, contact temperature probes such as thermocouples, thermistors, and RTDs can be used to accurately measure surface temperature. Small objects and thin surfaces, however, may not contain enough thermal mass to accurately measure using these devices. In these cases, contact probes can act as heat sinks and lower the temperature of the material, creating erroneous readings. Also, a good thermal bond must exist between the material and contact probe in order to transfer sufficient thermal energy to heat the probe to the same temperature as the material. In many cases, poor thermal bonding results in erroneous temperature measurements that are much lower than true temperature. Measurement errors due to low thermal mass and poor thermal bonding can result in errors as great 10, 20, or even 30°C when measuring an object at 60°C.

#emissivity #thermography #infraredthermography #thermalimaging #testo