Emissivity is a physics term describing the ability of a surface to emit or radiate energy in the infrared region of the electromagnetic spectrum. Surface emissivity is dependent on material makeup, surface characteristics, and measurement angle. Plastics and ceramics tend to have emissivity between 0.8 and 0.95. Metals can have with a much wider range of emissivity (between 0.02 and 0.95) depending on oxidation and surface roughness. The emissivity of many ceramics and plastics have lower emissivity when measured at high angles.
A blackbody is an ideal surface with emissivity equal to 1.00 (100%) and is called a perfect emitter. No surface can radiate more infrared energy at a given temperature than a blackbody. All real surfaces have emissivity below 1.00.
Real surfaces have emissivity that ranges between 0 and 1.00. The emissivity of a real surface represents the ratio of infrared energy that the surface emits at a given temperature to that of a blackbody. For example, a surface at 50°C with emissivity of 0.72 emits 72% of the infrared energy that a blackbody would emit at the same temperature. The temperature of an object and its emissivity define how much infrared energy an object radiates.
Reflectance and Transmittance
The reduced emissivity of a real surface is due to its partial reflectance, transmittance, or a combination of both. A surface with high reflectance by definition, has low emissivity. In fact, for opaque surfaces, emissivity is the inverse of reflectance. For example, an opaque surface with reflectance of 40% has emissivity of 60%. There are many opaque materials where this relationship holds such as most metals, plastics, ceramics, papers, paints, and tapes. Examples of semi-transparent surfaces where this relationship does not hold up include thin plastics (below 1mm thick) like plastic wrap, thin visually transparent electronic coatings, polyimide tape, and exotic refractory materials like Germanium and Silicon.
Reflective Surfaces: Although there is not perfect correspondence, if a surface exhibits reflectance in the visual spectrum, it usually has similar reflectance in the infrared spectrum. Surface transparency usually does not behave similarly. For example, although glass is transparent in the visual spectrum, it is nearly opaque in the infrared spectrum.
The apparent temperature of a surface is the temperature measured by a thermal imaging camera with no software emissivity compensation applied to the measurement. In other words, the software assumes that the surface is a black body with emissivity of 1.00. Apparent temperature is proportional to the total emitted, reflected, and transmitted infrared energy and is a function of surface temperature, emissivity, and background temperature.
An increase in surface temperature or an increase in surface emissivity both result in higher emittance and thus, higher apparent temperature. Given two surfaces with the same true temperature but different emissivity, a higher apparent temperature will be measured on the surface with higher emissivity. Likewise, given two surfaces with the same emissivity but different true temperature, a higher apparent temperature will be measured for the surface with higher true temperature. Note that these relationships are valid only if surface temperature is higher than background temperature.
The apparent temperature of a surface may be substantially different from its true temperature depending on surface emissivity. Only when surface emissivity is known can thermal imaging software compensate for emissivity and then calculate true temperature accurately.
True temperature is the actual temperature of an object and is a measure of the average heat or thermal energy of the particles in a substance.
Reflected Background Infrared Energy
Real surfaces reflect the infrared energy radiated by the surroundings to some degree. Polished aluminum with emissivity of 0.10, for example, reflects 90% of the background infrared energy striking its surface. Because apparent temperature is a function of reflected background energy, the apparent temperature is also dependent on the temperature of the background surroundings. Higher background temperatures will increase the aluminum's reflective energy, resulting in a higher apparent temperature.
Region emissivity can be used compensate for surface reflectance or transmittance. The emissivity of each Region can be set individually in the Region Settings window. When Region emissivity is used to calculate true temperature, an assumption is made that the emissivity is uniform over the area enclosed by the Region. The calculation also assumes that the Background Temperature is also uniform. The accuracy of these assumptions determines the accuracy of calculated temperature values.
Emissivity tables provide the ability to compensate for non-uniform emissivity over a surface pixel-by-pixel. Single temperature and double temperature point emissivity tables can be created. Like Region emissivity, single point emissivity table calculations assume a uniform Background Temperature. Double point tables, however, can account for non-uniform backgrounds.
Background Temperature is the temperature of a target's ambient background. Infrared emissions from the ambient background strike the target and are either absorbed, reflected, or transmitted. Background emissions greatly influence the apparent temperature of low-emissivity surfaces and therefore the Background Temperature must be set correctly in order for Thermalyze to accurately calculate true temperature.
Just as the emissivity algorithm can compensate for the reflectance of opaque surfaces, emissivity can also compensate for the reduction of infrared energy due to the partial transmittance of infrared windows. For example, a Region's emissivity can be set to 0.85 to compensate for an infrared window with 85% transmission.
Most surfaces have uniform emissivity throughout the long infrared wavelength range from 7 to 14 microns in which the IS640 camera operates. For example, the emissivity of most plastics, ceramics, and metals does not vary significantly in this wavelength range. And unless a physical or chemical material transformation takes place, surface emissivity is relatively stable over a reasonable range of temperatures.
|Paint (Flat Black)
The Emissivity Settings window (see Figure 1) contains controls that affect how emissivity compensation is calculated for regions and emissivity tables. To open the Emissivity Settings window, click the Emissivity Settings item under the Emissivity menu or press the button in the Emissivity section of the Shortcuts toolbar.
Figure 1: Emissivity Settings window
||Set this setting to the temperature of the ambient environment surrounding the object to be measured. This value is used to compensate for the energy that is emitted by the environment and reflected off of the object being measured into the camera. This setting is only used when a region Emissivity setting is below 1.00 and when pixel emissivity is below 1.00 in an emissivity table.
Note: When measuring objects whose temperature is greater than the value of the Background Temperature setting, the compensating algorithm in Themalyze will increase the calculated temperature as emissivity is decreased. In other words, because the total energy measured by the camera is constant for an object at a steady-state temperature, reducing the Background Temperature essentially instructs the software that the total measured energy is emitted by a higher-temperature object with a reduced emittance.
|Use Camera Temp
||Check this box when measuring objects that are close to the lens. When this box is checked, the real-time camera detector temperature is used in place of the Background Temperature setting when calculating emissivity compensation. The detector temperature is used at close-up measurements because in this case the majority of reflected energy from the target originates from the camera lens and housing.
Note: This is especially applicable when measuring specular surfaces at a perpendicular orientation because these mirror-like objects readily reflect the energy emitted from the camera lens and housing.
Most plastics, ceramics, and coated metals have relatively high emissivity and their true temperature can be accurately measured using emissivity compensation. Polished metals however, usually have very low emissivity (below 0.20) and measuring their true temperature accurately is difficult and prone to large errors. A small change in emissivity can result in a dramatic change in true temperature calculation. It is usually necessary therefore, to the increase surface emissivity of such materials by applying a coating or surface treatment.
The following are several common surface treatments that can be applied. For a given material, more than one of these approaches may be effective. The choice of treatment will depend on factors such as a treatment's ease of application and removal. When selecting and applying a surface treatment, it is important to apply the thinnest layer possible in order to minimize the thermal insulating effects of the treatment layer.
Apply a thin layer of adhesive tape such as masking tape
Apply a thin layer of paint, lacquer, or other high emissivity coating
Apply a surface treatment such as anodizing
Apply a thin layer of oil, water, or other high emissivity liquid
Wipe on a thin layer of baby powder or foot powder
Roughen the surface (may require substantial roughening)
Thermalyze enables you to measure both apparent temperature and true temperature. If the region emissivity is set to 1.00 (blackbody), the statistics in the Region Data Grid will display apparent temperature. If the Region's emissivity is set to a surface’s true emissivity however, then true temperature will be calculated and displayed.
The accuracy of true temperature calculations is determined by the precision to which emissivity and background temperature are known. The temperature of the ambient surroundings must be known because background infrared emissions reflect off targets, adding to the total energy detected by the camera. If the Background Temperature is held constant in Thermalyze, changes in ambient temperature will affect true temperature measurements. For example, an increase in room temperature of 5°C will increase the measurement of a 40°C object (with 0.80 emissivity) by approximately 1°C.
Two different techniques to measure emissivity will be described: surface treatment and precision heating. Surface treatment involves covering the surface with a thin, high-emissivity material (typically tape or paint) and then heating the object to any temperature higher than ambient. Precision heating involves heating an object to a precise, steady-state temperature. With both procedures, the most accurate emissivity measurements are achieved when objects are heated to actual, operating test temperatures. If performed correctly, accurate emissivity measurements can be obtained using either approach. The chosen method will depend on the size and shape of the object and its surface shape and texture.
This method can be used when an object’s size and surface characteristics allow a small piece of masking tape to be adhered to its surface. Masking tape is relatively thin and has high emissivity (~0.95) that is very uniform, consistent, and stable over time and temperature. Masking tape purchased at hardware stores can be used at temperatures up to 100°C. High temperature masking tape can be purchased from industrial suppliers and can withstand up to 150°C.
When measuring thin objects or objects with small thermal mass, however, masking tape may add a significant thermal barrier that could prevent heat from being transferred efficiently to the tape’s surface. After the tape has been adhered to the surface, heat from the surface transfers quickly to the surface of the tape, whose temperature can be accurately measured.
If tape is not available or cannot be applied due to a surface’s shape or small size, a thin layer of paint or white-out may be applied. A disadvantage of coatings is the difficulty in achieving uniform coating thickness, resulting in variations in emissivity and thermal barrier affects. Therefore, when working with coatings, take care to apply a thin, uniform layer.
Follow this procedure to determine emissivity using the surface treatment method:
Apply a small piece of masking tape to the area of interest, making sure to leave an area of the surface exposed next to the tape.
Heat the object to a relatively stable temperature that is as close as possible to an actual, operating test temperature at least 25°C above the Background Temperature. Heat can be applied by powering the device or by heating the surface using a hot plate or hot air gun.
Capture a thermal image of the heated surface. Important: Make sure the heat source is not reflecting off of the exposed surface when the image is captured.
Draw a small Region enclosing the tape and a second small Region enclosing the adjacent exposed surface.
Set the Region Emissivity of the Region enclosing the tape to 0.95.
Set the Background Temperature correctly.
Adjust the emissivity of the Region enclosing the exposed surface until the mean temperatures within both Regions are equal. Record the emissivity value.
This method should be employed when tape or paint cannot be applied to the surface due to an surface’s small size or rough, uneven texture. Precision heating is also useful when you need to determine the emissivity of many different surfaces on the same object. One of the most common methods of heating small or thin objects, such as semiconductor devices, is using a hot plate.
Follow this procedure to determine an object’s emissivity using the precision heating method:
Heat the surface to a known, steady-state temperature that is as close as possible to an actual, operating test temperature at least 25°C above the background temperature. Heat can be applied by powering the device or by heating the surface using a hot plate.
Measure the surface temperature using a contact temperature probe or by using thermal imaging to measure the temperature of a surface on the object having known emissivity.
Draw Regions enclosing each individual surface to be measured.
Adjust the emissivity of each Region until the mean temperatures within the Regions are equal to the temperature measured in step 2. Record the emissivity value of each surface.
Surface Temperature: The assumption that the hot plate and surface temperature are equal is valid only if there is good thermal coupling between the hot plate and device and if there is no thermal gradient between the bottom of the device and the surface to measure. Thermal pads or grease can be used to thermally couple devices to the hot plate.
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 or thin surfaces, however, may not contain enough thermal mass to be accurately measured using such devices. Contact probes act as heat sinks and can significantly lower the surface temperature of small or thin objects.
When fixing contact probes to a surface, a good thermal bond must exist between the surface and the probe. Thermal energy must be efficiently transferred from the surface to the probe in order to heat the probe to the same temperature as the surface.
Surface Temperature: Poor thermal bonding of contact probes to a surface will often result in erroneously low temperature measurements.