INFLUENCE ON THE MEASURE
In addition of being able to see totally in the dark without the need to use lighting (reduction of light pollution), thermal cameras offer the possibility to visualize the temperature difference of different objects in the field of view.
These differences can be visualized both from a qualitative point of view, different colors for different temperatures, or quantified by showing the temperature of the various objects.
Very often, especially in the field of video analysis and radiometry, the two are combined together.
In order to collect precise and accurate temperature measurements, there are several factors that could greatly affect the accuracy of those measurements.
Let's see what are the most common factors.
EMISSIVITY
Emissivity (ε) represents the ability of an object to first absorb heat and then to transmit and emit this thermal energy in the infrared field.
The emissivity can assume a value between 0.0 (when all the energy is reflected) and 1.0 (black body when all the energy is absorbed).
In reality, the two extremes have a purely theoretical value as in nature there are only gray bodies or materials that have intermediate emissivity values.
Generally speaking, an object’s emissivity is determined by the materials it’s composed of and its surface texture.
An object’s emissivity is highly dependent on its surface composition, and can also be affected by its roughness, oxidation, spectral wavelength, temperature, and possibly viewing angle.
Organic materials like human skin, soil, unpainted or varnished wood, and rocks are typically highly emissive.
Shiny or polished materials, on the other hand, are more reflective.
A measurement that does not account for the real emissivity of a surface will appear “colder” than it actually is.
As a reference, Table 1 demonstrates the wide range of emissivity values that may be encountered in radiometric applications.
| Material | ε |
|
Polished aluminum |
0.04 |
|
Stainless steel (Polished) |
0.10 to 0.15 |
|
Stainless steel (Oxidized) |
0.45 to 0.95 |
|
Snow |
0.83 |
|
Asphalt |
0.90 to 0.98 |
|
Wood |
0.90 |
|
Oil (Mineral) |
0.90 to 1.00 |
|
Concrete |
0.92 |
|
Water |
0.90 to 0.95 |
|
Human skin |
0.99 |
Table 1
REFLECTIVITY
Materials with very low emissivity (0,0-0,2), like highly polished metals, tend to be very reflective of ambient infrared energy and less effective at emitting their own electromagnetic waves.
If you were to take the temperature on the side of a stainless steel pan filled with boiling water with a thermal camera, for example, you could get a reading closer to 38°C (100°F) than 100°C (212°F). This is because shiny metal better reflects the room's ambient radiation rather than emitting its own infrared energy.
For higher emissivity objects, reflected temperature has less influence. For lower emissivity objects, however, it’s a critical factor that must be understood carefully in order to use infrared technology. As emissivity decreases, what you are measuring is coming more from the surfaces of surrounding objects, not the target you are inspecting.
Much like emissivity, the reflectivity of a surface is highly dependent on the surface morphology and roughness. Since reflectivity (R) is related to emissivity (ε) by R=1-ε, the importance of reflectivity can be greatly reduced by making measurements of surfaces with very high emissivity, ideally greater than 0,90.
For measurements of surfaces with low reflectivity, such as a steel tank, high emissivity / low reflectivity matte black paint can be used for more accurate measurements.
ENVIRONMENTAL FACTORS
Weather and environmental conditions will affect the thermal radiation emitted from an object and decrease the effective detection range due to the temperature difference that causes the lack of contrast between the background and foreground. An object with nearly the same thermal emission temperature as the background, such as a body on a moderately hot summer day, is harder to distinguish from its background than an object with a greater temperature difference, such as a car with a running engine on a cold winter day. The two most important environmental factors that affect the image of an object in the thermal camera are absorption and scattering. They reduce the thermal radiation that reaches the camera, thereby reducing the distance at which the imager can detect an object.
Water vapor (H2O) and carbon dioxide (CO2) in the air are the primary causes of absorption. During absorption, the heat radiated from the object is absorbed by water vapor and carbon dioxide and loses some of its energy before reaching the thermal camera. The water vapor content of the air affects image quality even in sunny and clear weather. In winter, if all other weather conditions are the same, the water vapor content of the air is lower than in summer. Since water vapor content is lower, less thermal radiation is absorbed by the water molecules, allowing more thermal radiation to reach the camera and resulting in better image quality when compared to a summer day.
During scattering, the thermal energy radiated from the object is dispersed when it hits particles in the air. The loss of radiation is directly related to the size and concentration of the particles, droplets or crystals that constitute polluting, condensing or precipitating conditions such as smog, fog, rain or snow.
The atmosphere can affect the temperature measurements in other unexpected ways. Measurements should always be performed in the absence of rain, snow, smoke, dust, or any other obscurants because they too will reduce the atmospheric transmission and change the background temperature. Finally, radiometric measurements only report the surface temperature and the surface temperature may be very sensitive to strong winds.
Authors:
Marco Saccardo & Matteo Torresani - Videotec Customer Technical Support
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