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Emissivity
Thermal emissivity is the ability of a material to emit thermal radiation. It is a measure of how well a material absorbs and re-emits heat. Materials with high emissivity absorb more heat and emit more thermal radiation than materials with low emissivity.
WHAT?
If you have ever seen glow in the dark paint, you have seen emissivity at work. in the thermal spectrum, the amount of energy something can give off is related to how hot it is. Kind of like exposing glow in the dark paint to a bright light.
How does this help?
Emissivity is a factor in idenfiying what is happening at the surface of a subject. Hot parts of images give off heat, but emissive surfaces give off heat faster. So a hot surface looks hotter, and a cooler but highly emissive surface will look warmer, longer than a cooler low emissive surface.
In Thermography class, we are taught how to adjust for emissivity to obtain a good estimate for temperature based on our own equipment. Without going into details, it involves a LOT of electrical tape, and a 5 gallon bucket of ice water!
How is emissivity used?
Thermographers use this concept to help identify temperatures in many different ways.
Some examples:
- Search and rescue: Thermal camera drones can be used to detect people and objects in low-light conditions or through smoke and fog. This can be useful for search and rescue missions in areas where visibility is poor, especially at night.
- Inspection: Thermal camera drones can be used to inspect structures or equipment. This can be useful for inspecting power lines, pipelines, or buildings. Solar panels!
- Agriculture: Thermal cameras can be used to monitor crops for heat stress or pests. This can help farmers to improve crop yields as well as look for nusance wildlife.
- Wildfire monitoring: Thermal cameras can be used to detect hot spots and track the spread of wildfires. This can help firefighters to fight wildfires more effectively.
- Building inspection: Thermal cameras can be used to inspect buildings for energy leaks. This can help building owners to save money on energy costs.
- Environmental science: Monitoring envnronments where any two temperatures mix
- Process monitoring: Thermal drones can determine the volume, relative or exact temperature, and thermal conditions of large scale processes, such as cemement or asphault processes.
- So much more
What uses do you want to see for thermal images?
Looking at this image of the USS Slater, the coolest and least emissive parts of the image are visible. Those are the sky and water.
If you look at the hull of the ship, you can see that even the different paint colors have differnt emissive values.
Of course, the highest heat items are fully exposed to the sun, and warmed cement will be visible for hours even after the sun has set.
