Thermography facts for kids
Infrared thermography (also called thermal imaging or thermal video) is a cool way to "see" heat. It uses a special camera called a thermal camera. This camera captures the invisible heat (infrared radiation) that objects give off. It then turns that heat into an image you can see, called a thermogram.
Every object that's warmer than absolute zero (the coldest possible temperature) gives off infrared radiation. This means thermal cameras can see things even in total darkness! Warmer objects give off more radiation. So, a thermal camera shows you where things are hot and where they are cold. Warm things, like people or animals, really stand out against cooler backgrounds, day or night. This makes thermal imaging super useful for things like surveillance and finding people.
Thermal imaging can also help doctors and vets. It can show changes in body temperature in people and animals. This helps them find problems or check on health. For example, it's used in veterinary medicine for animals. During the 2009 swine flu pandemic, thermal cameras were even used at airports. They helped find people with high body temperatures, which could be a sign of the virus.
Thermal imaging has been around for a long time. But it has become much more common in the last 50 years. Firefighters use it to see through smoke. It helps them find people and locate the hottest parts of a fire. Technicians use it to find hot spots in power lines. These hot spots can mean a part is about to break. Builders use it to find places where heat is escaping from buildings. This helps them make homes more energy-efficient.
Modern thermal cameras often look like regular camcorders. Sometimes, just seeing the live heat image is enough. You don't always need to record it.
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How Thermal Cameras See Heat
Thermal images, or thermograms, show all the infrared energy an object gives off, lets through, and reflects. It's like a heat map! Because heat comes from different places, it can be tricky to get an exact temperature reading. The camera uses smart computer programs to create a temperature image. Remember, it's an estimate of the temperature. The camera gathers information from around the object to figure it out.
Think of it this way:
- The heat the camera "sees" is a mix of:
- Heat the object itself is giving off (what we usually want to measure).
- Heat that passes through the object from something behind it.
- Heat that bounces off the object's surface from something else.
This happens all the time, everywhere. It's called radiant heat exchange. If an object is hotter than its surroundings, it sends heat out to the cooler areas. If an area in the thermogram looks cool, it means that object is absorbing heat from warmer things around it.
What is Emissivity?
Emissivity is how well a material can give off heat radiation. It's a property of the material itself. Emissivity can range from 0 (meaning it doesn't give off any heat) to 1 (meaning it gives off all the heat it can). For example, shiny silver has a very low emissivity (0.02). Asphalt, like on roads, has a high emissivity (0.98).
A "black body" is a perfect theoretical object. It has an emissivity of 1 and gives off heat perfectly based on its temperature. A normal object gives off less infrared radiation than a perfect black body. So, emissivity is the ratio of how much heat an object actually gives off compared to how much it could possibly give off.
Different materials have different emissivities. This can also change with temperature and the type of infrared light. For instance, clean metal surfaces give off less heat at longer infrared wavelengths.
How Thermal Cameras Measure Temperature
A thermal camera uses math to create a visible image. This is because it "sees" electromagnetic radiation that our eyes can't. The final image can be saved as a picture file, like a JPG.
The amount and type of heat radiation depend a lot on an object's surface temperature. This is how a thermal camera can show an object's temperature. But other things also affect the radiation, which can make the measurement less exact. For example, the radiation also depends on the object's emissivity.
To measure temperature without touching the object, you need to set the camera's emissivity correctly. If an object has low emissivity, the camera might think it's colder than it is. This is because the camera only detects the heat the object emits. For a quick guess, a thermographer (someone who uses thermal cameras) might look up the emissivity for that type of object in a table. Then they enter that value into the camera. The camera then calculates the object's temperature based on the emissivity and the heat it detects.
For a more accurate measurement, a thermographer might put a special material on the object's surface. This material has a known, high emissivity. It could be a special spray or even just black insulation tape, which has an emissivity of about 0.97. Then, the object's temperature can be measured using the known emissivity of the tape. If needed, you can then figure out the object's actual emissivity (on a part not covered by the tape). You do this by adjusting the camera's setting until it matches the known temperature. Sometimes, it's too dangerous or hard to do this test. In those cases, the thermographer has to rely on the tables.
Other things can also affect the measurement. These include how much the air absorbs the radiation and the air's temperature. Also, heat from the surroundings can reflect off the object. All these factors affect the final temperature shown by the camera.
Color Scales in Thermal Images
Thermal camera images usually look like they're in one color (monochrome). This is because the camera's sensor doesn't usually tell the difference between different infrared wavelengths. Our eyes see different wavelengths as different colors. But outside of visible light, colors don't have the same meaning.
Sometimes, these single-color images are shown in pseudo-color. This means that changes in color are used to show changes in the heat signal, instead of just changes in brightness. This is helpful because even though humans are good at seeing differences in brightness overall, it's hard to see small differences in very bright areas.
When measuring temperature, the hottest parts of the image are usually white. Medium temperatures are red and yellow. The coolest parts are black. A scale should always be shown next to a pseudo-color image. This scale tells you what temperatures the different colors represent.
Types of Thermal Cameras
A thermographic camera (also called an infrared camera or thermal imaging camera) is a device that creates an image using invisible infrared (IR) radiation. It's similar to a regular camera that uses visible light. But instead of seeing light from 400 to 700 nanometres, infrared cameras see wavelengths from about 1,000 nm (1 micrometre) to 14,000 nm (14 μm). Using and studying the information from these cameras is called thermography.
Thermal cameras change the energy from far infrared light into a visible image. All objects warmer than absolute zero give off thermal infrared energy. So, thermal cameras can "see" all objects, no matter how dark it is. However, most thermal cameras are sensitive to objects warmer than about -50°C.
Thermal cameras have lower image quality than regular cameras. They often have resolutions like 160x120 or 320x240 pixels. More expensive ones can go up to 1280x1024 pixels. Thermal cameras are also much more expensive than regular cameras. High-end models are often restricted from being sold to other countries. This is because the technology can be used by the military.
Cooled Infrared Detectors
