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Thermal infrared |
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Thermal infrared radiation energy is more frequently known as "heat". Everyone is familiar with heat because of our sense of touch. But what exactly is heat? Heat is a form of light invisible to our eyes, but noticeable with our skin. Visible light is part of a large spectrum of energy that includes other familiar electromagnetic energy regions, such as microwaves, radio waves, ultraviolet, and X-rays all are forms of light that we cannot see. The colors of a rainbow form a continuous spectrum of light in the visible wavelength region, as does the "light" in the other regions. Thermal infrared light occurs at wavelengths just below red light, hence the name, infra- (below) red.
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Near infrared is the "color" of the heating coil on an electric stove just before it glows red. The Thermal infrared colors are found at even longer wavelengths.
The light that we see with our eyes originates from a glowing source, a light bulb or the sun, for instance. But that light also can reflect off of surfaces and reach our eyes. This allows us to see things that usually do not emit their own light. Thermal infrared light also is produced and reflected. Unlike visible light though, any object that has a temperature above absolute zero emits Infrared light. With hotter temperatures comes brighter infrared light until and unless the object emits visible light.
We can determine the temperature of a glowing object from the color of the emitted light very easily. This is true in the case of the sun, a light bulb, or a rock heated by the sun. The color that we see in the Thermal infrared light results from the wavelength region where the maximum emitted energy occurs. In the first two cases, this radiance maximum occurs in the visible wavelengths that we call ‘yellow’. In the case of the rock, its peak radiance is in the thermal Infrared Radiation wavelengths. Determining temperature in this manner is a useful feature of infrared measurements.
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