Emissivity measures how well an object radiates absorbed energy. It can be measured by comparing the radiation emitted by the object to that of a black body. Reflective objects have low emissivity, while dark, opaque objects have high emissivity. Emissivity can be important for controlling temperature and insulation. The value can vary based on temperature, wavelength, and angle of energy. Common object values include .02 for silver, .93 for brick and marble, and .90 for wood.
Emissivity is a measure of how well something radiates the absorbed energy. Known measurements are readily available for many common objects, with graphs of their emissivity in various situations printed in many texts and references. For others, some measurements can be taken to determine the level of emissivity. This property can be important to measure and has a number of interesting applications that can make objects with unusually high or low emissivity artistically, scientifically, or commercially useful.
To measure this property, it is determined how much radiation is emitted from the surface of the material in question at a given temperature. This number is used to create a ratio to the amount of radiation emitted by a black body at the same temperature. For a black body, the value is one. For everything else, the number can range from zero to one. The higher the emissivity, the closer the ratio is to one; something with a level of .95, for example, has very high emissivity.
The more reflective something is, the less emissivity it has. Dark, opaque objects such as asphalt, basalt, wood, and paper tend to have high values. Reflective objects such as silver and foil have very low emissivity and high reflectivity. When added together, the two values for emissivity and reflectivity should equal one, illustrating the balance between reflection and the emission of radiant energy.
The ability to reflect more or less energy can be important. Windows, for example, can be wrapped or lined with low-e materials to control the temperature inside the home. Low-E glass, as it is known, reflects radiant energy from within, keeping heat in rather than letting it escape through windows, and it also reflects radiant energy from outside, keeping the house cool in the summer . Highly reflective materials are also used in the insulation to minimize heat transfer, making for a more efficient structure.
The ratio does not necessarily remain constant. Many things can affect the value. Temperature is an aspect, as are wavelength and angle of energy. Test materials under various conditions should provide a range of values. A material like ceramic, for example, can have a wide spread of values, which is important for people like artists to know.
Some examples of common object values include: .02 for silver, .93 for brick, .93 for marble, .91 for plaster, .85-.95 for concrete, and .90 for wood. The reflectivity values for these materials would be 98 for silver, 07 for brick and marble, 09 for plaster, 15-05 for concrete and 1 for wood.
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