Emissivity: Basics and Examples
Description of Emissivity
Emissivity is a measure of an object's ability to emit infrared energy compared to a perfect blackbody.
Emissivity plays a crucial role in various scientific and industrial applications. It determines how effectively a surface emits thermal radiation, which is essential for temperature measurements and thermal management.
What Affects Emissivity
Several factors influence an object's emissivity, including surface texture, color, and material composition. Smooth, dark surfaces typically have higher emissivity, making them more efficient at emitting infrared radiation.
Measuring Emissivity
Accurate measurement of emissivity is vital for tools like infrared thermometers. These devices rely on emissivity values to provide precise temperature readings by interpreting the emitted infrared energy from surfaces.
Applications of Emissivity
Emissivity is fundamental in fields such as meteorology, astronomy, and manufacturing. Understanding and controlling emissivity allows for better thermal regulation and energy efficiency in various technologies.
Infrared Thermometers
Infrared thermometers use emissivity settings to accurately measure surface temperatures without direct contact. Adjusting the emissivity value ensures the thermometer accounts for the specific material properties of the target surface.
Thermal Imaging
In thermal imaging, emissivity affects the accuracy of temperature maps. High emissivity materials appear brighter on thermal cameras, indicating higher temperatures, while low emissivity materials may require calibration for correct interpretation.
Emissivity Values of Common Materials
Emissivity is the ability of a surface to emit thermal radiation compared to that of a perfect black body (which has an emissivity of 1). Emissivity values range from 0 to 1, where 1 represents perfect emission (black body) and 0 represents no emission. The emissivity of a material depends on its surface finish, temperature, and wavelength of emitted radiation.
Here is a table of emissivity values for common materials:
|
Material |
Emissivity Value |
|
Blackbody |
1.00 |
|
Asphalt |
0.90 - 0.98 |
|
Aluminum (polished) |
0.03 - 0.05 |
|
Aluminum (oxidized) |
0.70 - 0.80 |
|
Copper (polished) |
0.02 - 0.05 |
|
Copper (oxidized) |
0.70 - 0.80 |
|
Brass |
0.40 - 0.60 |
|
Steel (polished) |
0.10 - 0.20 |
|
Steel (oxidized) |
0.60 - 0.70 |
|
Iron (polished) |
0.10 - 0.20 |
|
Iron (oxidized) |
0.60 - 0.80 |
|
Gold (polished) |
0.02 - 0.05 |
|
Silver (polished) |
0.02 - 0.05 |
|
Wood (painted) |
0.85 - 0.95 |
|
Wood (unpainted) |
0.90 - 0.95 |
|
Concrete |
0.80 - 0.90 |
|
Brick |
0.80 - 0.90 |
|
Glass (clear) |
0.85 - 0.95 |
|
Glass (frosted) |
0.70 - 0.90 |
|
Teflon |
0.95 - 0.98 |
|
PVC (plastic) |
0.85 - 0.90 |
|
Rubber (black) |
0.90 - 0.95 |
|
Paper |
0.85 - 0.95 |
|
Ice |
0.97 - 0.98 |
|
Snow |
0.80 - 0.90 |
|
Water |
0.90 - 0.95 |
For more information, please check Stanford Advanced Materials (SAM).
Frequently Asked Questions
What is emissivity in simple terms?
Emissivity measures how effectively a surface emits thermal radiation compared to a perfect emitter, known as a blackbody.
Why is emissivity important for infrared thermometers?
Emissivity settings ensure accurate temperature readings by accounting for the specific emission properties of different materials.
Can emissivity values change over time?
Yes, factors like surface oxidation, dirt accumulation, or wear can alter a material's emissivity.
How does surface texture affect emissivity?
Smoother surfaces generally have lower emissivity, while rough or matte surfaces have higher emissivity due to increased surface area for radiation.
Is emissivity the same for all wavelengths of infrared?
Emissivity can vary with different wavelengths, as materials may emit differently across the infrared spectrum.
