Emissivity is a measure of how efficiently a surface radiates heat energy compared to a theoretically perfect radiator. It’s expressed as a number between 0 and 1, where 1 means the surface emits the maximum possible thermal radiation at a given temperature, and 0 means it emits none. Polished aluminum has an emissivity of just 0.05, while common brick sits at 0.85 and glass reaches 0.92.
The Blackbody Comparison
To understand emissivity, you need to know what it’s being measured against. Physicists use an idealized object called a “blackbody” as the baseline. A blackbody absorbs all radiation that hits it (reflecting nothing, hence the name) and re-emits the maximum amount of energy possible at every wavelength. No real object behaves this way perfectly, but it serves as the gold standard for comparison.
Emissivity is simply the ratio: divide the energy a real surface radiates by the energy a blackbody would radiate at the same temperature. If a surface emits 85% as much energy as a blackbody, its emissivity is 0.85. The Sun and Earth aren’t perfect blackbodies, but they’re close enough that scientists often treat them as such when studying energy emission at planetary scales.
Why Emissivity Equals Absorptivity
There’s an elegant rule in thermal physics known as Kirchhoff’s Law: at any given wavelength, a material’s emissivity equals its absorptivity. In plain terms, a surface that’s good at absorbing thermal radiation is equally good at emitting it, and a surface that reflects most incoming radiation emits very little of its own. This is why a shiny metal thermos keeps drinks hot. The polished surface has low emissivity, so it barely radiates heat away, and its low absorptivity means it also resists absorbing heat from the environment.
What Changes a Material’s Emissivity
Emissivity isn’t a single fixed number stamped on a material. Several factors shift it up or down.
Surface roughness is one of the biggest influences. Research on aluminum alloys at Purdue University confirmed that rougher surfaces have significantly higher emissivity than smooth ones. A polished aluminum sheet (emissivity around 0.05) radiates far less heat than the same alloy with a rough, sandblasted finish. This happens because a rough surface creates tiny cavities that trap and re-emit radiation rather than reflecting it away.
Material composition matters at a fundamental level. Metals tend to have low emissivity because their free electrons reflect most incoming radiation. Non-metals like wood, concrete, and skin tend to have high emissivity because their molecular structure absorbs and re-emits radiation efficiently.
Temperature also plays a role. The optical properties of a material shift as it heats up, which changes how much radiation it emits at different wavelengths. In general, the agreement between predicted and measured emissivity values improves at higher temperatures, partly because hot objects emit more radiation overall, making the measurement more reliable.
Viewing angle can affect readings too. A surface measured head-on may show a different emissivity than the same surface measured at a steep angle, because the way radiation scatters off the surface geometry changes with direction.
Spectral vs. Total Emissivity
When engineers and scientists talk about emissivity, they sometimes mean two different things. Spectral emissivity describes how well a surface radiates at one specific wavelength of infrared light. Total emissivity combines the radiation across all wavelengths into a single number. Most everyday references, like the values listed on a chart for brick or glass, refer to total emissivity. But in specialized applications like satellite sensor design or industrial furnace monitoring, spectral emissivity at particular wavelengths is what matters.
How Emissivity Affects Temperature Measurement
If you’ve ever had your temperature taken with a no-contact forehead thermometer, emissivity was part of that measurement. Infrared thermometers work by detecting the thermal radiation coming off a surface and converting it into a temperature reading. But the calculation only works if the device knows the emissivity of whatever it’s pointed at.
Human skin has an emissivity of 0.98 in the long-wave infrared range, which is what medical thermometers use. That’s very close to a perfect blackbody, which is one reason skin temperature is relatively easy to measure remotely. Medical devices are typically pre-set to 0.98 for this reason. When the emissivity is at or above 0.98, temperature errors stay below 1°C.
Industrial infrared thermometers face a harder challenge. If you’re measuring the temperature of a polished metal surface with an emissivity of 0.05, a device set to the wrong emissivity value will give wildly inaccurate readings. The thermometer interprets low radiation as low temperature, when in reality the surface is just reflecting most of its thermal energy instead of emitting it. This is why industrial users need to look up or measure the emissivity of whatever material they’re targeting and manually set it on their device.
Emissivity Values for Common Materials
Here are some reference points to give you a sense of the range:
- Polished aluminum: 0.05
- Common brick: 0.85
- Glass: 0.92
- Human skin: 0.98
The pattern is clear: shiny metals sit at the bottom of the scale, while organic materials, ceramics, and rough surfaces cluster near the top. If you need precise values for a specific material, emissivity charts published by instrument manufacturers like Fluke cover hundreds of surfaces at various temperatures. For critical applications, ASTM International publishes standardized procedures for measuring emissivity directly using infrared imaging equipment, which can be done in either a lab or field setting.
Why It Matters Beyond the Lab
Emissivity has practical consequences in building design, where materials with different emissivity values gain or lose heat at different rates. A metal roof reflects most solar radiation and emits little thermal energy, which changes how the building heats and cools compared to a dark asphalt roof with high emissivity. It’s also central to climate science, where the emissivity of Earth’s surface, oceans, ice, and atmosphere determines how much heat the planet radiates back into space.
In manufacturing, controlling emissivity helps engineers manage heat in everything from semiconductor fabrication to jet engine components. Coatings can be applied to raise or lower a surface’s emissivity depending on whether the goal is to shed heat quickly or retain it. Even spacecraft use specialized high-emissivity and low-emissivity coatings on different surfaces to manage extreme temperature swings in orbit.