Spectral Directional Emissivity

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In summary, the direction of emission in the thermal band [7-14micron], particularly for non-metallic surfaces, can be influenced by the material's physical properties such as density and surface roughness. This can in turn affect the material's emissivity.
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sliwintm
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Hi all,

I am trying to understand why emissivity changes with direction. Specifically within the thermal band [7-14micron] and while looking at non-metallic surfaces (like a flat water surface).

What I'm thinking is that since vibrating atoms are basically accelerating charges and create EM waves that the density of the materials I'm observing cause restrictions to the atomic vibration and thus the emission angle of the light.

My background is in engineering. I'm just trying to get some enlightenment on the subject.
 
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Thank you for your post and your interest in understanding emissivity. Emissivity, or the ability of a material to emit thermal radiation, is affected by a number of factors including surface roughness, temperature, and direction. In the thermal band [7-14micron], the direction of emission can be influenced by the physical properties of the material, such as its density and molecular structure.

As you mentioned, the vibrating atoms in a material create electromagnetic waves, and the density of the material can affect the direction of these waves. For non-metallic surfaces like water, the density of the water molecules can vary depending on the direction of observation. This can impact the emissivity of the water surface, as the direction of emission may be more restricted in certain directions due to the arrangement of the molecules.

Additionally, the surface roughness of the material can also play a role in the direction of emission. A smoother surface may allow for more uniform emission in all directions, while a rougher surface may cause the emission to be more directional.

Overall, the combination of the material's physical properties, such as density and surface roughness, can affect the direction of emission and ultimately impact the emissivity of the material. I hope this helps to provide some insight into the topic.
 

1. What is spectral directional emissivity?

Spectral directional emissivity is a measure of the ability of a material to emit heat radiation at different wavelengths and in different directions. It is typically expressed as a ratio between the actual emitted radiation and the maximum possible emission for a given temperature and direction.

2. How is spectral directional emissivity measured?

Spectral directional emissivity can be measured using specialized equipment such as spectrometers, which measure the intensity of radiation emitted by a material at different wavelengths and angles. The data obtained from these measurements can then be used to calculate the emissivity at specific wavelengths and directions.

3. Why is spectral directional emissivity important?

Spectral directional emissivity is important in many areas of science and engineering, particularly in the study of heat transfer and energy conservation. It is also important in fields such as remote sensing and infrared thermography, where accurate knowledge of emissivity is necessary for accurate temperature measurements and material identification.

4. What factors affect spectral directional emissivity?

The spectral directional emissivity of a material can be affected by factors such as surface roughness, composition, and temperature. It can also vary with the wavelength of the emitted radiation and the direction of emission.

5. How can spectral directional emissivity be improved?

Spectral directional emissivity can be improved by using materials with smoother surfaces, as surface roughness can decrease emissivity. Coatings and treatments can also be applied to surfaces to alter their emissivity properties. Additionally, accurate measurements and modeling can help to improve our understanding of emissivity and how to control it.

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