Emissivity e varies with zenith angle according to e = E*cos(theta)

In summary, emissivity is a measure of how well a surface emits thermal radiation and varies with zenith angle due to changes in the angle at which radiation is emitted and received. The relationship between emissivity and zenith angle can be described by an equation and can affect thermal measurements by causing errors in temperature readings. This variation is influenced by factors such as material properties, surface roughness, temperature, and angle of incidence. To account for this variation in thermal calculations, appropriate correction factors and equations must be used.
  • #1
Callisto
41
0
If the emissivity e varies with zenith angle according to e = E*cos(theta) where E is the emissivity normal to the surface. Would this surface be an isotropic source of radiation?

Well, since e varies with angle then the flux density must vary accordingly so the surface would radiate anistropically.

Anybody disagree?
Callisto
 
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  • #2
This cosine law is like the one for intensity of incoming solar radiation, and that's a cylinder, so I agree, anisotropic.
 
  • #3
"Anisotropic" for a point on a surface; integrate intensity at any point above an infinite plane surface, and it's isotropic.
 

1. What is emissivity and why does it vary with zenith angle?

Emissivity is a measure of how well a surface emits thermal radiation. It is dependent on factors such as material composition, surface roughness, and temperature. The variation in emissivity with zenith angle is due to changes in the angle at which radiation is emitted and received by the surface.

2. What is the relationship between emissivity and zenith angle?

The relationship between emissivity and zenith angle is described by the equation e = E*cos(theta), where e is the emissivity, E is the maximum emissivity at normal incidence (zenith angle of 0 degrees), and theta is the zenith angle. As the zenith angle increases, the emissivity decreases due to the decrease in the effective surface area emitting and receiving radiation.

3. How does the variation in emissivity with zenith angle affect thermal measurements?

The variation in emissivity with zenith angle can affect thermal measurements by causing errors in temperature readings. This is because the measured temperature is dependent on the amount of radiation emitted and received by the surface, which is affected by the emissivity. Therefore, accurate corrections for emissivity must be applied in thermal measurements to ensure accurate results.

4. Are there any factors that can influence the variation of emissivity with zenith angle?

Yes, there are several factors that can influence the variation of emissivity with zenith angle. These include the material properties of the surface, the roughness of the surface, the temperature of the surface, and the wavelength of the radiation being emitted. Additionally, the angle of incidence of the radiation can also affect the variation of emissivity with zenith angle.

5. How can the variation in emissivity with zenith angle be taken into account in thermal calculations?

The variation in emissivity with zenith angle can be taken into account in thermal calculations by using the appropriate correction factors and equations. These corrections can be based on the material properties, roughness, and temperature of the surface, as well as the angle of incidence and wavelength of the radiation. It is important to accurately account for the variation in emissivity to ensure accurate and reliable thermal calculations.

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