Black and Grey Surfaces

• A black, or ideal surface, will have no surface resistance:

In this case the nodal Radiosity and emissive power will be equal.

This result gives some insight into the physical meaning of a black surface. Ideal surfaces radiate at the maximum possible level. Non-black surfaces will have a reduced potential, somewhat like a battery with a corroded terminal. They therefore have a reduced potential to cause heat/current flow.

• Large surfaces: Surfaces having a large surface area will behave as black surfaces, irrespective of the actual surface properties:

Physically, this corresponds to the characteristic of large surfaces that as they reflect energy, there is very little chance that energy will strike the smaller surfaces; most of the energy is reflected back to another part of the same large surface. After several partial absorptions most of the energy received is absorbed.

Gray Surfaces: The gray surface is a medium whose monochromatic emissivity (ε_λ) does not vary with wavelength. The monochromatic emissivity is defined as the ratio of the monochromatic emissive power of the body to monochromatic emissive power of a black body at same wave length and temperature.

ε_λ = E_λ / E_{λ, b}

But, we know the following.

Therefore,

Where: E_λ,b is the emissive power of a black body per unit wave length. If the gray body condition is imposed, that is, ε_λ = constant, so that: ε = ε_λ = constant