as you know, the ideal black-body has a lot of interesting properties. The most important of these is surely the Planck Law about its radiation:

enter image description here

Clearly the concept of black body does not exist perfectly in nature, but there are some objects that may approximate it. So we may say that a real non-ideal black body has a spectral radiance behaviour which is similar to that shown in the previous graph.

But now let's consider a generic object (not a non - ideal black body, something like a green object, a grey object etc). How does its spectral radiance appear? How is its shape? Is it similar to that of a blackbody, but less high at same temperature (that means less radiation)?

I have this question because I have always seen the analysis of blackbody radiation for several applications (for instance semiconductors) in which the object was not black.... so I do not understand the reason of this analysis.

So, does a generic body have something in common with the blackbody radiation?


1 Answer 1


The blackbody radiation curve supposes that the body radiates the same energy that it absorbs.

If a green object is at room temperature inside a closed cavity, the thermal energy it absorbs from the environment is at equilibrium with the infrared radiation it emits. It is a blackbody curve according to the room temperature.

But if it is outside, getting light of the sun, besides the BB radiation there is also scattered green light.

It happens because the situation is far from equilibium. There is a body (the sun) radiating at more that 5000 K, and the object radiating at 300 K.

  • $\begingroup$ In case the green body is not at equilibrium, which would be the shape of the spectral radiance? $\endgroup$
    – Kinka-Byo
    Jun 26, 2020 at 22:43
  • $\begingroup$ The greenness of an object is not a function of whether it is in thermal equilibrium. $\endgroup$
    – ProfRob
    Jun 27, 2020 at 0:13
  • $\begingroup$ @Kinka-Byo It depends on its emissivity. But anyway it is necessary to compensate for the scattered sunlight. $\endgroup$ Jun 27, 2020 at 0:37

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