- #1
mnb96
- 715
- 5
Hello,
I am considering an infinitesimal surface dA that receives a total radiant flux [itex]d\Phi[/itex], so basically we know the irradiance, that is given by [itex]dE = \frac{d\Phi}{dA}[/itex].
If I assume that this surface reflects 100% of the light, and it is Lambertian, how am I supposed to calculate the radiance coming out from it, in some direction?
-- My attempt was the following: --
The fact that dA is Lambertian means that the radiance emitted by it must be constant for any direction, moreover dA reflects all the radiation, so the radiant exitance is equal to its irradiance. However the radiance is defined as:
[tex]L=\frac{d^2\Phi}{dAcos\theta\cdot d\omega}[/tex]
where θ is the angle between the direction we are considering, and the normal of dA.
Since we know the radiant exitance is dE we can write:
[tex]L=\frac{dE}{cos\theta\cdot d\omega}[/tex]
and here something weird has happened, because L is supposed to be constant, dE is constant too, but we have a cosθ term, so the right term is dependent on the direction. Where is the mistake?
I am considering an infinitesimal surface dA that receives a total radiant flux [itex]d\Phi[/itex], so basically we know the irradiance, that is given by [itex]dE = \frac{d\Phi}{dA}[/itex].
If I assume that this surface reflects 100% of the light, and it is Lambertian, how am I supposed to calculate the radiance coming out from it, in some direction?
-- My attempt was the following: --
The fact that dA is Lambertian means that the radiance emitted by it must be constant for any direction, moreover dA reflects all the radiation, so the radiant exitance is equal to its irradiance. However the radiance is defined as:
[tex]L=\frac{d^2\Phi}{dAcos\theta\cdot d\omega}[/tex]
where θ is the angle between the direction we are considering, and the normal of dA.
Since we know the radiant exitance is dE we can write:
[tex]L=\frac{dE}{cos\theta\cdot d\omega}[/tex]
and here something weird has happened, because L is supposed to be constant, dE is constant too, but we have a cosθ term, so the right term is dependent on the direction. Where is the mistake?