Radiation and Lambert's Cosine Law

[Dewsquad]

Hi! I'm new to this forum and signed up to ask a few questions from my course. 

 

1. Lambert's Cosine Law says that radiative flux over an infinitely small area is largest at the perpendicular normal angle. But when the same flux is at an angle, the flux is spread over a larger area and is then proportionally smaller. This can be described with the equation

E [perpendicular] = E [perpendicular] * cos(zenith angle)

Is this correct? Have I described it right?

 

2. I have read about Lambertian surfaces, or perfectly diffuse surfaces. Is Lambert's Cosine Law assuming the target of the irradiance is a perfectly diffuse surface? Or is it assuming that the source of the radiation is a perfectly diffuse surface? 

 

3. Lambertian surfaces also talk about perfectly diffuse vs perfect radiator. What is the difference between these two? 

 

4. Is a Lambertian surface assumed to have zero transmission or absorption? Does perfectly diffuse only describe reflection?

Edited December 19, 2023 by Dewsquad

[swansont]

17 minutes ago, Dewsquad said:

Hi! I'm new to this forum and signed up to ask a few questions from my course. 

 

1. Lambert's Cosine Law says that radiative flux over an infinitely small area is largest at the perpendicular normal angle. But when the same flux is at an angle, the flux is spread over a larger area and is then proportionally smaller. This can be described with the equation

E [perpendicular] = E [perpendicular] * cos(zenith angle)

Is this correct? Have I described it right?

It’s not the perpendicular flux on the LHS of the equation 

17 minutes ago, Dewsquad said:

 

2. I have read about Lambertian surfaces, or perfectly diffuse surfaces. Is Lambert's Cosine Law assuming the target of the irradiance is a perfectly diffuse surface? Or is it assuming that the source of the radiation is a perfectly diffuse surface? 

The surface in question. Whether it’s reflecting or radiating.

17 minutes ago, Dewsquad said:

3. Lambertian surfaces also talk about perfectly diffuse vs perfect radiator. What is the difference between these two? 

I’ve not seen that phrasing; the surface is perfectly diffuse, and it either reflects or radiates.

17 minutes ago, Dewsquad said:

4. Is a Lambertian surface assumed to have zero transmission or absorption? Does perfectly diffuse only describe reflection?

Perfectly diffuse describes reflection or radiation. There is no transmission or absorption.