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I know the radiance is expressed as $$\text{[Radiance]} = \frac{W \cdot m^2}{\text{sterad}}$$

and

$$\text{[Irradiance]} = W \cdot m^2$$

But what's the relation between theese two quantities? Is the irradiance commonly used to referring to power reflected on a surface? And the radiance from a direct source?

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As you stated both radiometry units seem to be similar. While irradiance refers to incoming power, the radiance is used for two cases. It is used to describe angle dependant diffuse reflection and as well emission from light sources.

E.g. radiance in direction of the optical axis of a LED is higher, than it's radiance at an angle of 15°. Optical simulations / ray tracing calculate the irradiance on surfaces. Your last two questions are mainly correct. However I would rephrase the first statement: Irradiance commonly is used referring to power incident on a surface.

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