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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