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==== Radiant Exitance ====
**Radiant exitance**, denoted as $M$, refers to the radiant flux emitted per unit time from a unit area of a surface into the outward hemispherical space, with the unit being $W/m^2$.\\
Its mathematical expression is:
$$M = \frac{d\Phi}{dA}$$

**Relationship with Radiance**\\
Radiant exitance can be expressed as the integral of radiance over all outgoing directions from the surface:\\
$$M = \int_{\Omega} L(\omega) \cos\theta \, d\omega$$
where:\\
  * $L(\omega)$: radiance of the surface element in the direction $\omega$\\
  * $\theta$: angle between the surface normal and the direction of radiant exitance\\
  * $\Omega$: solid angle of the hemisphere\\

**Relationship with Irradiance**\\
For an ideal diffuse reflection surface (Lambertian surface), the radiant exitance and the incident irradiance satisfy:
$$M = \rho E$$

where:\\
  * $\rho$: surface reflectance\\
  * $E$: surface irradiance, in units of $\mathrm{W/m^2}$\\


**Special Case: Ideal Lambertian Surface**\\
For an ideal Lambertian surface, its radiance is identical in all directions, which simplifies to:\\
$$M = \pi L$$

Note:\\
In photometry, the corresponding quantity is luminous exitance, with the unit being $lm/m^2$.

**See Also**\\
[[yanding:成像基础知识:光学:辐射度学与光度学:光出射度]]