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
光出射度